Social distancing

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Social_distancing

People maintaining social distance while queuing to enter a supermarket in London during the 2020 COVID-19 pandemic. To ensure that shoppers are able to maintain distance once in the store, only a limited number are allowed inside at one time.

 

Social distancing reduces the rate of disease transmission and can stop an outbreak.

Social distancing, also called physical distancing, is a set of non-pharmaceutical interventions or measures taken to prevent the spread of a contagious disease by maintaining a physical distance between people and reducing the number of times people come into close contact with each other. It typically involves keeping a certain distance from others (the distance specified may differ from time to time and country to country) and avoiding gathering together in large groups.

By reducing the probability that a given uninfected person will come into physical contact with an infected person, the disease transmission can be suppressed, resulting in fewer deaths. The measures are used in combination with good respiratory hygiene and hand washing by a population. During the COVID-19 pandemic, the World Health Organization (WHO) suggested favoring the term "physical distancing" as opposed to "social distancing", in keeping with the fact that it is a physical distance which prevents transmission; people can remain socially connected via technology. To slow down the spread of infectious diseases and avoid overburdening healthcare systems, particularly during a pandemic, several social-distancing measures are used, including the closing of schools and workplaces, isolation, quarantine, restricting the movement of people and the cancellation of mass gatherings.

Although the term was only introduced in the twenty-first century, social-distancing measures date back to at least the fifth century BC. The Bible contains one of the earliest known references to the practice in the Book of Leviticus 13:46: "And the leper in whom the plague is ... he shall dwell alone; [outside] the camp shall his habitation be." During the Plague of Justinian of 541 to 542, emperor Justinian enforced an ineffective quarantine on the Byzantine Empire, including dumping bodies into the sea; he predominantly blamed the widespread outbreak on "Jews, Samaritans, pagans, heretics, Arians, Montanists, and homosexuals". In modern times, social distancing measures have been successfully implemented in several epidemics. In St. Louis, shortly after the first cases of influenza were detected in the city during the 1918 flu pandemic, authorities implemented school closures, bans on public gatherings and other social-distancing interventions. The case fatality rates in St. Louis were much less than in Philadelphia, which despite having cases of influenza, allowed a mass parade to continue and did not introduce social distancing until more than two weeks after its first cases. Authorities have encouraged or mandated social distancing during the COVID-19 pandemic. 

Social distancing measures are more effective when the infectious disease spreads via one or more of the following methods:

• droplet contact (coughing or sneezing)
• direct physical contact (including sexual contact)
• indirect physical contact (e.g., by touching a contaminated surface)
• airborne transmission (if the microorganism can survive in the air for long periods)

The measures are less effective when an infection is transmitted primarily via contaminated water or food or by vectors such as mosquitoes or other insects.

Drawbacks of social distancing can include loneliness, reduced productivity and the loss of other benefits associated with human interaction.

Definition

A poster (in Arabic, English and Urdu) encouraging social distancing during the COVID-19 pandemic

 

The Centers for Disease Control and Prevention (CDC) have described social distancing as a set of "methods for reducing frequency and closeness of contact between people in order to decrease the risk of transmission of disease". During the 2009 flu pandemic the WHO described social distancing as "keeping at least an arm's length distance from others, [and] minimizing gatherings". It is combined with good respiratory hygiene and hand washing, and is considered the most feasible way to reduce or delay a pandemic.

During the COVID-19 pandemic, the CDC revised the definition of social distancing as "remaining out of congregrate settings, avoiding mass gatherings, and maintaining distance (approximately six feet or two meters) from others when possible". It is not clear why six feet was chosen. Recent studies have suggested that droplets from a sneeze or forceful breathing during exercise can travel over six meters. Some have suggested the required distance is based on debunked research from the 1930s and 1940s or confusion regarding units of measurement. Researchers and science writers have recommended that larger social distances and/or both mask wearing and social distancing be required.

Measures

Social distancing helps prevent a sharp peak of infections ("flattens the epidemic curve") to help healthcare services deal with demand, and extends time for healthcare services to be increased and improved.

 

Knowing that a disease is circulating may trigger a change in behavior by people choosing to stay away from public places and other people. When implemented to control epidemics, such social distancing can result in benefits but with an economic cost. Research indicates that measures must be applied rigorously and immediately in order to be effective. Several social distancing measures are used to control the spread of contagious illnesses.

Avoiding physical contact

Social distancing includes eliminating the physical contact that occurs with the typical handshake, hug, or hongi; this New Zealand illustration offers eight alternatives.

 

Keeping at least two-metre (six-foot) distance (in the US or UK) or 1.5 metres distance (in Australia) or 1 metre distance (in France or Italy) from each other and avoiding hugs and gestures that involve direct physical contact, reduce the risk of becoming infected during flu pandemics and the coronavirus pandemic of 2020. These distances of separation, in addition to personal hygiene measures, are also recommended at places of work. Where possible it may be recommended to work from home.

Various alternatives have been proposed for the tradition of handshaking. The gesture of namaste, placing one's palms together, fingers pointing upwards, drawing the hands to the heart, is one non-touch alternative. During the COVID-19 pandemic in the United Kingdom, this gesture was used by Prince Charles upon greeting reception guests, and has been recommended by the Director-General of the WHO, Tedros Adhanom Ghebreyesus, and Israeli Prime Minister Benjamin Netanyahu. Other alternatives include the wave, the shaka (or "hang loose") sign, and placing a palm on your heart, as practiced in parts of Iran.

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  In this computer lab, every other workstation has been closed off to increase the distance between people working.
  
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  Floor markings can help people maintain distance in public places.
  

School closures

Swine flu cases per week in the United Kingdom in 2009; schools typically close for summer in mid-July and re-open in early September.

