Press Release

Safe social distance may be less than previously thought

Published on 12 June 2020

The current debate over safe social distancing may be based on a misunderstanding of studies examining infection risk, according to a University of Dundee statistician.

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Dr Mike Lonergan

The current debate over safe social distancing may be based on a misunderstanding of studies examining infection risk, according to a University of Dundee statistician.

Dr Mike Lonergan, senior statistician and epidemiologist at the University’s School of Medicine, believes that a recent study that showed the benefits of 2-metre guidelines for social distancing is flawed.

The World Health Organization recently commissioned a meta-analysis of 25 papers from around the world examining the effects of separation distance on infection risk. Although the WHO had previously recommended that people stay at least 1 metre away from each other, the researchers found that extending this distance significantly reduced the chances of infection.

Dr Lonergan, who previously estimated that lockdown could save around one million lives in the UK alone, re-examined the studies that informed the WHO report and believes these datasets contradict the researchers’ conclusion.

Each study estimated risk relative to that incurred when touching infected individuals. Dr Lonergan says that a 78% reduction in risk of infection occurs at distances below 1m. The data also implied that any subsequent reduction in risk resulting from further distances is likely to be minimal, although Dr Lonergan warns that the limitations of the dataset limit the strength of these conclusions in turn.

A pre-print of his paper can be found on ResearchGate. He has chosen to make it available in this way before submitting it for formal peer review given the importance of safe social distancing to the debate around how to keep transmission rates low while reopening economies.

“The most important way to prevent the spread of this coronavirus is to avoid touching an infected person, but questions about the safety of various distances between individuals remain,” said Dr Lonergan.

“Intuitively, we believe that the further we stay away from others, the less risk of coming into contact with matter projected through their mouth. That is true, but the datasets meta-analysed for the WHO, and that I re-examined, suggest that the breath effect is only important at very short distances.

“As epidemiologists, we can only work with the data available. The WHO funded a group of 40 experts to carry out a systematic review and meta-analysis of the evidence for social distancing. They looked at the best collection of papers on the subject and concluded that 2m is significantly better than 1m, and that 1m is better than not touching.

“From what we can see, however, they have misinterpreted the datasets. Our conclusion is that avoiding contact is very important and that a 1m distance might be slightly better than just avoiding contact, but the difference is unlikely to be much. These data give no indication that 2m is better than 1m or just avoiding contact.”

The original studies examined for the WHO report compared risk inside and beyond threshold distances. The risk ratios are all relative to those of touching, and Dr Lonergan believes the researchers should not have calculated them in the way they have as many individuals in the closer-in groups had physical contact with an infected individual.

If simply not touching infected individuals is likely to reduce infection risk by more than half, and longer separation distances add little to this, he furthermore argues there may be little benefit from long-range social distancing.

“These results could be taken as suggesting that the bulk of infection takes place through the direct transfer of material, rather than by an aerial route,” Dr Lonergan continued. “If that is so, then, provided we refrain from coughing or spitting on each other, public health policy might be better concentrating more on limiting the touching of surfaces than on keeping people apart.

“The observed benefits of social distancing may have resulted more from reductions in contact with things others have recently touched than in our avoidance of their exhalations. Touching requires proximity and not touching infected individuals is likely to mean spending less time very close to them. 

“These data cannot determine how much of the benefit actually comes from the first decimetres of separation, rather than simply the lack of physical contact.”

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Grant Hill

Press Officer

+44 (0)1382 384768

G.Hill@dundee.ac.uk