What is the science behind the UK’s coronavirus distancing rules?

Since the coronavirus outbreak reached the UK, health officials have recommended people stay at least 2 metres apart to reduce their risk of picking up the infection or spreading it to others.

But in recent weeks, as the impact of the lockdown on the economy has become clearer, the government has come under increasing pressure to relax the physical distancing rules so pubs, cafes and restaurants and other businesses can reopen fully.

Here’s what the science says about physical distancing.

Why does distance matter?

Coronavirus spreads when virus particles get into the body through the eyes, nose or mouth. One route is via contaminated surfaces, such as door handles that are touched by people with virus on their hands. But the more common route of infection appears to be tiny virus-bearing droplets that are released into the air by coughs and sneezes, and to a lesser extent when people talk or shout. These droplets fall to the ground with the heaviest falling nearest to the infected person and the lightest falling further away. The aim of physical distancing is to be far enough away from an infected person to avoid the airborne droplets.

What distance is recommended?

Health officials in the UK recommend people keep at least 2 metres from each other. At that distance, they believe the risk of infection during face-to-face conversation reduces to an acceptable level. The same distance applies in Canada and Spain. But other countries are less cautious. In the US people are advised to keep at least 6ft, or 1.8 metres, apart. Other countries, such as Germany, Italy, Greece, Australia and the Netherlands have opted for 1.5 metres. South Korea advises 1.4 metres, and in France, Denmark, China, Hong Kong and Singapore it is 1 metre.

Where do the numbers come from? 

In the 1930s, a Harvard sanitary engineer called William Wells proposed that some infectious diseases were spread by airborne droplets. His experiments showed that most droplets sprayed from the mouth fell to the ground within 3ft, and so beyond this distance infection was unlikely. The result shaped scientific opinion on what is a safe distance and underlies the World Health Organization’s 1-metre recommendation.

Will there be a second wave of coronavirus?

Epidemics of infectious diseases behave in different ways but the 1918 influenza pandemic that killed more than 50 million people is regarded as a key example of a pandemic that occurred in multiple waves, with the latter more severe than the first. It has been replicated – albeit more mildly – in subsequent flu pandemics.

How and why multiple-wave outbreaks occur, and how subsequent waves of infection can be prevented, has become a staple of epidemiological modelling studies and pandemic preparation, which have looked at everything from social behaviour and health policy to vaccination and the buildup of community immunity, also known as herd immunity.

This is being watched very carefully. Without a vaccine, and with no widespread immunity to the new disease, one alarm is being sounded by the experience of Singapore, which has seen a sudden resurgence in infections despite being lauded for its early handling of the outbreak.

Although Singapore instituted a strong contact tracing system for its general population, the disease re-emerged in cramped dormitory accommodation used by thousands of foreign workers with inadequate hygiene facilities and shared canteens.

Singapore’s experience, although very specific, has demonstrated the ability of the disease to come back strongly in places where people are in close proximity and its ability to exploit any weakness in public health regimes set up to counter it.

Conventional wisdom among scientists suggests second waves of resistant infections occur after the capacity for treatment and isolation becomes exhausted. In this case the concern is that the social and political consensus supporting lockdowns is being overtaken by public frustration and the urgent need to reopen economies.

The threat declines when susceptibility of the population to the disease falls below a certain threshold or when widespread vaccination becomes available.

In general terms the ratio of susceptible and immune individuals in a population at the end of one wave determines the potential magnitude of a subsequent wave. The worry right now is that with a vaccine still months away, and the real rate of infection only being guessed at, populations worldwide remain highly vulnerable to both resurgence and subsequent waves.

Peter Beaumont

But while 1 metre reduces the risk substantially, another coronavirus outbreak – Sars in 2003 – showed it may not be sufficient. Tests of passengers on one flight found that 90% who caught Sars from an infected passenger had been sitting more than three feet away from them. A distance of 6ft or 2 metres became the more cautious recommendation. The latest paper on physical distancing from the government’s scientific advisory group on emergencies recommends keeping the 2-metre rule but states it “should be seen as a ballpark guide to distancing rather than an absolute value.”

Is distance everything?

No. The way a person is exposed to the virus and how long for are important too. Because coughs are more forceful than conversations, they propel droplets further. According to UK science advisers, exposure to one cough at 2 metres has a similar risk to talking with someone for a minute at 1 metre and for half an hour at 2 metres. Likewise, a six-second chat at 1 metre is comparable to a one minute conversation at 2 metres. Explaining the importance of the 2-metre guidance, the government’s chief scientific adviser, Sir Patrick Vallance told the Commons health committee in May that the risk at 1 metre was 10 to 30 times higher than the risk at 2 metres. 

Is it safe to reduce the distance?

A recent Lancet study commissioned by the World Health Organization found that the chance of transmitting or becoming infected by the virus, with no other protection in place such as a face mask, fell dramatically with distance. At 1 metre, the risk was 12.8% but this dropped to 2.6% at 1.5 metres and to about 1.3% at 2 metres.

The decision on whether to reduce the distance must weigh up the direct public health risks from the virus and the harm caused by businesses folding or being unable to operate with 2-metre distancing enforced. “I think it would be reasonable to reduce it to 1.5 metres as long as strict guidelines are in place for what else needs to be done,” said Linda Bauld, a professor of public Health at the University of Edinburgh. 

What other measures do we need?

Beyond handwashing and distancing where possible, wearing face masks and ensuring venues are well ventilated will all help. Bauld says there is a case for bar and restaurant staff wearing masks to prevent them unwittingly spreading infections. And when people are in proximity for some time, such as in the queue for the toilets, the more cautious 2-metre distancing rule is preferable to standing closer. Another important move is to improve air circulation by opening doors and windows, which will be easier in the UK over the summer than later in the year. 



Is it safer being outdoors?

Yes. In an enclosed space such as a living room or bedroom there are no appreciable air currents to help disperse virus-containing droplets or extremely tiny aerosols. The latter tend to be smaller than five thousandths of a millimetre wide and can remain suspended in the air for some time. How important these are for spreading infection is unclear, however.

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