Two metres is a little better than one

“Cutting back to 1m ‘doubles infection risk’”

The Guardian (print edition), 2 June 2020

“One metre is enough!”

The Mail (print edition), 2 June 2020  

Yesterday the Guardian’s and the Mail’s front pages reported a new piece of research recently published in the Lancet medical journal.

“Reducing physical distancing advice from 2 metres to 1 metre could double the risk of coronavirus infection, according to the most comprehensive study to date,” said the Guardian.

“Keeping one metre apart can slash the risk of catching coronavirus by 80 per cent, according to a major study funded by the World Health Organization (WHO),” said the Mail.

Even though they seem to contradict each other, these are both broadly correct reports of the research.

What did the research say?

The research is a “meta-analysis” published in the Lancet on 1 June. This means the scientists pooled together many other pieces of research that had already been published, and used them to draw their conclusions.

In this case, that meant gathering evidence about how the chance of catching SARS, MERS or Covid-19 was related to someone’s physical distance from an infected person, both in healthcare settings and elsewhere. The researchers gathered what evidence they could find about masks and eye coverings as well.

From what they found, they concluded with "moderate certainty" that keeping over a metre away from people with probable Covid, SARS or MERS cuts your risk of infection from about 13% (without physical distance measures) to about 3%. This is the roughly 80% reduction in risk that the Mail mentioned.

The researchers also found that “for every 1 m further away in distancing, the relative effect might increase 2·02 times”. It is not clear from the Lancet paper exactly which relative effect is being referred to here. However, Professor Kevin McConway has told us that it would be consistent with figures elsewhere in the research paper to say that a change from keeping two metres apart (which is currently advised in the UK) to a distance of only one metre might therefore double the risk. This is what the Guardian said.

Wearing masks or eye protection might also substantially lower a person’s risk of catching SARS, MERS or Covid-19, according to the research, but the certainty on this point was “low”.

It is important to note that most of the research used in this study does not relate to Covid-19, but to SARS and MERS, diseases caused by other coronaviruses, which may spread differently. The study authors themselves say that “robust randomised trials are needed to better inform the evidence for these interventions, but this systematic appraisal of currently best available evidence might inform interim guidance.”

How far is far enough?

Deciding on the ideal social distance during this pandemic is a matter of weighing priorities against each other, so different people will reach different answers.

The researchers themselves concluded that “From a policy and public health perspective, current policies of at least 1m physical distancing seem to be strongly associated with a large protective effect, and distances of 2m could be more effective”.

The Mail isn’t wrong to say that one metre is enough—it’s an opinion. Likewise, in the opinion of the Guardian, it may be more important to warn about the risks of shortening the distance to one metre, instead of two.

If you want the risk of catching Covid-19 to be as low as possible, then this research suggests that people should stay as far apart as possible. However, you would get less extra protection for each extra metre that you added.

Since the Lancet research was published, both Professor McConway and Professor David Spiegelhalter have claimed that part of its statistical work was not carried out correctly. If so, the research would not in fact allow us to compare the risk of infection at a minimum of 1m with the risk at 2m.