Reducing the number of contacts between people must, to some extent, limit the transmission of the new coronavirus. The evidence for how best to stop transmission comes from preliminary studies of the current COVID-19 pandemic and from previous outbreaks of other infections, especially influenza.
Social distancing is used to protect public health during an outbreak. Its effects are not usually investigated in formal scientific experiments, so the strength of the evidence is limited. Scientists and policymakers have to take this into account, as well as the differences between COVID-19 and other infections such as influenza.
Types of social distancing
There are broadly two types of social distancing. The first aims to prevent the transmission of virus from infectious individuals to others who come into close contact. Among the measures that could work are keeping a minimum distance between people, wearing a face mask to prevent the transmission of infection through coughing and sneezing, and avoiding contact with contaminated surfaces.
The second set of measures aims to stop people from meeting at all, for example by closing schools, shops, and workplaces, banning mass gatherings, advising people to stay at home, and suspending public transport. These are usually more disruptive to society as a whole.
Social distancing is usually applied to everyone. But it can be targeted to specific categories of people, for example to protect the elderly and others who are most vulnerable to infection, and to quarantine people who have symptoms and who might pass infection on to others.
An outstanding question for research is whether the least disruptive means of social distancing can also be highly effective.
Social distancing to combat COVID-19
Scientific studies of social distancing for COVID-19 are in their early stages, so the evidence of what works is limited.
Guidance on what is the safe minimum distance and time of contact between individuals is informed by studies of the way viruses, particularly SARS Cov-2, are transmitted and their persistence in the environment. The UK government recommends keeping a minimum safe distance of at least two metres between people during the current COVID-19 outbreak. This is current best practice, but research groups are continuing to investigate how far virus-containing droplets are propelled by a cough or a sneeze.
As for the duration and type of social distancing required, one study found that the average time of SARS Cov-2 persistence in airborne droplets was 0.6 to 2.6 hours, but it could be much longer on some surfaces, with infectious viruses being recovered after 72 hours on plastic and stainless steel.
Because there have been few direct studies of social distancing for COVID-19, some of the evidence comes indirectly from mathematical models and simulations. Modelling suggests that the isolation of suspected cases, active case-finding, city-wide lockdowns, and screening measures at train stations and airports have all played a part in slowing the spread of COVID-19 in China and South Korea. Modelling also suggests that combinations of social distancing measures will work in the UK, including home isolation of suspected cases, home quarantine of those living in the same household, and the protection of elderly people and others at most risk of severe illness.
Lessons from previous pandemics
Previous pandemics also offer clues about what forms of social distancing might work for COVID-19. Most evidence comes from influenza, which caused a series of pandemics during the 20th and 21st centuries, including the 1918-20 ‘Spanish flu’ and the less extensive but more recent 2009 swine flu. Unlike influenza and COVID-19, Ebola is not a respiratory disease, but the 2014-15 West African Ebola outbreak offers lessons on social distancing too.
It is important to remember that Ebola, influenza and COVID-19 are different diseases, and also that society today may respond differently to how it did in the past. However, an advantage of studying earlier outbreaks is that investigators can look at longer-term effects of social distancing, over the whole course of an outbreak.
In general for influenza, social distancing measures reduced transmission of the virus and delayed its geographical spread. The specific effective methods included: isolating people who are ill, tracing and isolating their contacts, and advising quarantine for people who have been exposed to infection. As expected, combinations of measures were most effective, and more so when adopted early and continuously.
Closing schools during influenza pandemics has been effective too. School closure might not work as well for COVID-19 because children appear to be less susceptible to the new coronavirus than they are to flu.
For influenza, as for COVID-19, there has been concern about the knock-on effects of closing schools. When schools are closed, children can mix in other social situations; key workers, parents and other carers have to take time off to look after children, with health and economic costs; and there are risks of some children getting inadequate care or schooling.
Workplace measures such as remote working and staggered shifts can interrupt influenza transmission and delay the spread of infection. However, the workplace measures implemented in influenza outbreaks have seldom been as restrictive as those currently being deployed against COVID-19.
During a pandemic, multiple forms of social distancing are typically implemented at the same time. One study that examined 19 kinds of intervention in 17 US cities during the 1918 influenza pandemic found that cities, such as St Louis, which adopted social distancing early in the pandemic fared better than those that did not, such as Philadelphia. They cut peak death rates by about half on average and total deaths by about a fifth.