Vaccine

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Vaccine
Jonas Salk in 1955 holds two bottles of a culture used to grow polio vaccines.
MeSH	D014612

A vaccine is a biological preparation that provides active acquired immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism and is often made from weakened or killed forms of the microbe, its toxins, or one of its surface proteins. The agent stimulates the body's immune system to recognize the agent as a threat, destroy it, and to further recognize and destroy any of the microorganisms associated with that agent that it may encounter in the future. Vaccines can be prophylactic (example: to prevent or ameliorate the effects of a future infection by a natural or "wild" pathogen), or therapeutic (e.g., vaccines against cancer are being investigated).

The administration of vaccines is called vaccination. Vaccination is the most effective method of preventing infectious diseases;^[5] widespread immunity due to vaccination is largely responsible for the worldwide eradication of smallpox and the restriction of diseases such as polio, measles, and tetanus from much of the world. The effectiveness of vaccination has been widely studied and verified; for example, the influenza vaccine,^[6] the HPV vaccine,^[7] and the chicken pox vaccine.^[8] The World Health Organization (WHO) reports that licensed vaccines are currently available for twenty-five different preventable infections.^[9]

The terms vaccine and vaccination are derived from Variolae vaccinae (smallpox of the cow), the term devised by Edward Jenner to denote cowpox. He used it in 1798 in the long title of his Inquiry into the Variolae vaccinae known as the Cow Pox, in which he described the protective effect of cowpox against smallpox.^[10] In 1881, to honor Jenner, Louis Pasteur proposed that the terms should be extended to cover the new protective inoculations then being developed.^[11]

Effectiveness

Vaccines have historically been the most effective means to fight and eradicate infectious diseases. Limitations to their effectiveness, nevertheless, exist.^[12] Sometimes, protection fails because the host's immune system simply does not respond adequately or at all. Lack of response commonly results from clinical factors such as diabetes, steroid use, HIV infection or age.^{[citation needed]} It also might fail for genetic reasons if the host's immune system includes no strains of B cells that can generate antibodies suited to reacting effectively and binding to the antigens associated with the pathogen.

Even if the host does develop antibodies, protection might not be adequate; immunity might develop too slowly to be effective in time, the antibodies might not disable the pathogen completely, or there might be multiple strains of the pathogen, not all of which are equally susceptible to the immune reaction. However, even a partial, late, or weak immunity, such as a one resulting from cross-immunity to a strain other than the target strain, may mitigate an infection, resulting in a lower mortality rate, lower morbidity, and faster recovery.

Adjuvants commonly are used to boost immune response, particularly for older people (50–75 years and up), whose immune response to a simple vaccine may have weakened.^[13]

Maurice Hilleman's measles vaccine is estimated to prevent 1 million deaths every year.^[14]

The efficacy or performance of the vaccine is dependent on a number of factors:

• the disease itself (for some diseases vaccination performs better than for others)
• the strain of vaccine (some vaccines are specific to, or at least most effective against, particular strains of the disease)^[15]
• whether the vaccination schedule has been properly observed.
• idiosyncratic response to vaccination; some individuals are "non-responders" to certain vaccines, meaning that they do not generate antibodies even after being vaccinated correctly.
• assorted factors such as ethnicity, age, or genetic predisposition.

If a vaccinated individual does develop the disease vaccinated against (breakthrough infection), the disease is likely to be less virulent than in unvaccinated victims.^[16]

The following are important considerations in the effectiveness of a vaccination program:

1. careful modeling to anticipate the effect that an immunization campaign will have on the epidemiology of the disease in the medium to long term
2. ongoing surveillance for the relevant disease following introduction of a new vaccine
3. maintenance of high immunization rates, even when a disease has become rare.

In 1958, there were 763,094 cases of measles in the United States; 552 deaths resulted.^[17][18] After the introduction of new vaccines, the number of cases dropped to fewer than 150 per year (median of 56).^[18] In early 2008, there were 64 suspected cases of measles. Fifty-four of those infections were associated with importation from another country, although only 13% were actually acquired outside the United States; 63 of the 64 individuals either had never been vaccinated against measles or were uncertain whether they had been vaccinated.^[18]

Vaccines have contributed to the eradication of smallpox, one of the most contagious and deadly diseases in humans. Other diseases such as rubella, polio, measles, mumps, chickenpox, and typhoid are nowhere near as common as they were a hundred years ago. As long as the vast majority of people are vaccinated, it is much more difficult for an outbreak of disease to occur, let alone spread. This effect is called herd immunity. Polio, which is transmitted only between humans, is targeted by an extensive eradication campaign that has seen endemic polio restricted to only parts of three countries (Afghanistan, Nigeria, and Pakistan).^[19] However, the difficulty of reaching all children as well as cultural misunderstandings have caused the anticipated eradication date to be missed several times.

Vaccines also help prevent the development of antibiotic resistance. For example, by greatly reducing the incidence of pneumonia caused by Streptococcus pneumoniae, vaccine programs have greatly reduced the prevalence of infections resistant to penicillin or other first-line antibiotics.^[20]

Adverse effects

Vaccination given during childhood is generally safe.^[21] Adverse effects if any are generally mild.^[22] The rate of side effects depends on the vaccine in question.^[22] Some common side effects include fever, pain around the injection site, and muscle aches.^[22] Additionally, some individuals may be allergic to ingredients in the vaccine.^[23] MMR vaccine is rarely associated with febrile seizures.^[21]

Severe side effects are extremely rare.^[21] Varicella vaccine is rarely associated with complications in immunodeficient individuals and rotavirus vaccines are moderately associated with intussusception.^[21]

Types

Vaccine

Avian flu vaccine development by reverse genetics techniques.

Vaccines are dead or inactivated organisms or purified products derived from them.

There are several types of vaccines in use.^[24] These represent different strategies used to try to reduce the risk of illness while retaining the ability to induce a beneficial immune response.

Inactivated

Some vaccines contain inactivated, but previously virulent, micro-organisms that have been destroyed with chemicals, heat, or radiation.^[25] Examples include the polio vaccine, hepatitis A vaccine, rabies vaccine and some influenza vaccines.^{[citation needed]}

Attenuated

Some vaccines contain live, attenuated microorganisms. Many of these are active viruses that have been cultivated under conditions that disable their virulent properties, or that use closely related but less dangerous organisms to produce a broad immune response. Although most attenuated vaccines are viral, some are bacterial in nature. Examples include the viral diseases yellow fever, measles, mumps, and rubella, and the bacterial disease typhoid. The live Mycobacterium tuberculosis vaccine developed by Calmette and Guérin is not made of a contagious strain but contains a virulently modified strain called "BCG" used to elicit an immune response to the vaccine. The live attenuated vaccine containing strain Yersinia pestis EV is used for plague immunization. Attenuated vaccines have some advantages and disadvantages. They typically provoke more durable immunological responses and are the preferred type for healthy adults. But they may not be safe for use in immunocompromised individuals, and on rare occasions mutate to a virulent form and cause disease.^[26]

Toxoid

Toxoid vaccines are made from inactivated toxic compounds that cause illness rather than the micro-organism.^{[citation needed]} Examples of toxoid-based vaccines include tetanus and diphtheria. Toxoid vaccines are known for their efficacy.^{[citation needed]} Not all toxoids are for micro-organisms; for example, Crotalus atrox toxoid is used to vaccinate dogs against rattlesnake bites.^{[citation needed]}