 

Mathematical modeling has shown that transmission of an outbreak may be delayed by closing schools. However, effectiveness depends on the contacts children maintain outside of school. Often, one parent has to take time off work, and prolonged closures may be required. These factors could result in social and economic disruption.

Workplace closures

Modeling and simulation studies based on U.S. data suggest that if 10% of affected workplaces are closed, the overall infection transmission rate is around 11.9% and the epidemic peak time is slightly delayed. In contrast, if 33% of affected workplaces are closed, the attack rate decreases to 4.9%, and the peak time is delayed by one week. Workplace closures include closure of "non-essential" businesses and social services ("non-essential" means those facilities that do not maintain primary functions in the community, as opposed to essential services).

Canceling mass gatherings

Cancellation of mass gatherings includes sports events, films or musical shows. Evidence suggesting that mass gatherings increase the potential for infectious disease transmission is inconclusive. Anecdotal evidence suggests certain types of mass gatherings may be associated with increased risk of influenza transmission, and may also "seed" new strains into an area, instigating community transmission in a pandemic. During the 1918 influenza pandemic, military parades in Philadelphia and Boston may have been responsible for spreading the disease by mixing infected sailors with crowds of civilians. Restricting mass gatherings, in combination with other social distancing interventions, may help reduce transmission.

Travel restrictions

Border restrictions or internal travel restrictions are unlikely to delay an epidemic by more than two to three weeks unless implemented with over 99% coverage. Airport screening was found to be ineffective in preventing viral transmission during the 2003 SARS outbreak in Canada and the U.S. Strict border controls between Austria and the Ottoman Empire, imposed from 1770 until 1871 to prevent persons infected with the bubonic plague from entering Austria, were reportedly effective, as there were no major outbreaks of plague in Austrian territory after they were established, whereas the Ottoman Empire continued to suffer frequent epidemics of plague until the mid-nineteenth century.

A Northeastern University study published in March 2020 found that "travel restrictions to and from China only slow down the international spread of COVID-19 [when] combined with efforts to reduce transmission on a community and an individual level. [...] Travel restrictions aren't enough unless we couple it with social distancing." The study found that the travel ban in Wuhan delayed the spread of the disease to other parts of mainland China only by three to five days, although it did reduce the spread of international cases by as much as 80 percent. A primary reason travel restrictions were less effective is that many people with COVID-19 do not show symptoms during the early stages of infection.

Shielding

Social distancing markers and plexiglass shield at Whole Foods Market checkout in Toronto to reduce physical contact.

 

Shielding measures for individuals include limiting face-to-face contacts, conducting business by phone or online, avoiding public places and reducing unnecessary travel.

Quarantine

During the 2003 SARS outbreak in Singapore, approximately 8000 people were subjected to mandatory home quarantine and an additional 4300 were required to self-monitor for symptoms and make daily telephone contact with health authorities as a means of controlling the epidemic. Although only 58 of these individuals were eventually diagnosed with SARS, public health officials were satisfied that this measure assisted in preventing further spread of the infection. Voluntary self-isolation may have helped reduce transmission of influenza in Texas in 2009. Short and longterm negative psychological effects have been reported.

Stay-at-home orders

The objective of stay-at-home orders is to reduce day-to-day contact with between people and thereby reduce spread of infection

Cordon sanitaire

In 1995, a cordon sanitaire was used to control an outbreak of Ebola virus disease in Kikwit, Zaire. President Mobutu Sese Seko surrounded the town with troops and suspended all flights into the community. Inside Kikwit, the World Health Organization and Zaire's medical teams erected further cordons sanitaires, isolating burial and treatment zones from the general population and successfully containing the infection.

Protective sequestration

During the 1918 influenza epidemic, the town of Gunnison, Colorado, isolated itself for two months to prevent an introduction of the infection. Highways were barricaded and arriving train passengers were quarantined for five days. As a result of the isolation, no one died of influenza in Gunnison during the epidemic. Several other communities adopted similar measures.

Other measures

Other measures include shutting down or limiting mass transit and closure of sport facilities (community swimming pools, youth clubs, gymnasiums).

History

Leper colonies and lazarettos were established as a means of preventing the spread of leprosy and other contagious diseases through social distancing, until transmission was understood and effective treatments invented.

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  The Lazzaretto of Ancona was constructed in the 18th century on an artificial island to serve as a quarantine station and leprosarium for the port town of Ancona, Italy.
  
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  Two lepers denied entrance to town. Woodcut by Vincent of Beauvais, 14th century
  
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  New York City parks and playgrounds were closed during a 1916 polio epidemic.
  
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  Passenger without mask being refused boarding of a streetcar amid 1918 flu pandemic. (Seattle, Washington, 1918)
  

1916 New York City polio epidemic

During the 1916 New York City polio epidemic, when there were more than 27,000 cases and more than 6,000 deaths due to polio in the United States, with more than 2,000 deaths in New York City alone, movie theatres were closed, meetings were cancelled, public gatherings were almost non-existent, and children were warned not to drink from water fountains, and told to avoid amusement parks, swimming pools and beaches.

Influenza, 1918 to present

During the influenza pandemic of 1918, Philadelphia saw its first cases of influenza on 17 September. The city continued with its planned parade and gathering of more than 200000 people and over the subsequent three days, the city's 31 hospitals became fully occupied. Over one week, 4500 people died. Social distancing measures were introduced on 3 October, on the orders of St. Louis physician Max C. Starkloff, more than two weeks after the first case. Unlike Philadelphia, St. Louis experienced its first cases of influenza on 5 October and the city took two days to implement several social distancing measures, including closing schools, theatres, and other places where people get together. It banned public gatherings, including funerals. The actions slowed the spread of influenza in St. Louis and a spike in cases and deaths, as had happened in Philadelphia, did not occur. The final death rate in St. Louis increased following a second wave of cases, but remained overall less than in other cities. Bootsma and Ferguson analyzed social distancing interventions in sixteen U.S. cities during the 1918 epidemic and found that time-limited interventions reduced total mortality only moderately (perhaps 10–30%), and that the impact was often very limited because the interventions were introduced too late and lifted too early. It was observed that several cities experienced a second epidemic peak after social distancing controls were lifted, because susceptible individuals who had been protected were now exposed.

