Study doesn’t show masks cause 53% drop in Covid risk

“Mask-wearing cuts Covid incidence by 53%, says global study”.

A number of media outlets have have reported that a new study found wearing a mask cuts the risk of Covid-19 by 53%.

By our count, the claim was made by The Guardian, New Scientist, Forbes, The Times and Metro.

This isn’t what the study says. The study says that mask-wearing was “associated with” a reduction in risk but, due to the limitations of the research, “it was difficult for our review to draw a […] causal conclusion.” However, this was not mentioned in any of these articles. 

The Guardian, The Times and Metro have all now corrected their articles.

The study is a meta-analysis, which is where many studies on the same topic are pooled together and analysed collectively.

The researchers identified eight studies that they deemed appropriate to analyse the relationship between handwashing, mask-wearing and social distancing and the risk of catching Covid-19. 

All these interventions were associated with a lower risk, but in the case of handwashing, the result was not statistically significant.

The analysis of papers which looked at mask-wearing found it was associated with a 53% reduced risk of Covid-19 infection. 

Dr Baptiste Leurent, Assistant Professor in Medical Statistics, London School of Hygiene and Tropical Medicine told the Science Media Centre: “This was a well conducted review, and probably the best evidence on the effectiveness of public health measures to reduce SARS-CoV-2 transmissions so far.

“The main limitation is that nearly all the evidence comes from observational studies, where it is difficult to be certain that a difference seen is due to the intervention.”

He also warned that “caution is needed” when trying to put a single number on the effectiveness of things like mask-wearing.

An editorial in the British Medical Journal, which published the research, says: “The effects seen in observational studies of mask wearing will be confounded by other protective behaviours not accounted for during adjustment,” which the researchers acknowledge.

Confounding is where a third factor may be influencing, in this case, both the likelihood of mask-wearing and the likelihood of being infected with Covid-19. 

For example, if people who wear masks just tend to be more risk-averse, that risk-averseness might be what’s driving some of the reduction in Covid-19 infections, as opposed to mask wearing specifically.

That said, wider evidence does support the claim that masks reduce the risk of contracting Covid-19, to some extent. 

Nature reports that studies show masks can reduce the viral dose someone might receive, and can help protect others by reducing the amount of virus exhaled if the wearer has an infection.