Subunit

Protein subunit – rather than introducing an inactivated or attenuated micro-organism to an immune system (which would constitute a "whole-agent" vaccine), a fragment of it can create an immune response.^{[citation needed]} Examples include the subunit vaccine against Hepatitis B virus that is composed of only the surface proteins of the virus (previously extracted from the blood serum of chronically infected patients, but now produced by recombination of the viral genes into yeast)^{[citation needed]} or as an edible algae vaccine, the virus-like particle (VLP) vaccine against human papillomavirus (HPV) that is composed of the viral major capsid protein,^{[citation needed]} and the hemagglutinin and neuraminidase subunits of the influenza virus.^{[citation needed]} Subunit vaccine is being used for plague immunization.^{[citation needed]}

Conjugate

Conjugate – certain bacteria have polysaccharide outer coats that are poorly immunogenic. By linking these outer coats to proteins (e.g., toxins), the immune system can be led to recognize the polysaccharide as if it were a protein antigen.^{[citation needed]} This approach is used in the Haemophilus influenzae type B vaccine.^{[citation needed]}

Experimental

Electroporation System for experimental "DNA vaccine" delivery

A number of innovative vaccines are also in development and in use:

• Dendritic cell vaccines combine dendritic cells with antigens in order to present the antigens to the body's white blood cells, thus stimulating an immune reaction. These vaccines have shown some positive preliminary results for treating brain tumors ^[27] and are also tested in malignant melanoma.^[28]
• Recombinant Vector – by combining the physiology of one micro-organism and the DNA of the other, immunity can be created against diseases that have complex infection processes. An example is the RVSV-ZEBOV vaccine licensed to Merck that is being used in 2018 to combat ebola in Congo.^[29]
• DNA vaccination – an alternative, experimental approach to vaccination called DNA vaccination, created from an infectious agent's DNA, is under development. The proposed mechanism is the insertion (and expression, enhanced by the use of electroporation, triggering immune system recognition) of viral or bacterial DNA into human or animal cells. Some cells of the immune system that recognize the proteins expressed will mount an attack against these proteins and cells expressing them. Because these cells live for a very long time, if the pathogen that normally expresses these proteins is encountered at a later time, they will be attacked instantly by the immune system. One potential advantage of DNA vaccines is that they are very easy to produce and store. As of 2015, DNA vaccination is still experimental and is not approved for human use.
• T-cell receptor peptide vaccines are under development for several diseases using models of Valley Fever, stomatitis, and atopic dermatitis. These peptides have been shown to modulate cytokine production and improve cell-mediated immunity.
• Targeting of identified bacterial proteins that are involved in complement inhibition would neutralize the key bacterial virulence mechanism.^[30]

While most vaccines are created using inactivated or attenuated compounds from micro-organisms, synthetic vaccines are composed mainly or wholly of synthetic peptides, carbohydrates, or antigens.

Valence

Vaccines may be monovalent (also called univalent) or multivalent (also called polyvalent). A monovalent vaccine is designed to immunize against a single antigen or single microorganism.^[31] A multivalent or polyvalent vaccine is designed to immunize against two or more strains of the same microorganism, or against two or more microorganisms.^[32] The valency of a multivalent vaccine may be denoted with a Greek or Latin prefix (e.g., tetravalent or quadrivalent). In certain cases, a monovalent vaccine may be preferable for rapidly developing a strong immune response.^[33]

Heterotypic

Also known as heterologous or "Jennerian" vaccines, these are vaccines that are pathogens of other animals that either do not cause disease or cause mild disease in the organism being treated. The classic example is Jenner's use of cowpox to protect against smallpox. A current example is the use of BCG vaccine made from Mycobacterium bovis to protect against human tuberculosis.^[34]

Nomenclature

Various fairly standardized abbreviations for vaccine names have developed, although the standardization is by no means centralized or global. For example, the vaccine names used in the United States have well-established abbreviations that are also widely known and used elsewhere. An extensive list of them provided in a sortable table and freely accessible, is available at a US Centers for Disease Control and Prevention web page.^[35] The page explains that "The abbreviations [in] this table (Column 3) were standardized jointly by staff of the Centers for Disease Control and Prevention, ACIP Work Groups, the editor of the Morbidity and Mortality Weekly Report (MMWR), the editor of Epidemiology and Prevention of Vaccine-Preventable Diseases (the Pink Book), ACIP members, and liaison organizations to the ACIP."^[35] Some examples are "DTaP" for diphtheria and tetanus toxoids and acellular pertussis vaccine, "DT" for diphtheria and tetanus toxoids, and "Td" for tetanus and diphtheria toxoids. At its page on tetanus vaccination,^[36] the CDC further explains that "Upper-case letters in these abbreviations denote full-strength doses of diphtheria (D) and tetanus (T) toxoids and pertussis (P) vaccine. Lower-case "d" and "p" denote reduced doses of diphtheria and pertussis used in the adolescent/adult-formulations. The 'a' in DTaP and Tdap stands for 'acellular,' meaning that the pertussis component contains only a part of the pertussis organism."^[36] Another list of established vaccine abbreviations is at the CDC's page called "Vaccine Acronyms and Abbreviations", with abbreviations used on U.S. immunization records.^[37] The United States Adopted Name system has some conventions for the word order of vaccine names, placing head nouns first and adjectives postpositively. This is why the USAN for "OPV" is "poliovirus vaccine live oral" rather than "oral poliovirus vaccine".

Developing immunity

The immune system recognizes vaccine agents as foreign, destroys them, and "remembers" them. When the virulent version of an agent is encountered, the body recognizes the protein coat on the virus, and thus is prepared to respond, by (1) neutralizing the target agent before it can enter cells, and (2) recognizing and destroying infected cells before that agent can multiply to vast numbers.

When two or more vaccines are mixed together in the same formulation, the two vaccines can interfere. This most frequently occurs with live attenuated vaccines, where one of the vaccine components is more robust than the others and suppresses the growth and immune response to the other components. This phenomenon was first noted in the trivalent Sabin polio vaccine, where the amount of serotype 2 virus in the vaccine had to be reduced to stop it from interfering with the "take" of the serotype 1 and 3 viruses in the vaccine.^[38] This phenomenon has also been found to be a problem with the dengue vaccines currently being researched,^[when?] where the DEN-3 serotype was found to predominate and suppress the response to DEN-1, −2 and −4 serotypes.^[39]

Adjuvants and preservatives

Vaccines typically contain one or more adjuvants, used to boost the immune response. Tetanus toxoid, for instance, is usually adsorbed onto alum. This presents the antigen in such a way as to produce a greater action than the simple aqueous tetanus toxoid. People who have an adverse reaction to adsorbed tetanus toxoid may be given the simple vaccine when the time comes for a booster.

