New study estimates mask wearing could cut R number by 25%, not 0.25

“There’s a recent study from the University of Bristol that says wearing a face covering by large numbers of people can reduce the R rate by around 0.25.”

Andy Burnham, 13 July 2021.

Mayor of Greater Manchester Andy Burnham was asked on the Today programme about the government’s decision to no longer make it a legal requirement to wear a face covering in enclosed public spaces.

During the interview Mr Burnham quoted a study from the University of Bristol about the effectiveness of mask wearing against the spread of Covid-19, which he said shows large numbers of people wearing a face covering can reduce the R rate by 0.25. 

The reproduction (R) number, is the average number of people that a person infected with Covid-19 goes on to infect.  An R value of one, for example, means that on average every infected person will infect one other person.

Released at the end of June, the study by Universities of Bristol, Oxford and Copenhagen analysed the effect of mask-wearing on Covid-19 transmission of 20 million people across 92 regions around the world. It estimated that mask-wearing by an entire population led to an average (median) reduction in the R number of 25%, not a base reduction of 0.25 as Mr Burnham’s comments may have suggested. 

A reduction in the R number of 0.25 could produce a very different result from a 25% decrease. While a 25% decrease could amount to a reduction in the R number of 0.25, the two measurements are distinct. 

For example, if the R number was 2 then a reduction of 0.25 would result in an R number of 1.75, while a 25% reduction would lead to an R number of 1.5. Currently, the latest R range in England is between 1.2 and 1.5. A reduction of 0.25 in the R number would therefore mean a range between 0.95 and 1.25, while a 25% reduction would change it to between 0.9 and 1.125. 

The study, which is still a pre-print, meaning it hasn’t been peer-reviewed, also estimates masks would have to be worn by everyone within a population to achieve this reduction, not just by “large numbers of people” as Mr Burnham said.

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