Trisha Greenhalgh and colleagues argue that it is time to apply the precautionary principle

No abstract

The question of whether and when to mandate the wearing of masks or face coverings by the lay public to prevent the spread of Covid-19 remains controversial. A vast research literature, across a range of academic disciplines, has accumulated in the past six months. We summarise that literature, which (whilst not universally accepted) points consistently to some important conclusions. First, there is growing evidence that the SARS-CoV-2 virus is airborne and hence may spread long distances and be inhaled. Infection control policy must therefore go beyond ‘contact and droplet’ measures such as hand-washing and cleaning surfaces. Second, masks and face coverings, if widely worn, appear to significantly reduce population transmission of the virus. Third, randomized controlled trials of the preventive impact of population masking in Covid-19 remain sparse and have yet to address the question of source control. Fourth, the harms of wearing masks appear to be relatively minor (though by no means trivial) and were over-estimated in the early months of the pandemic; harms are outweighed by benefits when COVID-19 is spreading in a population. Fifth, face shields, valved respirators and flimsy or ill-fitting face coverings are unsuitable for source control. Finally, mandated masking involves a trade-off with personal freedom so such policies should be pursued only if the threat is severe and the benefits cannot be achieved through less intrusive means.

No abstract