In the preparation for the 1990 Persian Gulf campaign, whole cell pertussis vaccine was used as an adjuvant for anthrax vaccine. This produces a more rapid immune response than giving only the anthrax vaccine, which is of some benefit if exposure might be imminent.^{[citation needed]}

Vaccines may also contain preservatives to prevent contamination with bacteria or fungi. Until recent years, the preservative thimerosal was used in many vaccines that did not contain live virus. As of 2005, the only childhood vaccine in the U.S. that contains thimerosal in greater than trace amounts is the influenza vaccine,^[40] which is currently recommended only for children with certain risk factors.^[41] Single-dose influenza vaccines supplied in the UK do not list thiomersal (its UK name) in the ingredients. Preservatives may be used at various stages of production of vaccines, and the most sophisticated methods of measurement might detect traces of them in the finished product, as they may in the environment and population as a whole.^[42]

Schedule

In order to provide the best protection, children are recommended to receive vaccinations as soon as their immune systems are sufficiently developed to respond to particular vaccines, with additional "booster" shots often required to achieve "full immunity". This has led to the development of complex vaccination schedules. In the United States, the Advisory Committee on Immunization Practices, which recommends schedule additions for the Centers for Disease Control and Prevention, recommends routine vaccination of children against:^[43] hepatitis A, hepatitis B, polio, mumps, measles, rubella, diphtheria, pertussis, tetanus, HiB, chickenpox, rotavirus, influenza, meningococcal disease and pneumonia.^[44] A large number of vaccines and boosters recommended (up to 24 injections by age two) has led to problems with achieving full compliance. In order to combat declining compliance rates, various notification systems have been instituted and a number of combination injections are now marketed (e.g., Pneumococcal conjugate vaccine and MMRV vaccine), which provide protection against multiple diseases.

Besides recommendations for infant vaccinations and boosters, many specific vaccines are recommended for other ages or for repeated injections throughout life—most commonly for measles, tetanus, influenza, and pneumonia. Pregnant women are often screened for continued resistance to rubella. The human papillomavirus vaccine is recommended in the U.S. (as of 2011)^[45] and UK (as of 2009).^[46] Vaccine recommendations for the elderly concentrate on pneumonia and influenza, which are more deadly to that group. In 2006, a vaccine was introduced against shingles, a disease caused by the chickenpox virus, which usually affects the elderly.

History

Edward Jenner

Prior to the introduction of vaccination with material from cases of cowpox (heterotypic immunisation), smallpox could be prevented by deliberate inoculation of smallpox virus, later referred to as variolation to distinguish it from smallpox vaccination. The earliest hints of the practice of inoculation for smallpox in China come during the 10th century.^[47] The Chinese also practiced the oldest documented use of variolation, dating back to the fifteenth century. They implemented a method of "nasal insufflation" administered by blowing powdered smallpox material, usually scabs, up the nostrils. Various insufflation techniques have been recorded throughout the sixteenth and seventeenth centuries within China.^[48]:60 Two reports on the Chinese practice of inoculation were received by the Royal Society in London in 1700; one by Dr. Martin Lister who received a report by an employee of the East India Company stationed in China and another by Clopton Havers.^[49]

Jenner's handwritten draft of the first vaccination

Sometime during the late 1760s whilst serving his apprenticeship as a surgeon/apothecary Edward Jenner learned of the story, common in rural areas, that dairy workers would never have the often-fatal or disfiguring disease smallpox, because they had already had cowpox, which has a very mild effect in humans. In 1796, Jenner took pus from the hand of a milkmaid with cowpox, scratched it into the arm of an 8-year-old boy, James Phipps, and six weeks later inoculated (variolated) the boy with smallpox, afterwards observing that he did not catch smallpox.^[50][51] Jenner extended his studies and in 1798 reported that his vaccine was safe in children and adults and could be transferred from arm-to-arm reducing reliance on uncertain supplies from infected cows.^[10] Since vaccination with cowpox was much safer than smallpox inoculation,^[52] the latter, though still widely practised in England, was banned in 1840.^[53]

French print in 1896 marking the centenary of Jenner's vaccine

The second generation of vaccines was introduced in the 1880s by Louis Pasteur who developed vaccines for chicken cholera and anthrax,^[11] and from the late nineteenth century vaccines were considered a matter of national prestige, and compulsory vaccination laws were passed.^[50]

The twentieth century saw the introduction of several successful vaccines, including those against diphtheria, measles, mumps, and rubella. Major achievements included the development of the polio vaccine in the 1950s and the eradication of smallpox during the 1960s and 1970s. Maurice Hilleman was the most prolific of the developers of the vaccines in the twentieth century. As vaccines became more common, many people began taking them for granted. However, vaccines remain elusive for many important diseases, including herpes simplex, malaria, gonorrhea and HIV.^[50][54]

Timeline

Vaccine timeline

Economics of development

One challenge in vaccine development is economic: Many of the diseases most demanding a vaccine, including HIV, malaria and tuberculosis, exist principally in poor countries. Pharmaceutical firms and biotechnology companies have little incentive to develop vaccines for these diseases, because there is little revenue potential. Even in more affluent countries, financial returns are usually minimal and the financial and other risks are great.^[55]

Most vaccine development to date has relied on "push" funding by government, universities and non-profit organizations.^[56] Many vaccines have been highly cost effective and beneficial for public health.^[57] The number of vaccines actually administered has risen dramatically in recent decades.^[58] This increase, particularly in the number of different vaccines administered to children before entry into schools may be due to government mandates and support, rather than economic incentive.

Patents

The filing of patents on vaccine development processes can also be viewed as an obstacle to the development of new vaccines. Because of the weak protection offered through a patent on the final product, the protection of the innovation regarding vaccines is often made through the patent of processes used in the development of new vaccines as well as the protection of secrecy.^[59]

According to the World Health Organization, the biggest barrier to local vaccine production in less developed countries has not been patents, but the substantial financial, infrastructure, and workforce expertise requirements needed for market entry. Vaccines are complex mixtures of biological compounds, and unlike the case of drugs, there are no true generic vaccines. The vaccine produced by a new facility must undergo complete clinical testing for safety and efficacy similar to that undergone by that produced by the original manufacturer. For most vaccines, specific processes have been patented. These can be circumvented by alternative manufacturing methods, but this required R&D infrastructure and a suitably skilled workforce. In the case of a few relatively new vaccines such as the human papillomavirus vaccine, the patents may impose an additional barrier.^[60]

Production

Two workers make openings in chicken eggs in preparation for production of measles vaccine.

Vaccine production has several stages. First, the antigen itself is generated. Viruses are grown either on primary cells such as chicken eggs (e.g., for influenza) or on continuous cell lines such as cultured human cells (e.g., for hepatitis A).^[61] Bacteria are grown in bioreactors (e.g., Haemophilus influenzae type b). Likewise, a recombinant protein derived from the viruses or bacteria can be generated in yeast, bacteria, or cell cultures. After the antigen is generated, it is isolated from the cells used to generate it. A virus may need to be inactivated, possibly with no further purification required. Recombinant proteins need many operations involving ultrafiltration and column chromatography. Finally, the vaccine is formulated by adding adjuvant, stabilizers, and preservatives as needed. The adjuvant enhances the immune response of the antigen, stabilizers increase the storage life, and preservatives allow the use of multidose vials.^[62][63] Combination vaccines are harder to develop and produce, because of potential incompatibilities and interactions among the antigens and other ingredients involved.^[64]