School closures were shown to reduce morbidity from the Asian flu by 90% during the 1957–1958 pandemic, and up to 50% in controlling influenza in the U.S., 2004–2008. Similarly, mandatory school closures and other social distancing measures were associated with a 29% to 37% reduction in influenza transmission rates during the 2009 flu epidemic in Mexico.

During the swine flu outbreak in 2009 in the UK, in an article titled "Closure of schools during an influenza pandemic" published in The Lancet Infectious Diseases, a group of epidemiologists endorsed the closure of schools in order to interrupt the course of the infection, slow further spread and buy time to research and produce a vaccine. Having studied previous influenza pandemics including the 1918 flu pandemic, the influenza pandemic of 1957 and the 1968 flu pandemic, they reported on the economic and workforce effect school closure would have, particularly with a large percentage of doctors and nurses being women, of whom half had children under the age of 16. They also looked at the dynamics of the spread of influenza in France during French school holidays and noted that cases of flu dropped when schools closed and re-emerged when they re-opened. They noted that when teachers in Israel went on strike during the flu season of 1999–2000, visits to doctors and the number of respiratory infections dropped by more than a fifth and more than two fifths respectively.

SARS 2003

During the SARS outbreak of 2003, social distancing measures such as banning large gatherings, closing schools and theaters, and other public places, supplemented public health measures such as finding and isolating affected people, quarantining their close contacts, and infection control procedures. This was combined with wearing masks for certain people. During this time in Canada, "community quarantine" was used to reduce transmission of the disease with moderate success.

COVID-19 pandemic

Simulations comparing rate of spread of infection, and number of deaths due to overrun of hospital capacity, when social interactions are "normal" (left, 200 people moving freely) and "distanced" (right, 25 people moving freely).
Green = Healthy, uninfected individuals
Red = Infected individuals
Blue = Recovered individual
Black = Dead individuals

During the COVID-19 pandemic, social distancing and related measures are emphasised by several governments as alternatives to an enforced quarantine of heavily affected areas. According to UNESCO monitoring, more than a hundred countries have implemented nationwide school closures in response to COVID-19, impacting over half the world's student population. In the United Kingdom, the government advised the public to avoid public spaces, and cinemas and theatres voluntarily closed to encourage the government's message.

With many people disbelieving that COVID-19 is any worse than the seasonal flu, it has been difficult to convince the public—especially teens and young adults—to voluntarily adopt social distancing practices. In Belgium, media reported a rave was attended by at least 300 before it was broken up by local authorities. In France teens making nonessential trips are fined up to US$150. Beaches were closed in Florida and Alabama to disperse partygoers during spring break. Weddings were broken up in New Jersey and an 8 p.m. Curfew was imposed in Newark. New York, New Jersey, Connecticut and Pennsylvania were the first states to adopt coordinated social distancing policies which closed down non-essential businesses and restricted large gatherings. Shelter in place orders in California were extended to the entire state on 19 March. On the same day Texas declared a public disaster and imposed statewide restrictions.

These preventive measures such as social-distancing and self-isolation prompted the widespread closure of primary, secondary, and post-secondary schools in more than 120 countries. As of 23 March 2020, more than 1.2 billion learners were out of school due to school closures in response to COVID-19. Given low rates of COVID-19 symptoms among children, the effectiveness of school closures has been called into question. Even when school closures are temporary, it carries high social and economic costs. However, the significance of children in spreading COVID-19 is unclear. While the full impact of school closures during the coronavirus pandemic are not yet known, UNESCO advises that school closures have negative impacts on local economies and on learning outcomes for students.

In early March 2020, the sentiment "Stay The Fuck Home" was coined by Florian Reifschneider, a German engineer and was quickly echoed by notable celebrities such as Taylor Swift, Ariana Grande and Busy Philipps in hopes of reducing and delaying the peak of the outbreak. Facebook, Twitter and Instagram also joined the campaign with similar hashtags, stickers and filters under #staythefhome, #stayhome, #staythefuckhome and began trending across social media. The website claims to have reached about two million people online and says the text has been translated into 17 languages.

Drawbacks

There are concerns that social distancing can have adverse affects on participants' mental health. It may lead to stress, anxiety, depression or panic, especially for individuals with preexisting conditions such as anxiety disorders, obsessive compulsive disorders, and paranoia. Widespread media coverage about a pandemic, its impact on economy, and resulting hardships may create anxiety. Change in daily circumstances and uncertainty about the future may add onto the mental stress of being away from other people.

Portrayal in literature

In his 1957 science fiction novel The Naked Sun, Isaac Asimov portrays a planet where people live with social distancing. They are spread out, miles from each other, across a sparsely-populated world. Communication is primarily through technology. A male and a female still need to engage in sex to make a baby, but because of the risk of disease transmission it is a dangerous, nasty chore. In contrast, when communication is through technology the situation is the reverse: there is no modesty, and casual nudity is frequent. The novel's point of departure is a murder: this seemingly idyllic world in fact has serious social problems.

