The science behind social-distancing measures

THE WORLD HEALTH ORGANISATION (WHO) recommends that people stay at least one metre (about three feet) away from each other to reduce the chance of breathing in the exhalations of someone infected with the coronavirus. Several countries, however, advise larger distances. Germany suggests staying 1.5 metres apart. In America, which abjures the metric system, the rule is six feet, or 1.8 metres. Britain’s government, which was slow to enact other measures, is more risk-averse, recommending a gap of two metres between breathers, although it is considering reducing this, under pressure from business and some Conservative MPs. Denmark advised two metres before halving the distance in May. (A popular joke in Scandinavia celebrates the end of the two-metre rule: “Now we can go back to staying five metres away from each other.”) Why do countries use different measures? Is it safe to go from two metres to one?

Like a lot of health advice, social-distancing recommendations are better thought of as guidelines than as hard-and-fast rules. For example, many European countries recommend eating five portions of fruit and vegetables a day. That is based on the WHO’s daily guideline of 400 grams, divided into five manageable chunks. Some countries suggest even bigger portions. It is self-evident that eating more fruit and veg is better than eating lots of red meat or cheese. Similarly, it does not take a medical degree to understand that the farther you are from an infected person, the lower the chance of your catching the bug. The WHO’s one-metre rule originates in a study from the 1930s. When someone infected with covid-19 speaks (or, worse, coughs) they exhale tiny virus-containing droplets, which can be inhaled by interlocutors or others nearby. More research has been carried out over the years. Mostly, scientists have found what any layperson might assume: longer distances equal lower risk, though with diminishing returns.

Distance is not the only factor that determines the chances of someone getting covid-19. The most obvious element is whether people nearby have the disease. If they don’t, then clearly any distance is safe. However, without widespread testing it is impossible to know, which is why governments around the world have put in place risk-mitigating measures. Physical barriers such as masks or plexiglass screens have been shown to reduce the risk of transmission. So does the setting: it is safer to be outdoors than indoors, and it is better to be in a well-ventilated room than a closed-off one. People’s positioning makes a difference as well. Those standing back-to-back or side-by-side are less likely to infect each other than those directly facing each other. And the shorter the time two people are in contact, the better. So to stand the best chance of contracting the virus, a person would need to be speaking face-to-face to an infected person, for a long time, without any barriers and in a poorly ventilated room.

The debate in Britain over shortening the recommended distance has sparked a fierce argument about whether the government is taking unnecessary risks in order to open up the economy. But the debate ignores other elements of health policy, such as making mask-wearing compulsory. Moreover, as the number of people with covid-19 goes down, the risk of contracting it from a random member of the public does too. And as the pandemic drags on, there is some evidence that people are tiring of the rules. Worried that people are already breaking the rules, governments are considering relaxing them. If they do, they ought to double-down on other measures to reduce the virus's spread, such as mandating the use of masks on public transport and indoor public areas, and identifying those who may be infected and quarantining them.