Vaccine production techniques are evolving. Cultured mammalian cells are expected to become increasingly important, compared to conventional options such as chicken eggs, due to greater productivity and low incidence of problems with contamination. Recombination technology that produces genetically detoxified vaccine is expected to grow in popularity for the production of bacterial vaccines that use toxoids. Combination vaccines are expected to reduce the quantities of antigens they contain, and thereby decrease undesirable interactions, by using pathogen-associated molecular patterns.^[64]

In 2010, India produced 60 percent of the world's vaccine worth about $900 million(€670 million).^[65]

Excipients

Beside the active vaccine itself, the following excipients and residual manufacturing compounds are present or may be present in vaccine preparations:^[66]

• Aluminum salts or gels are added as adjuvants. Adjuvants are added to promote an earlier, more potent response, and more persistent immune response to the vaccine; they allow for a lower vaccine dosage.
• Antibiotics are added to some vaccines to prevent the growth of bacteria during production and storage of the vaccine.
• Egg protein is present in influenza and yellow fever vaccines as they are prepared using chicken eggs. Other proteins may be present.
• Formaldehyde is used to inactivate bacterial products for toxoid vaccines. Formaldehyde is also used to inactivate unwanted viruses and kill bacteria that might contaminate the vaccine during production.
• Monosodium glutamate (MSG) and 2-phenoxyethanol are used as stabilizers in a few vaccines to help the vaccine remain unchanged when the vaccine is exposed to heat, light, acidity, or humidity.
• Thimerosal is a mercury-containing antimicrobial that is added to vials of vaccine that contain more than one dose to prevent contamination and growth of potentially harmful bacteria. Due to the controversy surrounding thimerosal it has been removed from most vaccines except multi-use influenza, where it was reduced to levels so that a single dose contained less than 1 microgram of mercury, a level similar to eating 10g of canned tuna.^[67]

Role of preservatives

Many vaccines need preservatives to prevent serious adverse effects such as Staphylococcus infection, which in one 1928 incident killed 12 of 21 children inoculated with a diphtheria vaccine that lacked a preservative.^[68] Several preservatives are available, including thiomersal, phenoxyethanol, and formaldehyde. Thiomersal is more effective against bacteria, has a better shelf-life, and improves vaccine stability, potency, and safety; but, in the U.S., the European Union, and a few other affluent countries, it is no longer used as a preservative in childhood vaccines, as a precautionary measure due to its mercury content.^[69] Although controversial claims have been made that thiomersal contributes to autism, no convincing scientific evidence supports these claims.^[70]

Delivery systems

Woman receiving rubella vaccination, Brazil, 2008.

The development of new delivery systems raises the hope of vaccines that are safer and more efficient to deliver and administer. Lines of research include liposomes and ISCOM (immune stimulating complex).^[71]

Notable developments in vaccine delivery technologies have included oral vaccines. Early attempts to apply oral vaccines showed varying degrees of promise, beginning early in the 20th century, at a time when the very possibility of an effective oral antibacterial vaccine was controversial.^[72] By the 1930s there was increasing interest in the prophylactic value of an oral typhoid fever vaccine for example.^[73]

An oral polio vaccine turned out to be effective when vaccinations were administered by volunteer staff without formal training; the results also demonstrated increased ease and efficiency of administering the vaccines. Effective oral vaccines have many advantages; for example, there is no risk of blood contamination. Vaccines intended for oral administration need not be liquid, and as solids, they commonly are more stable and less prone to damage or to spoilage by freezing in transport and storage.^[74] Such stability reduces the need for a "cold chain": the resources required to keep vaccines within a restricted temperature range from the manufacturing stage to the point of administration, which, in turn, may decrease costs of vaccines.

A microneedle approach, which is still in stages of development, uses "pointed projections fabricated into arrays that can create vaccine delivery pathways through the skin".^[75]

An experimental needle-free^[76] vaccine delivery system is undergoing animal testing.^[77][78] A stamp-size patch similar to an adhesive bandage contains about 20,000 microscopic projections per square cm.^[79] This dermal administration potentially increases the effectiveness of vaccination, while requiring less vaccine than injection.^[80]

Plasmids

The use of plasmids has been validated in preclinical studies as a protective vaccine strategy for cancer and infectious diseases. However, in human studies, this approach has failed to provide clinically relevant benefit. The overall efficacy of plasmid DNA immunization depends on increasing the plasmid's immunogenicity while also correcting for factors involved in the specific activation of immune effector cells.^[81]

Veterinary medicine

Goat vaccination against sheep pox and pleural pneumonia

Vaccinations of animals are used both to prevent their contracting diseases and to prevent transmission of disease to humans.^[82] Both animals kept as pets and animals raised as livestock are routinely vaccinated. In some instances, wild populations may be vaccinated. This is sometimes accomplished with vaccine-laced food spread in a disease-prone area and has been used to attempt to control rabies in raccoons.

Where rabies occurs, rabies vaccination of dogs may be required by law. Other canine vaccines include canine distemper, canine parvovirus, infectious canine hepatitis, adenovirus-2, leptospirosis, bordatella, canine parainfluenza virus, and Lyme disease, among others.

Cases of veterinary vaccines used in humans have been documented, whether intentional or accidental, with some cases of resultant illness, most notably with brucellosis.^[83] However, the reporting of such cases is rare and very little has been studied about the safety and results of such practices. With the advent of aerosol vaccination in veterinary clinics for companion animals, human exposure to pathogens that are not naturally carried in humans, such as Bordetella bronchiseptica, has likely increased in recent years.^[83] In some cases, most notably rabies, the parallel veterinary vaccine against a pathogen may be as much as orders of magnitude more economical than the human one.

DIVA vaccines

DIVA (Differentiating Infected from Vaccinated Animals) vaccines make it possible to differentiate between infected and vaccinated animals.

DIVA vaccines carry at least one epitope less than the microorganisms circulating in the field. An accompanying diagnostic test that detects antibody against that epitope allows us to actually make that differentiation.

First DIVA vaccines

The first DIVA vaccines (formerly termed marker vaccines and since 1999 coined as DIVA vaccines) and companion diagnostic tests have been developed by J.T. van Oirschot and colleagues at the Central Veterinary Institute in Lelystad, The Netherlands.^{[84] [85]} They found that some existing vaccines against pseudorabies (also termed Aujeszky's disease) had deletions in their viral genome (among which the gE gene). Monoclonal antibodies were produced against that deletion and selected to develop an ELISA that demonstrated antibodies against gE. In addition, novel genetically engineered gE-negative vaccines were constructed.^[86] Along the same lines, DIVA vaccines and companion diagnostic tests against bovine herpesvirus 1 infections have been developed.^[87][88]

Use in practice

The DIVA strategy has been applied in various countries and successfully eradicated pseudorabies virus. Swine populations were intensively vaccinated and monitored by the companion diagnostic test and, subsequently, the infected pigs were removed from the population. Bovine herpesvirus 1 DIVA vaccines are also widely used in practice.