Theoretical basis

A look at the math behind social distancing amid coronavirus where the goal is to decrease the effective reproduction number, ${\displaystyle R}$, which starts off equal to ${\displaystyle R_{0}}$, the basic reproduction number, which is the average number of secondary infected individuals generated from one primary infected individual in a population where all individuals are equally susceptible to COVID-19

 

From the perspective of epidemiology, the basic goal behind social distancing is to decrease the effective reproduction number, ${\displaystyle R_{e}}$ or ${\displaystyle R}$, which in the absence of social distancing would equate to the basic reproduction number, i.e. the average number of secondary infected individuals generated from one primary infected individual in a population where all individuals are equally susceptible to a disease. In a basic model of social distancing, where a proportion ${\displaystyle f}$ of the population engages in social distancing to decrease their interpersonal contacts to a fraction ${\displaystyle a}$ of their normal contacts, the new effective reproduction number ${\displaystyle R}$ is given by:

$${\displaystyle R=[1-(1-a^{2})f]R_{0}}$$

For example, 25% of the population reducing their social contacts to 50% of their normal level gives an effective reproduction number about 81% of the basic reproduction number. A seemingly small reduction has a statistically significant effect in delaying the exponential growth and spread of a disease.

Where the value of ${\displaystyle R}$ can be brought below 1 for sufficiently long, containment is achieved, and the number infected should decrease.

Passive immunity

From Wikipedia, the free encyclopedia

 

Passive immunity is the transfer of active humoral immunity of ready-made antibodies. Passive immunity can occur naturally, when maternal antibodies are transferred to the fetus through the placenta, and it can also be induced artificially, when high levels of antibodies specific to a pathogen or toxin (obtained from humans, horses, or other animals) are transferred to non-immune persons through blood products that contain antibodies, such as in immunoglobulin therapy or antiserum therapy. Passive immunization is used when there is a high risk of infection and insufficient time for the body to develop its own immune response, or to reduce the symptoms of ongoing or immunosuppressive diseases. Passive immunization can be provided when people cannot synthesize antibodies, and when they have been exposed to a disease that they do not have immunity against.

Naturally acquired

Maternal passive immunity is a type of naturally acquired passive immunity, and refers to antibody-mediated immunity conveyed to a fetus or infant by its mother. Naturally acquired passive immunity can be provided during pregnancy, and through breastfeeding. In humans, maternal antibodies (MatAb) are passed through the placenta to the fetus by an FcRn receptor on placental cells. This occurs predominately during the third trimester of pregnancy, and thus is often reduced in babies born prematurely. Immunoglobulin G (IgG) is the only antibody isotype that can pass through the human placenta, and is the most common antibody of the five types of antibodies found in the body. IgG antibodies protects against bacterial and viral infections in fetuses. Immunization is often required shortly following birth to prevent diseases in newborns such as tuberculosis, hepatitis B, polio, and pertussis, however, maternal IgG can inhibit the induction of protective vaccine responses throughout the first year of life. This effect is usually overcome by secondary responses to booster immunization. Maternal antibodies protect against some diseases, such as measles, rubella, and tetanus, more effectively than against others, such as polio and pertussis. Maternal passive immunity offers immediate protection, though protection mediated by maternal IgG typically only lasts up to a year.

Passive immunity is also provided through colostrum and breast milk, which contain IgA antibodies that are transferred to the gut of the infant, providing local protection against disease causing bacteria and viruses until the newborn can synthesize its own antibodies. Protection mediated by IgA is dependent on the length of time that an infant is breastfed, which is one of the reasons the World Health Organization recommends breastfeeding for at least the first two years of life.

Other species besides humans transfer maternal antibodies before birth, including primates and lagomorphs (which includes rabbits and hares). In some of these species IgM can be transferred across the placenta as well as IgG. All other mammalian species predominantly or solely transfer maternal antibodies after birth through milk. In these species, the neonatal gut is able to absorb IgG for hours to days after birth. However, after a period of time the neonate can no longer absorb maternal IgG through their gut, an event that is referred to as "gut closure". If a neonatal animal does not receive adequate amounts of colostrum prior to gut closure, it does not have a sufficient amount of maternal IgG in its blood to fight off common diseases. This condition is referred to as failure of passive transfer. It can be diagnosed by measuring the amount of IgG in a newborn's blood, and is treated with intravenous administration of immunoglobulins. If not treated, it can be fatal.

Artificially acquired

Artificially acquired passive immunity is a short-term immunization achieved by the transfer of antibodies, which can be administered in several forms; as human or animal blood plasma or serum, as pooled human immunoglobulin for intravenous (IVIG) or intramuscular (IG) use, as high-titer human IVIG or IG from immunized donors or from donors recovering from the disease, and as monoclonal antibodies (MAb). Passive transfer is used to prevent disease or used prophylactically in the case of immunodeficiency diseases, such as hypogammaglobulinemia. It is also used in the treatment of several types of acute infection, and to treat poisoning. Immunity derived from passive immunization lasts for a few weeks to three to four months. There is also a potential risk for hypersensitivity reactions, and serum sickness, especially from gamma globulin of non-human origin. Passive immunity provides immediate protection, but the body does not develop memory, therefore the patient is at risk of being infected by the same pathogen later unless they acquire active immunity or vaccination.

History and applications of artificial passive immunity

A vial of diphtheria antitoxin, dated 1895

In 1888 Emile Roux and Alexandre Yersin showed that the clinical effects of diphtheria were caused by diphtheria toxin and, following the 1890 discovery of an antitoxin-based immunity to diphtheria and tetanus by Emil Adolf von Behring and Kitasato Shibasaburō, antitoxin became the first major success of modern therapeutic immunology. Shibasaburo and von Behring immunized guinea pigs with the blood products from animals that had recovered from diphtheria and realized that the same process of heat treating blood products of other animals could treat humans with diphtheria. By 1896, the introduction of diphtheria antitoxin was hailed as "the most important advance of the [19th] Century in the medical treatment of acute infective disease".

Prior to the advent of vaccines and antibiotics, specific antitoxin was often the only treatment available for infections such as diphtheria and tetanus. Immunoglobulin therapy continued to be a first line therapy in the treatment of severe respiratory diseases until the 1930s, even after sulfonamides were introduced.

