In recent weeks, face masks have become an unremarkable sight in streets and supermarkets.

Many people are choosing to cover their mouth and nose with online-bought or homemade masks or scarves, in spite of the advice of the World Health Organization and, in the UK, from Public Health England, that they are no protection against coronavirus.

Yet one country after another has decided to depart from the WHO guidance and rule that masks should be worn – most notably the US.

To add to the confusion, on Monday, Sir Patrick Vallance, the UK government’s chief scientific adviser, said the UK position on masks was under review and would change if the scientific evidence warranted it.

This followed remarks by David Nabarro, a UN special envoy on Covid-19, who appeared to depart from the WHO’s line by suggesting the UK would have to get used to wearing masks.

“The virus isn’t going to go away and we don’t know if people who have had the virus stay immune afterwards,” he told the BBC, adding: “Yes, we will have to wear masks.”

Despite the mixed messages, the WHO guidance updated a week ago, has remained consistent.

It has stuck to the line that masks are for healthcare workers – not the public.

“Wearing a medical mask is one of the prevention measures that can limit the spread of certain respiratory viral diseases, including Covid-19. However, the use of a mask alone is insufficient to provide an adequate level of protection, and other measures should also be adopted,” it stated.

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Underlying WHO’s concerns are the shortage of high-quality protective masks for frontline healthcare workers.

There is also concern the public will not understand how to use a mask and may get infected if they come into contact with the virus when they take it off and then touch their faces.

Most of all, there is no robust scientific evidence – in the form or trials – that ordinary masks block the virus from infecting people who wear them.

N95 masks, worn by doctors and nurses who are treating people with Covid-19, certainly do. But the virus particles are thought to pass through other types of mask.

Nevertheless, masks do have a role when used by people who are already infected by the virus.

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It is accepted that they can block the transmission to other people. Given that many people with Covid-19 do not show any symptoms for the first days after they are infected, masks clearly have a potential role to play if everyone wears them.

In the US, Donald Trump has advised people to wear masks whenever they go out, while stressing it is not compulsory, citing advice from the Centers for Disease Control. In France, the Academy of Medicine has recommended the use of a “general public” mask – to differentiate it from the mask used by healthcare workers.

There is now increasing pressure for the UK stance on masks to change. Even marginal gains could make a difference is at the heart of the argument.

Prof Trisha Greenhalgh from the Nuffield department of primary care health sciences at the University of Oxford, and colleagues wrote a paper published by the British Medical Journal last week that argued in favour of “the precautionary principle”.

The standard level of scientific evidence is no good in this issue, she told the Guardian. Randomised controlled trials are the gold standard in drug development, but not appropriate to face masks in a pandemic, she said.

“The point is we have now got a hugely complex issue going on. The last thing we need is a controlled experiment. We need to follow the logic of complex systems,.”

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Greenhalgh thinks the whole population of the UK should wear masks – just as they do in South Korea, where the epidemic curve is far lower than ours.

We could make that acceptable, she said, by making it fun – encouraging people to make and personalise their own.

Even if masks do not become the norm, it is quite possible they will anyway become part of the strategy for the exit from lockdown, at which point not only will every possible way of preventing new infections be considered, but the public will be so keen to resume “normal” life that it is hard to imagine anyone refusing to comply.