Other DIVA vaccines (under development)

Scientists have put and still, are putting much effort in applying the DIVA principle to a wide range of infectious diseases, such as, for example, classical swine fever,^[89] avian influenza,^[90] Actinobacillus pleuropneumonia^[91] and Salmonella infections in pigs.^[92]

Trends

Vaccine development has several trends:^[93]

• Until recently,^[when?] most vaccines were aimed at infants and children, but adolescents and adults are increasingly being targeted.^[93][94]
• Combinations of vaccines are becoming more common; vaccines containing five or more components are used in many parts of the world.^[93]
• New methods of administering vaccines are being developed,^[when?] such as skin patches, aerosols via inhalation devices, and eating genetically engineered plants.^[93]
• Vaccines are being designed to stimulate innate immune responses, as well as adaptive.^[93]
• Attempts are being made to develop vaccines to help cure chronic infections, as opposed to preventing disease.^[93]
• Vaccines are being developed to defend against bioterrorist attacks such as anthrax, plague, and smallpox.^[93]
• Appreciation for sex and pregnancy differences in vaccine responses "might change the strategies used by public health officials".^[95]
• Scientists are now trying to develop synthetic vaccines by reconstructing the outside structure of a virus, this will help prevent vaccine resistance.^[96]

Principles that govern the immune response can now be used in tailor-made vaccines against many noninfectious human diseases, such as cancers and autoimmune disorders.^[97] For example, the experimental vaccine CYT006-AngQb has been investigated as a possible treatment for high blood pressure.^[98] Factors that affect the trends of vaccine development include progress in translatory medicine, demographics, regulatory science, political, cultural, and social responses.^[99]

Plants as bioreactors for vaccine production

Transgenic plants have been identified as promising expression systems for vaccine production. Complex plants such as tobacco, potato, tomato, and banana can have genes inserted that cause them to produce vaccines usable for humans.^[100] Bananas have been developed that produce a human vaccine against Hepatitis B.^[101] Another example is the expression of a fusion protein in alfalfa transgenic plants for the selective directioning to antigen presenting cells, therefore increasing vaccine potency against Bovine Viral Diarrhea Virus (BVDV).[

The Future of Nanotechnology: Molecular Manufacturing

April 14, 2003 by K. Eric Drexler
Original link:  http://www.kurzweilai.net/the-future-of-nanotechnology-molecular-manufacturing
Originally published on EurekaAlert! April 2003. Published on KurzweilAI.net April 14, 2003.

The future generations of nanotechnology will rely on being able to effectively arrange atoms. Molecular manufacturing, and the use of molecular assemblers to hold and position molecules, will be key to the future, controlling how molecules react and allowing scientists to build complex structures with atomically precise control. In this essay, Dr. Drexler discusses the benefits and challenges of future molecular manufacturing.

 

The future of technology is sometimes easy to predict. Computers will compute faster, materials will become stronger, and medicine will cure more diseases. Nanotechnology, which works on the nanometer scale of molecules and atoms, will be a large part of this future, enabling great improvements in all these fields. Advanced nanotechnology will work with molecular precision, making a wide range of products that are impossible to make today.

 

Institute of Molecular Manufacturing

A proposed fine-motion controller for guiding molecular assembly. Molecular machinery driving rotation of the blue rings (below) would move the tool-holder (above) with precise control of angle and position.

Why focus on molecular manufacturing? Every manufacturing method is a method for arranging atoms. Most methods arrange atoms crudely; even the finest commercial microchips are grossly irregular at the atomic scale, and much of today’s nanotechnology faces the same limit.

Chemistry and biology, however, make molecules defined by particular arrangements of atoms—always the same numbers, kinds, and linkages. Chemists use clever tricks that don’t scale up well. Biology, however, uses a more powerful method: cells contain molecular machines that read genetic data to guide the assembly of large molecules (proteins) that they serve as parts of molecular machines. Molecular manufacturing will likewise use stored data to guide construction work done by molecular machines, greatly extending abilities in nanotechnology.

The molecular assembler concept

The basic idea is simple: where chemists mix molecules in solution, allowing them to wander and bump together at random, molecular assemblers will instead position molecules, bringing them together to the specific location at the desired time. Letting molecules bump at random leads to unwanted reactions—a problem that grows worse as products get larger. By holding and positioning molecules, assemblers will control how the molecules react, building up complex structures with atomically precise control.


Theoretical and Computational Biophysics Group/Beckman Institute, University of Illinois at Urbana-Champaign

The energy-carrying molecule ATP is manufactured by a molecular machine, ATPase, mounted in a membrane. The lower part is a motor powered by protons flowing through it across the membrane; the motor turns a shaft that drives the mechanosynthesis of ATP in the upper part.

Picture an industrial robot arm standing next to an unfinished work piece. A conveyor belt supplies the arm with parts, each mounted on a handle. Step after step, the belt advances, the robot grips a fresh handle, plugs the attached part into the work piece, then puts the empty handle back on the belt. Eventually, the work piece is finished and another belt moves it away, shifting a new unfinished work piece into place.

To picture a molecular assembler in a manufacturing system, imagine that all the parts are measured in nanometers, and that the transferred parts are just a few atoms, shifting from handle to work piece through a chemical reaction at a specific site. An assembler works as part of a larger system that prepares tools, puts them on the conveyor, and controls the robotic positioning mechanism.

This will be a complex system that no one will build any time soon. Indeed, no one is even trying to build molecular assemblers today, because nanotechnology is still in its infancy. We can see a path to assemblers, just as the rocketry pioneers of the 1930s and 1940s could see a path to the Moon. But like those pioneers, we aren’t ready to attempt the final goal. They knew they must first launch many satellites, just as we must first build many molecular machines.

Confusion and controversy: Developing assemblers

While nano assembly has been described as "building things atom by atom," an expression that has caught on in the press, this is a misconception. Molecular assemblers will build with atomic precision by mechanically guiding chemical reactions that typically add a few atoms at a time, but some researchers have criticized this misconception as if it were the actual proposal. It is correct that assemblers can’t build things by using tiny tweezers to pick up and put down atoms one at a time, but even from the start this was never the idea.

The apparent controversy over "molecular assemblers" is thus an illusion: the critics are talking about something else. The idea of building things by mechanically guiding chemical reactions has withstood scientific scrutiny for over 20 years, and seems sound. It is time to move on, to consider the consequences of molecular assemblers and what they will be able to build.

Understanding advanced capabilities

We can catch a glimpse of future technologies because we sometimes can understand things that we can’t yet build. Chemistry, biology, engineering, and applied physics all provide useful perspectives.

Chemistry shows what can happen when reactive molecules meet. By using molecular machinery to guide reactive molecules, similar structures can be built at larger scales. The products can be stronger, tougher, and more capable than the delicate structures found in living cells.

Biology shows that molecular machines can exist, can be programmed with genetic data, and can build more molecular machines. Biology shows that the products of molecular machine systems can be as cheap as potatoes. Molecular manufacturing will make a far wider range of products for similarly low costs.

Engineering shows that strong, precise parts can be combined to make computers, robots, and a host of useful gadgets. Applied physics, aided by computer modeling, shows that these sorts of devices can be built from atomically precise parts of nanometer scale. These glimpses of future technologies are enough to establish the potential for molecular manufacturing.

Directions and applications

Molecular manufacturing will bring both great opportunities and great dangers. Nanocomputers will extend desktop computational power by a factor of a billion or more. Nanoscale sensors, computers, and tools will bring surgical control to the molecular level, enabling a revolution in medicine. Light, strong, and inexpensive aerospace structures will make spaceflight easy.

But the future’s faster, cheaper, cleaner production of better products will also bring disruption. Advanced lethal and nonlethal weapons, deployed quickly and cheaply, could make the world a more dangerous place. The list of consequences is long, much of it sounding like science fiction.