This image is from the Historical Medical Library of The College of Physicians of Philadelphia. This displays the administration of diphtheria antitoxin from horse serum to young child, dated 1895.

In 1890 antibody therapy was used to treat tetanus, when serum from immunized horses was injected into patients with severe tetanus in an attempt to neutralize the tetanus toxin, and prevent the dissemination of the disease. Since the 1960s, human tetanus immune globulin (TIG) has been used in the United States in unimmunized, vaccine-naive or incompletely immunized patients who have sustained wounds consistent with the development of tetanus. The administration of horse antitoxin remains the only specific pharmacologic treatment available for botulism. Antitoxin also known as heterologous hyperimmune serum is often also given prophylactically to individuals known to have ingested contaminated food. IVIG treatment was also used successfully to treat several victims of toxic shock syndrome, during the 1970s tampon scare.

Antibody therapy is also used to treat viral infections. In 1945, hepatitis A infections, epidemic in summer camps, were successfully prevented by immunoglobulin treatment. Similarly, hepatitis B immune globulin (HBIG) effectively prevents hepatitis B infection. Antibody prophylaxis of both hepatitis A and B has largely been supplanted by the introduction of vaccines; however, it is still indicated following exposure and prior to travel to areas of endemic infection.

In 1953, human vaccinia immunoglobulin (VIG) was used to prevent the spread of smallpox during an outbreak in Madras, India, and continues to be used to treat complications arising from smallpox vaccination. Although the prevention of measles is typically induced through vaccination, it is often treated immuno-prophylactically upon exposure. Prevention of rabies infection still requires the use of both vaccine and immunoglobulin treatments.

During a 1995 Ebola virus outbreak in the Democratic Republic of Congo, whole blood from recovering patients, and containing anti-Ebola antibodies, was used to treat eight patients, as there was no effective means of prevention, though a treatment was discovered recently in the 2013 Ebola epidemic in Africa. Only one of the eight infected patients died, compared to a typical 80% Ebola mortality, which suggested that antibody treatment may contribute to survival. Immune globulin or immunoglobulin has been used to both prevent and treat reactivation of the herpes simplex virus (HSV), varicella zoster virus, Epstein-Barr virus (EBV), and cytomegalovirus (CMV).^[12]

FDA licensed immunoglobulins

The following immunoglobulins are the immunoglubulins currently approved for use for infectious disease prophylaxis and immunotherapy, in the United States.

FDA approved products for passive immunization and immunotherapy

Disease	Product	Source	Use
Botulism	Specific equine IgG	horse	Treatment of wound and food borne forms of botulism, infant botulism is treated with human botulism immune globulin (BabyBIG).
Cytomegalovirus (CMV)	hyper-immune IVIG	human	Prophylaxis, used most often in kidney transplant patients.
Diphtheria	Specific equine IgG	horse	Treatment of diphtheria infection.
Hepatitis A, measles	Pooled human Ig	human serum	Prevention of Hepatitis A and measles infection, treatment of congenital or acquired immunodeficiency.
Hepatitis B	Hepatitis B Ig	human	Post-exposure prophylaxis, prevention in high-risk infants (administered with Hepatitis B vaccine).
ITP, Kawasaki disease, IgG deficiency	Pooled human IgG	human serum	Treatment of ITP and Kawasaki disease, prevention/treatment of opportunistic infection with IgG deficiency.
Rabies	Rabies Ig	human	Post-exposure prophylaxis (administered with rabies vaccine).
Tetanus	Tetanus Ig	human	Treatment of tetanus infection.
Vaccinia	Vaccinia Ig	human	Treatment of progressive vaccinia infection including eczema and ocular forms (usually resulting from smallpox vaccination in immunocompromised individuals).
Varicella (chicken-pox)	Varicella-zoster Ig	human	Post-exposure prophylaxis in high risk individuals.

Passive transfer of cell-mediated immunity

The one exception to passive humoral immunity is the passive transfer of cell-mediated immunity, also called adoptive immunization which involves the transfer of mature circulating lymphocytes. It is rarely used in humans, and requires histocompatible (matched) donors, which are often difficult to find, and carries severe risks of graft-versus-host disease. This technique has been used in humans to treat certain diseases including some types of cancer and immunodeficiency. However, this specialized form of passive immunity is most often used in a laboratory setting in the field of immunology, to transfer immunity between "congenic", or deliberately inbred mouse strains which are histocompatible.

Advantages and disadvantages

An individual's immune response of passive immunity is "faster than a vaccine" and can instill immunity in an individual that does not "respond to immunization", often within hours or a few days. In addition to conferring passive immunities, breastfeeding has other lasting beneficial effects on the baby's health, such as decreased risk of allergies and obesity.

A disadvantage to passive immunity is that producing antibodies in a laboratory is expensive and difficult to do. In order to produce antibodies for infectious diseases, there is a need for possibly thousands of human donors to donate blood or immune animals' blood would be obtained for the antibodies. Patients who are immunized with the antibodies from animals may develop serum sickness due to the proteins from the immune animal and develop serious allergic reactions. Antibody treatments can be time consuming and are given through an intravenous injection or IV, while a vaccine shot or jab is less time consuming and has less risk of complication than an antibody treatment. Passive immunity is effective, but only lasts a short amount of time.

Herd immunity

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Herd_immunity

The top box shows an outbreak in a community in which a few people are infected (shown in red) and the rest are healthy but unimmunized (shown in blue); the illness spreads freely through the population. The middle box shows a population where a small number have been immunized (shown in yellow); those not immunized become infected while those immunized do not. In the bottom box, a large proportion of the population have been immunized; this prevents the illness from spreading significantly, including to unimmunized people. In the first two examples, most healthy unimmunized people become infected, whereas in the bottom example only one fourth of the healthy unimmunized people become infected.

