Passing through the new Beijing Daxing airport on my way to an international scientific meeting on 25 January, two days after Wuhan had been put under lockdown, one sight in particular was unsettling. Close to 100% of people were wearing masks (myself included). Even in pollution-stricken China, this was highly unusual. Arriving in Europe, masks were nowhere to be seen.

Three months on, even as more than 30 countries, including Germany, have mandated face coverings outdoors for the public, the UK government has been adamant that there’s not enough evidence to support such a move here. The government, advised by the scientific advisory group on emergencies (Sage), is expected to make a decision shortly on whether face masks will be mandatory. Reports suggest that the advice will be a compromise, with the public allowed to choose to wear scarves or face-coverings rather than medical-grade masks. So why has there been such a divergence on this issue?

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Most of us would probably ask how well a mask, especially a one made from cloth, may protect us from Covid-19. But the notion of “personal protection” misses the major reason why calls for increased use of masks and face-coverings have gained momentum around the world. From February, data was emerging that, unlike Sars – where patients were most contagious several days after showing severe symptoms – people infected by Sars-Cov-2, the cause of Covid-19, were “shedding” most viruses (that could infect someone else) just before or at the time symptoms first appeared, often in very mild form. This had two implications: first, efforts to “contain” coronavirus would be orders of magnitude more difficult than for Sars. Second, and as it has turned out more contentiously, the official World Health Organization and UK government advice that masks should only be worn by members of the public showing symptoms, and members of their immediate households, is probably wrong.

In the field of infectious diseases, stopping a contagious patient infecting others is known as source control. For respiratory infections, source control would include wearing a mask when not under isolation. By advising symptomatic individuals to wear a mask, the government was advocating source control. But what if symptomatic individuals were only half the story? When people have been systematically tested for Covid-19, be it passengers on the Diamond Princess cruise ship or women about to give birth in New York, the number of individuals who test positive but display no symptoms is about half the total number of those who test positive (the range is 20-90%, but most agree it’s 40-60%).

So can completely “well” people transmit Covid-19? We know that even normal speech can spread droplets, and this is dramatically reduced by the use of cloth masks. And for symptomatic people infected with common-cold coronaviruses and influenza, normal breathing also results in shedding of virus, which again, is blocked by wearing a mask. While we don’t know for sure how contagious a truly asymptomatic Covid-19 patient might be, evidence suggests that around 40% of cases of transmission arise from people without symptoms.

So what are the objections that have been raised against masks in public as policy? There have been repeated statements that the evidence in favour of masks is flimsy and ambiguous. For medics, the “gold standard” of evidence is the randomised controlled trial (some people take the medicine or undergo the intervention, and some don’t, and we look to see if there were benefits or risks). There have been a few trials looking at masks for preventing influenza, and results were disappointing – although crucially, most of the group meant to wear masks actually didn’t: it turns out we’re just not that scared of catching the flu.

But if one looks at the same level of evidence for the other measures strongly advocated by the government, for example hand-washing, studies have been similarly disappointing. And for other advice, such as the two-metre rule, or lockdowns, there have been no trials at all. So masks seem subject to a different standard of evidence than other measures. And while controlled trial data is not available, “natural experiments” do suggest at least a correlation between mask-wearing and reduced Covid-19 transmission. The city of Jena in Germany introduced mandatory mask-wearing on 31 March and recorded no new infections for eight days, while surrounding cities continued to see a rise.

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Other objections include claims that wearing a mask increases other risky behaviours, such as a relaxed attitude to distancing (comparable to the idea that dangerous driving would increase when seatbelts became law), or that contaminated masks will increase rather than decrease infections. In Hong Kong, where mask uptake is an astonishing 98%, there have been no reports of harms associated with the practice, indeed, only four people have died from Covid-19 since the beginning of the outbreak.

That perhaps leaves the final objection, which is that the public wearing masks will divert this scarce resource away from hospitals and care homes. In this sense, the government may simply be dragging its feet because of scarcity – which is a political and economic issue, not a scientific one. It might also explain the recent focus on cloth masks or face-coverings over masks: by advocating these, there is no competition with the NHS. And although less PR-worthy than collaborations with Rolls-Royce to produce ventilators, other partnerships between government and industry could ensure high-quality cloth masks for everyone in the country.

As the government equivocates, unlike our European neighbours, the British public may end up being the “control group” for the face mask experiment the government has been demanding all along. But do we want to be?