The tools required to develop nanotechnologies are typically small and unobtrusive. The pace of research is accelerating worldwide. Some suggest stopping it, but it is hard to imagine how. Thus, it seems that this technology, with all its challenges and opportunities, is an unavoidable part of our future.

Sponsored by the U.S. Department of Energy. For more information about nanotechnology, visit EurekAlert!’s Nanotechnology In-Context Module.

© 2002 American Association for the Advancement of Science. Reprinted with permission.

Antiviral drug

From Wikipedia, the free encyclopedia

Antiviral drugs are a class of medication used specifically for treating viral infections rather than bacterial ones. Most antivirals are used for specific viral infections, while a broad-spectrum antiviral is effective against a wide range of viruses. Unlike most antibiotics, antiviral drugs do not destroy their target pathogen; instead they inhibit their development.

Antiviral drugs are one class of antimicrobials, a larger group which also includes antibiotic (also termed antibacterial), antifungal and antiparasitic drugs,^[3] or antiviral drugs based on monoclonal antibodies.^[4] Most antivirals are considered relatively harmless to the host, and therefore can be used to treat infections. They should be distinguished from viricides, which are not medication but deactivate or destroy virus particles, either inside or outside the body. Natural antivirals are produced by some plants such as eucalyptus.^[5]

Medical uses

Most of the antiviral drugs now available are designed to help deal with HIV, herpes viruses, the hepatitis B and C viruses, and influenza A and B viruses. Researchers are working to extend the range of antivirals to other families of pathogens.

Designing safe and effective antiviral drugs is difficult, because viruses use the host's cells to replicate. This makes it difficult to find targets for the drug that would interfere with the virus without also harming the host organism's cells. Moreover, the major difficulty in developing vaccines and anti-viral drugs is due to viral variation.

The emergence of antivirals is the product of a greatly expanded knowledge of the genetic and molecular function of organisms, allowing biomedical researchers to understand the structure and function of viruses, major advances in the techniques for finding new drugs, and the pressure placed on the medical profession to deal with the human immunodeficiency virus (HIV), the cause of acquired immunodeficiency syndrome (AIDS).

The first experimental antivirals were developed in the 1960s, mostly to deal with herpes viruses, and were found using traditional trial-and-error drug discovery methods. Researchers grew cultures of cells and infected them with the target virus. They then introduced into the cultures chemicals which they thought might inhibit viral activity, and observed whether the level of virus in the cultures rose or fell. Chemicals that seemed to have an effect were selected for closer study.

This was a very time-consuming, hit-or-miss procedure, and in the absence of a good knowledge of how the target virus worked, it was not efficient in discovering effective antivirals which had few side effects. Only in the 1980s, when the full genetic sequences of viruses began to be unraveled, did researchers begin to learn how viruses worked in detail, and exactly what chemicals were needed to thwart their reproductive cycle.

Virus life cycle

Viruses consist of a genome and sometimes a few enzymes stored in a capsule made of protein (called a capsid), and sometimes covered with a lipid layer (sometimes called an 'envelope'). Viruses cannot reproduce on their own, and instead propagate by subjugating a host cell to produce copies of themselves, thus producing the next generation.

Researchers working on such "rational drug design" strategies for developing antivirals have tried to attack viruses at every stage of their life cycles. Some species of mushrooms have been found to contain multiple antiviral chemicals with similar synergistic effects.^[6] Viral life cycles vary in their precise details depending on the type of virus, but they all share a general pattern:

• Attachment to a host cell.
• Release of viral genes and possibly enzymes into the host cell.
• Replication of viral components using host-cell machinery.
• Assembly of viral components into complete viral particles.
• Release of viral particles to infect new host cells.

Limitations and Policy Implications

Several factors including cost, vaccination stigma, and acquired resistance limit the effectiveness of antiviral therapies. These issues are explored via a health policy perspective.

Research and prices

Rising Costs

Cost is an important factor that limits access to antivirals therapies in the United States and internationally.^[7][8][9] The recommended treatment regimen for hepatitis C virus infection, for example, includes sofosbuvir-velpatasvir (Epclusa) and ledipasvir-sofosbuvir (Harrvoni).^[10] A twelve-week supply of these drugs amount to $113,400 and $89,712, respectively.^[11][12] These drugs can be manufactured generically at a cost of $100 - $250 per 12 week treatment.^[13] Pharmaceutical companies attribute the majority of these costs to research and development expenses. However, critics point to monopolistic market conditions that allow manufacturers to increase prices without facing a reduction in sales, leading to higher profits at patient's expense.^[14] Intellectual property laws, anti-importation policies,^[15] and the slow pace of FDA review limit alternative options. Recently, private-public research partnerships have been established to promote expedited, cost-effective research.^[16]

Vaccinations and stigma

Vaccines and population health

While most antivirals treat viral infection, vaccines are a preemptive first line of defense against pathogens. Vaccination involves the introduction (i.e. via injection) of a small amount of typically inactivated or attenuated antigenic material to stimulate an individual's immune system. The immune system responds by developing white blood cells to specifically combat the introduced pathogen, resulting in adaptive immunity.^[17] Vaccination in a population results in herd immunity and greatly improved population health, with significant reductions in viral infection and disease.^[18]

Vaccination policy

Vaccination policy in the United States consists of public and private vaccination requirements. For instance, public schools require students to receive vaccinations (termed "vaccination schedule") for viruses and bacteria such as diphtheria, pertussis, and tetanus (DTaP), measles, mumps, rubella (MMR), varicella (chickenpox), hepatitis B, rotavirus, polio, and more. Private institutions might require annual influenza vaccination. The Center for Disease Control and Prevention has estimated that routine immunization of newborns prevents about 42,000 deaths and 20 million cases of disease each year, saving about $13.6 billion.^[19]

Vaccination controversy

Despite their successes, there is plenty of stigma surrounding vaccines that cause people to be incompletely vaccinated. These "gaps" in vaccination result in unnecessary infection, death, and costs.^[20] There are two major reasons for incomplete vaccination:

1. Vaccines, like other medical treatments, have a risk of causing serious complications in some individuals (i.e. severe allergic reactions). While these complications are less common than the risks faced when not vaccinated, negative media coverage can instill fear in a population.^[21] Other controversies involve the association of autism with vaccines, although the Center for Disease Control and Prevention,^[22] Institute of Medicine,^[23] and National Health Service^[24] regard this link as unfounded.
2. Low vaccine-preventable disease rates as a result of herd immunity also make vaccines seem unnecessary and leave many unvaccinated.^[25][26]

Although the American Academy of Pediatrics endorses universal immunization,^[27] they note that physicians should respect parents' refusal to vaccinate their children after sufficient advising and provided the child does not face a significant risk of infection. Parents can also cite religious reasons to avoid public school vaccination mandates, but this reduces herd immunity and increases risk of viral infection.^[18]

Limitations of vaccines

Vaccines bolster the body's immune system to better attack viruses in the "complete particle" stage, outside of the organism's cells. They traditionally consist of an attenuated (a live weakened) or inactivated (killed) version of the virus. These vaccines can, in very rare cases, harm the host by inadvertently infecting the host with a full-blown viral occupancy. Recently "subunit" vaccines have been devised that consist strictly of protein targets from the pathogen. They stimulate the immune system without doing serious harm to the host. In either case, when the real pathogen attacks the subject, the immune system responds to it quickly and blocks it.