 

Herd immunity (also called herd effect, community immunity, population immunity, or social immunity) is a form of indirect protection from infectious disease that occurs when a large percentage of a population has become immune to an infection, whether through previous infections or vaccination, thereby providing a measure of protection for individuals who are not immune. In a population in which a large proportion of individuals possess immunity, such people being unlikely to contribute to disease transmission, chains of infection are more likely to be disrupted, which either stops or slows the spread of disease. The greater the proportion of immune individuals in a community, the smaller the probability that non-immune individuals will come into contact with an infectious individual, helping to shield non-immune individuals from infection.

Individuals can become immune by recovering from an earlier infection or through vaccination. Some individuals cannot become immune because of medical conditions, such as an immunodeficiency or immunosuppression, and for this group herd immunity is a crucial method of protection. Once a certain threshold has been reached, herd immunity gradually eliminates a disease from a population. This elimination, if achieved worldwide, may result in the permanent reduction in the number of infections to zero, called eradication. Herd immunity created via vaccination contributed to the eventual eradication of smallpox in 1977 and has contributed to the reduction of the frequencies of other diseases. Herd immunity does not apply to all diseases, just those that are contagious, meaning that they can be transmitted from one individual to another. Tetanus, for example, is infectious but not contagious, so herd immunity does not apply.

Herd immunity was recognized as a naturally occurring phenomenon in the 1930s when it was observed that after a significant number of children had become immune to measles, the number of new infections temporarily decreased, including among susceptible children. Mass vaccination to induce herd immunity has since become common and proved successful in preventing the spread of many infectious diseases. Opposition to vaccination has posed a challenge to herd immunity, allowing preventable diseases to persist in or return to communities that have inadequate vaccination rates.

Effects

Protection of those without immunity

Some individuals either cannot develop immunity after vaccination or for medical reasons cannot be vaccinated. Newborn infants are too young to receive many vaccines, either for safety reasons or because passive immunity renders the vaccine ineffective. Individuals who are immunodeficient due to HIV/AIDS, lymphoma, leukemia, bone marrow cancer, an impaired spleen, chemotherapy, or radiotherapy may have lost any immunity that they previously had and vaccines may not be of any use for them because of their immunodeficiency.

A portion of those vaccinated may not develop long-term immunity. Vaccine contraindications may prevent certain individuals from being vaccinated. In addition to not being immune, individuals in one of these groups may be at a greater risk of developing complications from infection because of their medical status, but they may still be protected if a large enough percentage of the population is immune.

High levels of immunity in one age group can create herd immunity for other age groups. Vaccinating adults against pertussis reduces pertussis incidence in infants too young to be vaccinated, who are at the greatest risk of complications from the disease. This is especially important for close family members, who account for most of the transmissions to young infants. In the same manner, children receiving vaccines against pneumococcus reduces pneumococcal disease incidence among younger, unvaccinated siblings. Vaccinating children against pneumococcus and rotavirus has had the effect of reducing pneumococcus- and rotavirus-attributable hospitalizations for older children and adults, who do not normally receive these vaccines. Influenza (flu) is more severe in the elderly than in younger age groups, but influenza vaccines lack effectiveness in this demographic due to a waning of the immune system with age. The prioritization of school-age children for seasonal flu immunization, which is more effective than vaccinating the elderly, however, has been shown to create a certain degree of protection for the elderly.

For sexually transmitted infections (STIs), high levels of immunity in one sex induces herd immunity for both sexes. Vaccines against STIs that are targeted at one sex result in significant declines in STIs in both sexes if vaccine uptake in the target sex is high. Herd immunity from female vaccination does not, however, extend to homosexual males. If vaccine uptake among the target sex is low, then the other sex may need to be immunized so that the target sex can be sufficiently protected. High-risk behaviors make eliminating STIs difficult since even though most infections occur among individuals with moderate risk, the majority of transmissions occur because of individuals who engage in high-risk behaviors. For these reasons, in certain populations it may be necessary to immunize high-risk persons or individuals of both sexes to establish herd immunity.

Evolutionary pressure

Herd immunity itself acts as an evolutionary pressure on certain viruses, influencing viral evolution by encouraging the production of novel strains, in this case referred to as escape mutants, that are able to "escape" from herd immunity and spread more easily. At the molecular level, viruses escape from herd immunity through antigenic drift, which is when mutations accumulate in the portion of the viral genome that encodes for the virus's surface antigen, typically a protein of the virus capsid, producing a change in the viral epitope. Alternatively, the reassortment of separate viral genome segments, or antigenic shift, which is more common when there are more strains in circulation, can also produce new serotypes. When either of these occur, memory T cells no longer recognize the virus, so people are not immune to the dominant circulating strain. For both influenza and norovirus, epidemics temporarily induce herd immunity until a new dominant strain emerges, causing successive waves of epidemics. As this evolution poses a challenge to herd immunity, broadly neutralizing antibodies and "universal" vaccines that can provide protection beyond a specific serotype are in development.

Serotype replacement

Serotype replacement, or serotype shifting, may occur if the prevalence of a specific serotype declines due to high levels of immunity, allowing other serotypes to replace it. Initial vaccines against Streptococcus pneumoniae significantly reduced nasopharyngeal carriage of vaccine serotypes (VTs), including antibiotic-resistant types, only to be entirely offset by increased carriage of non-vaccine serotypes (NVTs). This did not result in a proportionate increase in disease incidence though, since NVTs were less invasive than VTs. Since then, pneumococcal vaccines that provide protection from the emerging serotypes have been introduced and have successfully countered their emergence. The possibility of future shifting remains, so further strategies to deal with this include expansion of VT coverage and the development of vaccines that use either killed whole-cells, which have more surface antigens, or proteins present in multiple serotypes.

