As the UK continues to ease out of lockdown, the government has been reminding us to maintain social distancing as we take advantage of new freedoms.

One measure that looks set to be in place for some time still is mandatory mask-wearing in many public places.

But the evidence to support mask-wearing policies in “community settings” – that means in everyday life outside hospitals and care homes – still isn’t clear-cut.

The UK government was reluctant to make masks compulsory at the beginning of the pandemic, saying the evidence wasn’t strong enough.

Masks were only mandated on public transport and in NHS settings in June, and in shops in July.

Ten months on, what have we learned about masks?

‘Limited and inconsistent’ evidence

There are many studies on the use of masks to control respiratory diseases.

But few of them look at SARS-CoV-2, the virus that causes Covid-19, and few are randomised control trials – often considered the “gold standard” of scientific evidence.

To do this kind of trial with masks you would have to divide participants at random into two groups – one with masks and one without – and record any differences in health outcomes.

Many scientists argue that it would be unethical to do this because it could risk the lives of those in the no-mask group.

So far, there have been only a tiny number of randomised control trials on mask-wearing for Covid-19.

This partly explains why scientists who have attempted to sift through all the available evidence on masks often conclude that it is low-quality and patchy.

Despite recommending their use in most indoor public places, the World Health Organization said in December: “At present there is only limited and inconsistent scientific evidence to support the effectiveness of masking of healthy people in the community to prevent infection with respiratory viruses, including SARS-CoV-2”.

The WHO highlights a “large randomized community-based trial” in Denmark with over 4,000 healthy participants that found “no difference in infection with SARS-CoV-2” between those who wore “medical/surgical” masks and those who did not.

The WHO notes a review of nine studies – two of healthcare workers, seven in the community – which “concluded that wearing a mask may make little or no difference to the prevention of influenza-like illness […] or laboratory confirmed illness”.

But it also points to conflicting data from a study in Beijing, and a number of other papers and meta-analyses – including some that make country-to-country comparisons – that seem to show masks are helpful at reducing the spread of disease.

‘Modest impact’

In November, researchers at Bond University in Australia reviewed the results of 67 papers that considered mask wearing.

Two of the review’s authors said in a summary of their findings: “We know masks are effective in laboratory studies, and we know they are effective as part of personal protective equipment for health care workers. But that effect appears diminished in community usage.”

“Based on the current evidence, we believe the community impact is modest and it may be better to focus on mask-wearing in high-risk situations”, they wrote.

Though it’s worth saying that none of the studies in their review looked at SARS-CoV-2. As with much of the data in this debate, the figures relate to other respiratory viruses.

What do other experts say?

Dr Julian Tang, honorary associate professor in the department of respiratory sciences at the University of Leicester, told FactCheck: “the WHO stance on this is very frustrating, and I feel, misleading and damaging to some extent”.

There is, he says, “definite physical evidence that masks both contain aerosols produced by an infected person, as well as blocking incoming aerosols when worn by a susceptible person”.

(Aerosols are particles or droplets that hang in the air. In the right conditions, they can transmit respiratory viruses between people.)

Dr Tang points us to several studies that demonstrate the use of masks against various viruses, and one conducted in the early stages of the Covid-19 pandemic that found even a single-layer cloth mask blocked 51 per cent of the particles from a lab-simulated cough.

Paul Hunter, professor in medicine at the University of East Anglia, told FactCheck: “I do think that the evidence does support use of face coverings. But the impact is probably not huge”.

He pointed us to a 2020 paper he co-authored, which concluded that “wearing face masks may reduce primary respiratory infection risk probably by 6–15%.” The article also notes that “COVID-19-specific studies are required.”

A pre-print (not yet peer-reviewed) study from UK researchers found that “a stricter mask-wearing policy”, by which they mean “mandatory in most or all shared/public spaces”, reduced the R number by 12 per cent.

While Professor Hunter praised the draft research, he noted that “any formal analysis would still classify this as weak evidence” in favour of mask-wearing policies. He told us face coverings are “not going to be the thing that makes the difference. Other social distance measures are probably still really important.”

Why does the WHO recommend masks if it says the evidence is weak?

Professor Ben Cowling, head of the epidemiology and biostatistics division at the University of Hong Kong School of Public Health, told FactCheck it might be informed by the “precautionary principle”.

That’s the idea that even if the evidence isn’t definitive, it’s still worth doing if there’s a chance it could save lives.

Professor Cowling also pointed to “mechanistic evidence” of the type he and his colleagues published last year in the journal Nature Medicine. Their study found surgical face masks “significantly reduced” the amount of coronavirus RNA detected in aerosols.

They didn’t look specifically at SARS-CoV-2, but Professor Cowling says the coronaviruses they did examine “should share a lot of similarities” with the one behind the current pandemic.

FactCheck verdict

There is plenty of data to support the idea that face masks block particles of respiratory viruses in the lab.

Where it gets complicated is the evidence for mask-wearing policies in the real world to prevent the spread of Covid-19.

As it stands, the data is conflicting and often shows only modest results. That’s partly because it can be difficult to measure the effect of mask-wearing independent of other factors. The World Health Organization says the evidence is “limited and inconsistent”, though recommends their use all the same.

Many experts say that even if it’s not the definitive factor in battling Covid-19, it’s still worth adopting masks.

They argue that there does not seem to be any major downside from making them mandatory in most indoor public settings, and that waiting until the gold standard of evidence has been reached could end up costing lives.