Social distancing, or 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 involves keeping a distance of six feet or two meters from others and avoiding gathering together in large groups.
Social distancing measures date back to at least the fifth century BCE. The biblical book of Leviticus contains one of the earliest known references to the practice, likely as response to leprosy. During the Plague of Justinian, emperor Justinian enforced an ineffective quarantine on the Byzantine Empire, including dumping bodies into the sea, blaming the widespread outbreak predominately on "Jews, Samaritans, pagans, heretics, Arians, Montanists, and homosexuals". In modern times, social distancing measures have been successfully implemented in several previous 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. Social distancing has also been used during the 2019-20 coronavirus epidemic.
Social distancing measures are more effective when the infectious disease spreads via droplet contact (coughing or sneezing); direct physical contact, including sexual contact; indirect physical contact (e.g., by touching a contaminated surface); or 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.
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 2019–2020 coronavirus 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".
Previously, in 2009, 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.
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 behaviour 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 illustration offers eight alternatives.
Keeping at least two-metre (six-foot) distance 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.
Floor markings can help people maintain distance in public places
People queueing to enter a supermarket in Naas, Ireland. Numbers entering the supermarket are restricted, and queuers are standing at least 2 metres apart.
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.
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.
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.
Self-shielding measures for individuals include limiting face-to-face contacts, conducting business by phone or online, avoiding public places and reducing unnecessary travel.
Quarantine of possible cases
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.
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 include shutting down or limiting mass transit and closure of recreational facilities (community swimming pools, youth clubs, gymnasiums).
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.
New York City parks and playgrounds were closed during a 1916 polio epidemic.
Passenger without mask being refused boarding of a streetcar (Seattle, Washington, 1918)
1916 New York City polio epidemic
During the 1916 New York City polio epidemic, when there were more than 27000 cases and more than 6000 deaths due to polio in the United States, with more than 2000 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 3October, more than two weeks after the first case. Unlike Philadelphia, St. Louis experienced its first cases of influenza on 5October 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 outbreak, 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 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.
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.
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 2019–2020 coronavirus pandemic, social distancing and related measures were 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 8p.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.
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.
From the perspective of epidemiology, the basic goal behind social distancing is to decrease 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 a disease. In a basic model of social distancing, where a proportion of the population engages in social distancing to decrease their interpersonal contacts to a fraction of their normal contacts, the new effective reproduction number is given by:
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 profound[clarification needed] effect in delaying the exponential growth and spread of a disease.
Where the value of can be brought below1 for sufficiently long, containment is achieved, and the number infected should decrease.
^Ishola, David A.; Phin, Nick (December 2011). "Could Influenza Transmission Be Reduced by Restricting Mass Gatherings? Towards an Evidence-Based Policy Framework". Journal of Epidemiology and Global Health. 1 (1): 33–60. doi:10.1016/j.jegh.2011.06.004. PMID23856374.