Eradication of diseases

A cow with rinderpest in the "milk fever" position, 1982. The last confirmed case of rinderpest occurred in Kenya in 2001, and the disease was officially declared eradicated in 2011.

 

If herd immunity has been established and maintained in a population for a sufficient time, the disease is inevitably eliminated—no more endemic transmissions occur. If elimination is achieved worldwide and the number of cases is permanently reduced to zero, then a disease can be declared eradicated. Eradication can thus be considered the final effect or end-result of public health initiatives to control the spread of infectious disease.

The benefits of eradication include ending all morbidity and mortality caused by the disease, financial savings for individuals, health care providers, and governments, and enabling resources used to control the disease to be used elsewhere. To date, two diseases have been eradicated using herd immunity and vaccination: rinderpest and smallpox. Eradication efforts that rely on herd immunity are currently underway for poliomyelitis, though civil unrest and distrust of modern medicine have made this difficult. Mandatory vaccination may be beneficial to eradication efforts if not enough people choose to get vaccinated.

Free riding

Herd immunity is vulnerable to the free rider problem. Individuals who lack immunity, particularly those who choose not to vaccinate, free ride off the herd immunity created by those who are immune. As the number of free riders in a population increases, outbreaks of preventable diseases become more common and more severe due to loss of herd immunity. Individuals may choose to free ride for a variety of reasons, including the perceived ineffectiveness of a vaccine, believing that the risks associated with vaccines are greater than those associated with infection, mistrust of vaccines or public health officials, bandwagoning or groupthinking, social norms or peer pressure, and religious beliefs. Certain individuals are more likely to choose not to receive vaccines if vaccination rates are high enough so as to convince a person that he or she may not need to be vaccinated, since a sufficient percentage of others are already immune.

Mechanics

Estimated R₀ and HITs (herd immunity threshold) of well-known infectious diseases

Disease	Transmission	R0	HIT
Measles	Airborne	12–18	92–95%
Pertussis	Airborne droplet	12–17	92–94%
Diphtheria	Saliva	6–7	83–86%
Rubella	Airborne droplet
Smallpox		5–7	80–86%
Polio	Fecal-oral route
Mumps	Airborne droplet	4–7	75–86%
SARS (2002–2004 SARS outbreak)		2–5	50–80%
COVID-19 (COVID-19 pandemic)		1.4–3.9	29–74%
Ebola (Ebola virus epidemic in West Africa)	Bodily fluids	1.5–2.5	33–60%
Influenza (influenza pandemics)	Airborne droplet	1.5–1.8	33–44%

Further information: Mathematical modelling of infectious disease

Individuals who are immune to a disease act as a barrier in the spread of disease, slowing or preventing the transmission of disease to others. An individual's immunity can be acquired via a natural infection or through artificial means, such as vaccination. When a critical proportion of the population becomes immune, called the herd immunity threshold (HIT) or herd immunity level (HIL), the disease may no longer persist in the population, ceasing to be endemic. 

The critical value, or threshold, in a given population, is the point where the disease reaches an endemic steady state, which means that the infection level is neither growing nor declining exponentially. This threshold can be calculated by taking R₀, the basic reproduction number, or the average number of new infections caused by each case in an entirely susceptible population that is homogeneous, or well-mixed, meaning each individual can come into contact with every other susceptible individual in the population, multiplying it by S, the proportion of the population who are susceptible to infection, and setting this product to be equal to 1:

${\displaystyle \ R_{0}\cdot S=1.}$

S can be rewritten as (1 − p) because p is the proportion of the population that is immune and p + S equals one. Then, the equation can be rearranged to place p by itself as follows:

${\displaystyle \ R_{0}\cdot (1-p)=1,}$ → ${\displaystyle 1-p={\frac {1}{R_{0}}},}$ → ${\displaystyle p_{c}=1-{\frac {1}{R_{0}}}.}$

With p being by itself on the left side of the equation, it can now be written as p_c to represent the critical proportion of the population needed to become immune to stop the transmission of disease, or the "herd immunity threshold". R₀ functions as a measure of contagiousness, so low R₀ values are associated with lower HITs, whereas higher R₀s result in higher HITs. For example, the HIT for a disease with an R₀ of 2 is theoretically only 50%, whereas with disease with an R₀ of 10 the theoretical HIT is 90%.

These calculations assume that the entire population is susceptible, meaning no individuals are immune to the disease. In reality, varying proportions of the population are immune to any given disease at any given time. To account for this, the effective reproductive number R_e, also written as R_t, or the average number of infections caused at time t, can found by multiplying R₀ by the fraction of the population that is still susceptible. When R_e is reduced to and sustained below 1, the number of cases occurring in the population gradually decreases until the disease has been eliminated. If a population is immune to a disease in excess of that disease's HIT, the number of cases reduces at a faster rate, outbreaks are even less likely to happen, and outbreaks that occur are smaller than they would be otherwise. If R_e increases to above 1, then the disease is neither in a steady state nor decreasing in incidence, but is actively spreading through the population and infecting a larger number of people than usual.

A second assumption in these calculations is that populations are homogeneous, or well-mixed, meaning that every individual comes into contact with every other individual, when in reality populations are better described as social networks as individuals tend to cluster together, remaining in relatively close contact with a limited number of other individuals. In these networks, transmission only occurs between those who are geographically or physically close to one another. The shape and size of a network is likely to alter a disease's HIT, making incidence either more or less common.

In heterogeneous populations, R₀ is now considered to be a measure of the number of cases generated by a "typical" infectious person, which depends on how individuals within a network interact with each other. Interactions within networks are more common than between networks, in which case the most highly connected networks transmit disease more easily, resulting in a higher R₀ and a higher HIT than would be required in a less connected network. In networks that either opt not to become immune or are not immunized sufficiently, diseases may persist despite not existing in better-immunized networks.

