November 17, 2021

expert reaction to systematic review and meta-analysis on public health measures to reduce incidence of COVID-19

A systematic review and meta-analysis published in The BMJ looks at the effectiveness of public health measures in reducing the incidence of COVID-19, SARS-CoV-2 transmission, and COVID-19 mortality.

Dr Baptiste Leurent, Assistant Professor in Medical Statistics, London School of Hygiene and Tropical Medicine (LSHTM), said:

“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. For example, in a “case-control” study, COVID-19 cases are asked how much they washed their hands, and this is compared to controls (COVID-19 negative). But any difference could be due to other factors, for example those who washed their hands more often could be those who are more careful and limited their social contacts. Only one study of mask wearing compared two otherwise identical groups (a randomised trial) and had found a risk reduction of around -18%. This is compatible with, but much smaller, than the 53% reduction reported in this review.

“In addition, all these studies are very different. For example, “mask wearing” or “hand washing”, or what it is compared to, will be very different from one study to another. In my opinion, it does not make much sense to combine all these results together to get an “overall effect”, and would be cautious about putting any single number to the effectiveness of these measures.

“However, all the studies looking at “hand washing” and “masks wearing” suggested a protective effect, and together give reasonable evidence that these measures are effective.

“The press release states: “Results from 8 of these 35 studies were analysed in detail, which indicated a statistically significant 53% reduction in the incidence of covid-19 with mask wearing and a 25% reduction with physical distancing. Statistically significant means that it is unlikely to have found this difference by chance alone (with a less than 5% chance, which is commonly accepted as “sufficient evidence”). But finding a “significant reduction” does not mean much if the quality of the study is limited, and the difference seen could still be explained by other factors.

“Overall, this review provides good evidence that handwashing and mask wearing are effective to reduce the spread of SARS-CoV-2. This is particularly relevant regarding mask wearing, for which many people in the UK may have received mixed messages. But caution is needed when trying to put a single number to their effectiveness.”

‘Effectiveness of public health measures in reducing the incidence of covid-19, SARS-CoV-2 transmission, and covid-19 mortality: systematic review and meta-analysis’ by Stella Talic et al. was published in The BMJ at 23:30 UK time on Wednesday 17 November.

All our previous output on this subject can be seen at this weblink:

www.sciencemediacentre.org/tag/covid-19

Declared interests

None received.