Vaccines are very effective on stable viruses, but are of limited use in treating a patient who has already been infected. They are also difficult to successfully deploy against rapidly mutating viruses, such as influenza (the vaccine for which is updated every year) and HIV. Antiviral drugs are particularly useful in these cases.

Antiretroviral Therapy as HIV prevention

Following the HPTN 052 study and PARTNER study, there is significant evidence to demonstrate that antiretroviral drugs inhibit transmission when the person living with HIV has been undetectable for 6 months or longer.

Public policy

Use and Distribution

Guidelines regarding viral diagnoses and treatments change frequently and limit quality care.^[28] Even when physicians diagnose older patients with influenza, use of antiviral treatment can be low.^[29] Provider knowledge of antiviral therapies can improve patient care, especially in geriatric medicine. Furthermore, in local health departments (LHDs) with access to antivirals, guidelines may be unclear, causing delays in treatment.^[30] With time-sensitive therapies, delays could lead to lack of treatment. Overall, national guidelines regarding infection control and management standardize care and improve patient and health care worker safety. Guidelines such as those provided by the Centers for Disease Control and Prevention (CDC) during the 2009 flu pandemic caused by the H1N1 virus, recommend antiviral treatment regimens, clinical assessment algorithms for coordination of care, and antiviral chemoprophylaxis guidelines for exposed persons, among others.^[31] Roles of pharmacists and pharmacies have also expanded to meet the needs of public during public health emergencies.^[32]

Stockpiling

Public Health Emergency Preparedness initiatives are managed by the CDC via the Office of Public Health Preparedness and Response.^[33] Funds aim to support communities in preparing for public health emergencies, including pandemic influenza. Also managed by the CDC, the Strategic National Stockpile (SNS) consists of bulk quantities of medicines and supplies for use during such emergencies.^[34] Antiviral stockpiles prepare for shortages of antiviral medications in cases of public health emergencies. During the H1N1 pandemic in 2009-2010, guidelines for SNS use by local health departments was unclear, revealing gaps in antiviral planning.^[30] For example, local health departments that received antivirals from the SNS did not have transparent guidance on the use of the treatments. The gap made it difficult to create plans and policies for their use and future availabilities, causing delays in treatment.

Anti-viral targeting

The general idea behind modern antiviral drug design is to identify viral proteins, or parts of proteins, that can be disabled. These "targets" should generally be as unlike any proteins or parts of proteins in humans as possible, to reduce the likelihood of side effects. The targets should also be common across many strains of a virus, or even among different species of virus in the same family, so a single drug will have broad effectiveness. For example, a researcher might target a critical enzyme synthesized by the virus, but not the patient, that is common across strains, and see what can be done to interfere with its operation.

Once targets are identified, candidate drugs can be selected, either from drugs already known to have appropriate effects, or by actually designing the candidate at the molecular level with a computer-aided design program.

The target proteins can be manufactured in the lab for testing with candidate treatments by inserting the gene that synthesizes the target protein into bacteria or other kinds of cells. The cells are then cultured for mass production of the protein, which can then be exposed to various treatment candidates and evaluated with "rapid screening" technologies.

Approaches by life cycle stage

Before cell entry

One anti-viral strategy is to interfere with the ability of a virus to infiltrate a target cell. The virus must go through a sequence of steps to do this, beginning with binding to a specific "receptor" molecule on the surface of the host cell and ending with the virus "uncoating" inside the cell and releasing its contents. Viruses that have a lipid envelope must also fuse their envelope with the target cell, or with a vesicle that transports them into the cell, before they can uncoat.

This stage of viral replication can be inhibited in two ways:

1. Using agents which mimic the virus-associated protein (VAP) and bind to the cellular receptors. This may include VAP anti-idiotypic antibodies, natural ligands of the receptor and anti-receptor antibodies.^{[clarification needed]}
2. Using agents which mimic the cellular receptor and bind to the VAP. This includes anti-VAP antibodies, receptor anti-idiotypic antibodies, extraneous receptor and synthetic receptor mimics.

This strategy of designing drugs can be very expensive, and since the process of generating anti-idiotypic antibodies is partly trial and error, it can be a relatively slow process until an adequate molecule is produced.

Entry inhibitor

A very early stage of viral infection is viral entry, when the virus attaches to and enters the host cell. A number of "entry-inhibiting" or "entry-blocking" drugs are being developed to fight HIV. HIV most heavily targets the immune system's white blood cells known as "helper T cells", and identifies these target cells through T-cell surface receptors designated "CD4" and "CCR5". Attempts to interfere with the binding of HIV with the CD4 receptor have failed to stop HIV from infecting helper T cells, but research continues on trying to interfere with the binding of HIV to the CCR5 receptor in hopes that it will be more effective.

HIV infects a cell through fusion with the cell membrane, which requires two different cellular molecular participants, CD4 and a chemokine receptor (differing depending on the cell type). Approaches to blocking this virus/cell fusion have shown some promise in preventing entry of the virus into a cell. At least one of these entry inhibitors—a biomimetic peptide called Enfuvirtide, or the brand name Fuzeon—has received FDA approval and has been in use for some time. Potentially, one of the benefits from the use of an effective entry-blocking or entry-inhibiting agent is that it potentially may not only prevent the spread of the virus within an infected individual but also the spread from an infected to an uninfected individual.

One possible advantage of the therapeutic approach of blocking viral entry (as opposed to the currently dominant approach of viral enzyme inhibition) is that it may prove more difficult for the virus to develop resistance to this therapy than for the virus to mutate or evolve its enzymatic protocols.

Uncoating inhibitor

Inhibitors of uncoating have also been investigated.^[35][36]

Amantadine and rimantadine have been introduced to combat influenza. These agents act on penetration and uncoating.^[37]

Pleconaril works against rhinoviruses, which cause the common cold, by blocking a pocket on the surface of the virus that controls the uncoating process. This pocket is similar in most strains of rhinoviruses and enteroviruses, which can cause diarrhea, meningitis, conjunctivitis, and encephalitis.

Some scientists are making the case that a vaccine against rhinoviruses, the predominant cause of the common cold, is achievable. Vaccines that combine dozens of varieties of rhinovirus at once are effective in stimulating antiviral antibodies in mice and monkeys, researchers have reported in Nature Communications in 2016.

The quest for a vaccine against rhinoviruses may have seemed quixotic, because there are more than 100 varieties circulating around the world. But the immune system can handle the challenge.

Rhinoviruses are the most common cause of the common cold; other viruses such as respiratory syncytial virus, parainfluenza virus and adenoviruses can cause them too. Rhinoviruses also exacerbate asthma attacks. Although rhinoviruses come in many varieties, they do not drift to the same degree that influenza viruses do. A mixture of 50 inactivated rhinovirus types should be able to stimulate neutralizing antibodies against all of them to some degree.

During viral synthesis

A second approach is to target the processes that synthesize virus components after a virus invades a cell.

Reverse transcription

One way of doing this is to develop nucleotide or nucleoside analogues that look like the building blocks of RNA or DNA, but deactivate the enzymes that synthesize the RNA or DNA once the analogue is incorporated. This approach is more commonly associated with the inhibition of reverse transcriptase (RNA to DNA) than with "normal" transcriptase (DNA to RNA).

