Social distancing is a term applied to certain nonpharmaceutical infection control actions that are taken by public health officials to stop or slow down the spread of a highly contagious disease. The objective of social distancing is to reduce the probability of contact between persons carrying an infection, and others who are not infected, so as to minimize disease transmission, morbidity and ultimately, mortality. Social distancing may be ineffective in cases where the infection is transmitted primarily via contaminated water or food or by a vector such as mosquitoes or other insects, and less frequently from person to person.
One of the earliest references to social distancing dates to the seventh century BC in the Book of Leviticus, 13:45: "And the leper in whom the plague is...he shall dwell alone; [outside] the camp shall his habitation be."
Historically, 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 were invented.
Some examples of social distancing used to control the spread of contagious illnesses include:
- school closure (proactive or reactive)
- workplace closure 
- isolation (health care)
- cordon sanitaire
- protective sequestration
- cancellation of mass gatherings such as sports events, films or musical shows.
- shutting down or limiting mass transit
- closing “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.
- closure of recreational facilities (community swimming pools, youth clubs, gymnasiums).
- "self-shielding" measures for individuals include limiting face-to-face contacts, conducting business by phone or online, avoiding public places and reducing unnecessary travel. The "elbow bump" and the "Dracula sneeze" are additional measures to reduce direct person-to-person transmission of microorganisms.
Research indicates that measures must be applied rigorously and immediately in order to be effective. During the 1918 flu pandemic, authorities in the US implemented school closures, bans on public gatherings, and other social distancing interventions in Philadelphia and in St. Louis, but in Philadelphia the delay of five days in initiating these measures allowed transmission rates to double three to five times, whereas a more immediate response in St. Louis was significant in reducing transmission there. Bootsma and Ferguson analyzed social distancing interventions in 16 US 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 1957-58 outbreak, and up to 50% in controlling influenza in the US, 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.
Modeling and simulation studies based on US 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 1 week.
Quarantine of contacts and suspected cases
During the 2003 SARS outbreak in Singapore, some 8,000 persons were subjected to mandatory home quarantine and an additional 4,300 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.
Canceling mass gatherings
Evidence suggesting that mass gatherings increase the potential for infectious disease transmission is inconclusive. There is some evidence that certain types of mass gatherings may be associated with increased risk of influenza transmission. They may also "seed" new strains into an area, and may instigate community transmission in a pandemic. Restricting mass gatherings, in combination with other social distancing interventions, may help reduce transmission.
Border restrictions and/or internal travel restrictions are unlikely to delay an epidemic by more than 2–3 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 US. 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.
- Cordon sanitaire
- Infection control
- Isolation (health care)
- Leper colony
- Protective sequestration
- Transmission (medicine)
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