What is the evidence to support the 2-metre social distancing rule to reduce COVID-19 transmission? – a lay summary
The UK government’s rule that social distancing should be set at two metres, is set to be eased to "one metre plus" in England on 4th July. The 2 metre rule had been based on the assumption that SARS-CoV-2 – the virus responsible for the disease COVID-19 – is transmitted mainly via large droplets sneezed or coughed onto other people or surfaces. The reality, we now know, is more complex. What effect could the easing of the rule have on safety?
SARS-CoV-2 virus can be carried in droplets of all sizes. The largest ones are big enough to see, and are sprayed by coughs and sneezes onto bystanders and nearby surfaces. But it can also travel in tiny invisible droplets (sometimes referred to as "aerosols") that drift in the air, even beyond 2 metres, after being exhaled by infected individuals. It is not known for certain that live, infectable SARS-CoV-2 viruses are spread in the most distant airborne particles, but it cannot be ruled out.
Social distancing rules on communicable diseases were traditionally set at between 1-2 metres based on a 1942 photographic study which found that most expelled droplets travelled within 1 metre. But the pictures only captured the largest droplets – we now know that the tiniest droplets may travel as far as 8 metres.
There are several clues pointing towards the idea that the SARS-CoV-2 virus is spread by these airborne particles. In one study SARS-CoV-2 virus was found deep in the airways of hospitalized patients, which suggests it had been inhaled. This makes it different to other infections of the upper respiratory tract, such as the common cold, which tend to reach the nose and eyes via fingers touching a surface contaminated with large droplets. Further, we know that coronavirus can be spread by people without symptoms, who are not coughing and sneezing. And one study reported finding particles of the virus in air-vents.
Evidence from community studies suggests that clusters of infection may result from groups of people spending a prolonged time together in an enclosed indoor space, such as an office, restaurant, hospital or elderly care setting, where poor ventilation allows levels of airborne virus to build up.
What people are doing there also matters – activities that involve heavy breathing produce more airborne particles. In one study, a two and a half hour choir rehearsal with one symptomatic person led to 32 confirmed and 20 probable COVID-19 cases among the 61 singers, despite limited physical contact. Other indoor case clusters have been recorded in churches and fitness gyms.
Most evidence specific to SARS-CoV-2 that has been collected so far was taken from widely different populations and settings, and is not of good enough quality to be able to draw hard lines on the relative risk of SARS-CoV-2 at different distances. But what is clear is that the risk of infection with SARS-CoV-2 grows when the physical distance between people is smaller. So, reducing distance restrictions from 2 to 1 metre could increase risk if other measures are not taken.
The size of a "safe" distance between people will be affected by many factors such as:
- how long you are exposed for
- how many infected people are in the vicinity, and the stage and severity of their disease
- whether you are indoors or outdoors
- the effectiveness of ventilation and surface cleaning measures if indoors
- whether face coverings and other personal protection is worn
- an individual’s susceptibility to infection
- what you are doing – activities that project airborne particles over greater distances, such as singing, coughing or heavy breathing, seem likely to increase risk
Social distancing alone is not a magic bullet, but is one risk-reducing factor to be used alongside good indoor ventilation, regular and effective hand washing, keeping surfaces clean, wearing face coverings where appropriate, and prompt isolation of infected individuals.
For those seeking advice on schools, we should add that the Oxford evidence review team surveyed all the available evidence on social distancing studies, but among these no studies were found looking at risk vs distance in school settings, or looking specifically at children or babies, so the conclusions here do not necessarily apply to younger age groups.
Mandy Payne, 23rd June 2020
This is a lay summary of a report prepared for the Oxford COVID-19 Evidence Service by the Oxford Centre for Evidence Based Medicine by Zeshan Qureshi, Nick Jones, Robert Temple, Jessica Larwood, Trisha Greenhalgh and Lydia Bourouiba, and posted on 22nd June 2020.
Disclaimer: the article has not been peer-reviewed; it should not replace individual clinical judgement and the sources cited in the original report should be checked. The views expressed in this commentary represent an interpretation by HealthWatch and do not necessarily represent those of Oxford CEBM, the NHS, the NIHR, or the Department of Health and Social Care. The views are not a substitute for professional medical advice.