Do face coverings reduce the spread of Covid-19? Surgical-grade face coverings and respirators offer high levels of protection against coronavirus infection for the wearer. However, these robust masks have to be combined with goggles or a visor, and they are costly, in limited supply, and uncomfortable to wear for long periods. The evidence around homemade masks or face coverings, which the UK public will now be required to wear on public transport, is less clearcut. Studies in laboratories have shown that they can reduce the distance and number of droplets spread by coughs, but the impact of masks in “field conditions” has been harder to assess. The ambiguity around the science probably explains the different approaches different countries have taken – and the evolving rules around wearing face masks. Was advice to wear face masks previously considered in the UK? Yes. As early as 3 February one of the key bodies advising the government on its approach to tackling the pandemic, the New and Emerging Respiratory Virus Threats Advisory Group (Nervtag) concluded there was some evidence to support the wearing of masks by those with symptoms to prevent transmission. The group recommended to the Department of Health and Social Care that symptomatic people should wear fluid-resistant masks “if tolerated”. This never became a recommendation to the general public. The same group said there was limited evidence of the benefits of the general public wearing face masks as a preventative measure. Separately, in April, behavioural scientists sitting on the government’s Scientific Advisory Group for Emergencies (Sage) considered the pros and cons of recommending face masks. The group highlighted the risk that wearing masks could make people feel invincible and therefore be less likely to adhere to other rules around socialising and staying at home. At the time, some also had concerns that recommending the wider use of face masks could exacerbate the acute shortages of PPE among healthcare workers that the UK was grappling with earlier in the pandemic. Has the science on face masks changed? Previously, the government has defended its decision not to recommend the use of face masks – while other countries have urged people to adopt masks – saying: “For the majority of people in more general settings, there is little evidence of widespread benefit of using face masks to prevent transmission.” In Thursday’s announcement, Grant Shapps, the transport secretary, said: “The evidence suggests that face coverings offer some, albeit limited, protection against the virus.” So has the science changed? Although there have been concerted efforts to answer the question of whether face masks make a difference, there has not been a dramatic shift in the science: there is some evidence of a benefit, but it remains fairly weak. This week, the Lancet published the most comprehensive paper to date on the subject, a systematic review of 172 studies looking at the role of physical distancing measures, face masks, and eye protection to prevent transmission. It found weak support for the use of face coverings, with the risk of infection or transmission when wearing a mask being estimated at 3% versus 17% when not wearing a mask. Evidence in the study was looking mainly at mask use within households, rather than on public transport, and the authors note that the certainty of the evidence is low and other measures, such as physical distancing, are by far the most effective at reducing the spread of coronavirus. The science is still uncertain, but as the most robust infection control measures are lifted, the more marginal gains that may be achieved through the use of face coverings may have simply become more relevant.