Boosts

Vaccination

The primary way to boost levels of immunity in a population is through vaccination. Vaccination is originally based on the observation that milkmaids exposed to cowpox were immune to smallpox, so the practice of inoculating people with the cowpox virus began as a way to prevent smallpox. Well-developed vaccines provide protection in a far safer way than natural infections, as vaccines generally do not cause the diseases they protect against and severe adverse effects are significantly less common than complications from natural infections.

The immune system does not distinguish between natural infections and vaccines, forming an active response to both, so immunity induced via vaccination is similar to what would have occurred from contracting and recovering from the disease. To achieve herd immunity through vaccination, vaccine manufacturers aim to produce vaccines with low failure rates, and policy makers aim to encourage their use. After the successful introduction and widespread use of a vaccine, sharp declines in the incidence of diseases it protects against can be observed, which decreases the number of hospitalizations and deaths caused by such diseases.

Assuming a vaccine is 100% effective, then the equation used for calculating the herd immunity threshold can be used for calculating the vaccination level needed to eliminate a disease, written as V_c. Vaccines are usually imperfect however, so the effectiveness, E, of a vaccine must be accounted for:

${\displaystyle V_{c}={\frac {1-{\frac {1}{R_{0}}}}{E}}.}$

From this equation, it can be observed that if E is less than (1 − 1/R₀), then it is impossible to eliminate a disease, even if the entire population is vaccinated. Similarly, waning vaccine-induced immunity, as occurs with acellular pertussis vaccines, requires higher levels of booster vaccination to sustain herd immunity. If a disease has ceased to be endemic to a population, then natural infections no longer contribute to a reduction in the fraction of the population that is susceptible. Only vaccination contributes to this reduction. The relation between vaccine coverage and effectiveness and disease incidence can be shown by subtracting the product of the effectiveness of a vaccine and the proportion of the population that is vaccinated, p_v, from the herd immunity threshold equation as follows: 

Measles vaccine coverage and reported measles cases in Eastern Mediterranean countries. As coverage increased, the number of cases decreased.

 

${\displaystyle \left(1-{\frac {1}{R_{0}}}\right)-(E\times p_{v}).}$

It can be observed from this equation that, all other things being equal ("ceteris paribus"), any increase in either vaccine coverage or vaccine effectiveness, including any increase in excess of a disease's HIT, further reduces the number of cases of a disease. The rate of decline in cases depends on a disease's R₀, with diseases with lower R₀ values experiencing sharper declines.

Vaccines usually have at least one contraindication for a specific population for medical reasons, but if both effectiveness and coverage are high enough herd immunity can protect these individuals. Vaccine effectiveness is often, but not always, adversely affected by passive immunity, so additional doses are recommended for some vaccines while others are not administered until after an individual has lost his or her passive immunity.

Passive immunity

Individual immunity can also be gained passively, when antibodies to a pathogen are transferred from one individual to another. This can occur naturally, whereby maternal antibodies, primarily immunoglobulin G antibodies, are transferred across the placenta and in colostrum to fetuses and newborns. Passive immunity can also be gained artificially, when a susceptible person is injected with antibodies from the serum or plasma of an immune person.

Protection generated from passive immunity is immediate, but wanes over the course of weeks to months, so any contribution to herd immunity is temporary. For diseases that are especially severe among fetuses and newborns, such as influenza and tetanus, pregnant women may be immunized in order to transfer antibodies to the child. In the same way, high-risk groups that are either more likely to experience infection, or are more likely to develop complications from infection, may receive antibody preparations to prevent these infections or to reduce the severity of symptoms.

Cost–benefit analysis

Herd immunity is often accounted for when conducting cost–benefit analyses of vaccination programs. It is regarded as a positive externality of high levels of immunity, producing an additional benefit of disease reduction that would not occur had no herd immunity been generated in the population. Therefore, herd immunity's inclusion in cost–benefit analyses results both in more favorable cost-effectiveness or cost–benefit ratios, and an increase in the number of disease cases averted by vaccination. Study designs done to estimate herd immunity's benefit include recording disease incidence in households with a vaccinated member, randomizing a population in a single geographic area to be vaccinated or not, and observing the incidence of disease before and after beginning a vaccination program. From these, it can be observed that disease incidence may decrease to a level beyond what can be predicted from direct protection alone, indicating that herd immunity contributed to the reduction. When serotype replacement is accounted for, it reduces the predicted benefits of vaccination.

History

Measles cases in the United States before and after mass vaccination against measles began.

Herd immunity was first recognized as a naturally occurring phenomenon in the 1930s when A. W. Hedrich published research on the epidemiology of measles in Baltimore, and took notice that after many children had become immune to measles, the number of new infections temporarily decreased, including among susceptible children. In spite of this knowledge, efforts to control and eliminate measles were unsuccessful until mass vaccination using the measles vaccine began in the 1960s. Mass vaccination, discussions of disease eradication, and cost–benefit analyses of vaccination subsequently prompted more widespread use of the term herd immunity. In the 1970s, the theorem used to calculate a disease's herd immunity threshold was developed. During the smallpox eradication campaign in the 1960s and 1970s, the practice of ring vaccination, of which herd immunity is integral to, began as a way to immunize every person in a "ring" around an infected individual to prevent outbreaks from spreading.

Since the adoption of mass and ring vaccination, complexities and challenges to herd immunity have arisen. Modeling of the spread of infectious disease originally made a number of assumptions, namely that entire populations are susceptible and well-mixed, which do not exist in reality, so more precise equations have been developed. In recent decades, it has been recognized that the dominant strain of a microorganism in circulation may change due to herd immunity, either because of herd immunity acting as an evolutionary pressure or because herd immunity against one strain allowed another already-existing strain to spread. Emerging or ongoing fears and controversies about vaccination have reduced or eliminated herd immunity in certain communities, allowing preventable diseases to persist in or return to these communities.