The first successful antiviral, aciclovir, is a nucleoside analogue, and is effective against herpesvirus infections. The first antiviral drug to be approved for treating HIV, zidovudine (AZT), is also a nucleoside analogue.

An improved knowledge of the action of reverse transcriptase has led to better nucleoside analogues to treat HIV infections. One of these drugs, lamivudine, has been approved to treat hepatitis B, which uses reverse transcriptase as part of its replication process. Researchers have gone further and developed inhibitors that do not look like nucleosides, but can still block reverse transcriptase.

Another target being considered for HIV antivirals include RNase H – which is a component of reverse transcriptase that splits the synthesized DNA from the original viral RNA.

On 10 August 2011 researchers at MIT announced the publication^[38] of a new method of inhibiting RNA, the process selectively affected infected cells. The team named the process "Double-stranded RNA Activated Caspase Oligomerizer" (DRACO). According to the lead researcher "In theory, [DRACO] should work against all viruses."^[39]

Integrase

Another target is integrase, which integrate the synthesized DNA into the host cell genome.

Transcription

Once a virus genome becomes operational in a host cell, it then generates messenger RNA (mRNA) molecules that direct the synthesis of viral proteins. Production of mRNA is initiated by proteins known as transcription factors. Several antivirals are now being designed to block attachment of transcription factors to viral DNA.

Translation/antisense

Genomics has not only helped find targets for many antivirals, it has provided the basis for an entirely new type of drug, based on "antisense" molecules. These are segments of DNA or RNA that are designed as complementary molecule to critical sections of viral genomes, and the binding of these antisense segments to these target sections blocks the operation of those genomes. A phosphorothioate antisense drug named fomivirsen has been introduced, used to treat opportunistic eye infections in AIDS patients caused by cytomegalovirus, and other antisense antivirals are in development. An antisense structural type that has proven especially valuable in research is morpholino antisense.

Morpholino oligos have been used to experimentally suppress many viral types:

• caliciviruses^[40]
• flaviviruses (including WNV)^[41]
• dengue^[42]
• HCV^[43]
• coronaviruses^[44]

Translation/ribozymes

Yet another antiviral technique inspired by genomics is a set of drugs based on ribozymes, which are enzymes that will cut apart viral RNA or DNA at selected sites. In their natural course, ribozymes are used as part of the viral manufacturing sequence, but these synthetic ribozymes are designed to cut RNA and DNA at sites that will disable them.

A ribozyme antiviral to deal with hepatitis C has been suggested,^[45] and ribozyme antivirals are being developed to deal with HIV.^[46] An interesting variation of this idea is the use of genetically modified cells that can produce custom-tailored ribozymes. This is part of a broader effort to create genetically modified cells that can be injected into a host to attack pathogens by generating specialized proteins that block viral replication at various phases of the viral life cycle.

Protein processing and targeting

Interference with post translational modifications or with targeting of viral proteins in the cell is also possible.^[47]

Protease inhibitors

Some viruses include an enzyme known as a protease that cuts viral protein chains apart so they can be assembled into their final configuration. HIV includes a protease, and so considerable research has been performed to find "protease inhibitors" to attack HIV at that phase of its life cycle.^[48] Protease inhibitors became available in the 1990s and have proven effective, though they can have unusual side effects, for example causing fat to build up in unusual places.^[49] Improved protease inhibitors are now in development.

Protease inhibitors have also been seen in nature. A protease inhibitor was isolated from the Shiitake mushroom (Lentinus edodes).^[50] The presence of this may explain the Shiitake mushrooms noted antiviral activity in vitro.^[51]

Assembly

Rifampicin acts at the assembly phase.^[52]

Release phase

The final stage in the life cycle of a virus is the release of completed viruses from the host cell, and this step has also been targeted by antiviral drug developers. Two drugs named zanamivir (Relenza) and oseltamivir (Tamiflu) that have been recently introduced to treat influenza prevent the release of viral particles by blocking a molecule named neuraminidase that is found on the surface of flu viruses, and also seems to be constant across a wide range of flu strains.

Immune system stimulation

A second category of tactics for fighting viruses involves encouraging the body's immune system to attack them, rather than attacking them directly. Some antivirals of this sort do not focus on a specific pathogen, instead stimulating the immune system to attack a range of pathogens.

One of the best-known of this class of drugs are interferons, which inhibit viral synthesis in infected cells.^[53] One form of human interferon named "interferon alpha" is well-established as part of the standard treatment for hepatitis B and C,^[54] and other interferons are also being investigated as treatments for various diseases.

A more specific approach is to synthesize antibodies, protein molecules that can bind to a pathogen and mark it for attack by other elements of the immune system. Once researchers identify a particular target on the pathogen, they can synthesize quantities of identical "monoclonal" antibodies to link up that target. A monoclonal drug is now being sold to help fight respiratory syncytial virus in babies,^[55] and antibodies purified from infected individuals are also used as a treatment for hepatitis B.^[56]

Acquired resistance

Antiviral resistance can be defined by a decreased susceptibility to a drug through either a minimally effective, or completely ineffective, treatment response to prevent associated illnesses from a particular virus.^[57] The issue inevitably remains a major obstacle to antiviral therapy as it has developed to almost all specific and effective antimicrobials, including antiviral agents.^[58]

The Centers for Disease Control and Prevention (CDC) inclusively recommends those six months and older to get a yearly vaccination to protect from influenza A viruses (H1N1) and (H3N2) and up to two influenza B viruses (depending on the vaccination).^[57] Comprehensive protection starts by ensuring vaccinations are current and complete. The three FDA-approved neuraminidase antiviral flu drugs available in the United States, recommended by the CDC, include: oseltamivir (Tamiflu), zanamivir (Relenza), and peramivir (Rapivab).^[57]

A study published in 2009 in Nature Biotechnology emphasized the urgent need for augmentation of oseltamivir (Tamiflu) stockpiles with additional antiviral drugs including zanamivir (Relenza). This finding was based on a performance evaluation of these drugs supposing the 2009 H1N1 'Swine Flu' neuraminidase (NA) were to acquire the Tamiflu-resistance (His274Tyr) mutation which is currently widespread in seasonal H1N1 strains.^[59]

Origin of antiviral resistance

The genetic makeup of viruses is constantly changing and therefore may alter the virus resistant to the treatments currently available.^[60] Viruses can become resistant through spontaneous or intermittent mechanisms throughout the course of an antiviral treatment.^[57] Immunocompromised patients, more often than immunocompetent patients, hospitalized with pneumonia are at the highest risk of developing oseltamivir resistance during treatment.^[57] Subsequent to exposure to someone else with the flu, those who received oseltamivir for "post-exposure prophylaxis" are also at higher risk of resistance.^[61]

Detection of antiviral resistance

National and international surveillance is performed by the CDC to determine effectiveness of the current FDA-approved antiviral flu drugs.^[57] Public health officials use this information to make current recommendations about the use of flu antiviral medications. WHO further recommends in-depth epidemiological investigations to control potential transmission of the resistant virus and prevent future progression.^[62] As novel treatments and detection techniques to antiviral resistance are enhanced so can the establishment of strategies to combat the inevitable emergence of antiviral resistance.