Masks work, our comprehensive review has found

When a Texan farm worker caught bird flu from cattle recently, social media was abuzz with rumours. While bird flu is not a human pandemic, scientists and policymakers the world over are keen to prepare as best they can for when such a pandemic emerges – a tricky task, given that science is messy, policy must be pragmatic and people’s values don’t always align.

It’s time for masks to enter the chat. At the beginning of a pandemic caused by a novel or newly mutated virus, there may be no vaccine, no firm knowledge about how bad things will get and no specific treatment. Slowing transmission until more is known will be critical.

Getting most people to wear a mask could nip the outbreak in the bud, preventing a pandemic or lessening its impact. Wearing a mask is inconvenient, but not as inconvenient as lockdowns.

But do masks work? A review of masks and respirators, that looked only at clinical trials, concluded that there was not enough evidence to assess whether mask wearing reduces the risk of spreading or contracting respiratory diseases. However, we disagree with that.

The review, by the not-for-profit Cochrane Collaboration, failed to influence recent guidance issued by the US Centers for Disease Control and Prevention (CDC) in response to the troubling news of bird flu transmission to humans. The CDC recommended well-fitting respirators – along with overalls and safety goggles – for anyone working with potentially infected cattle until the bird flu threat subsides.

Is this latest guidance based on sound evidence? According to our new review of the evidence, yes. Like the Cochrane team, we pooled data from randomised controlled trials (RCTs) and analysed the combined data – a so-called meta-analysis.

Unlike them, we also examined non-RCT evidence, including dozens of laboratory studies which showed that respiratory infections, including the common cold, COVID, flu, measles and TB, spread mainly through the air.

Laboratory evidence showed that different mask materials are better or worse at filtering tiny particles, and more or less breathable – especially when damp. This explains why a cloth or paper mask that’s become soggy from the moisture in exhaled air gets harder to breathe through and may be less protective.

Whereas medical masks are typically tied loosely around the face (hence air can bypass the filter), respirators fit snugly and if worn at work must be fit-tested to make sure that all air inhaled or exhaled passes through a high-grade filter.

All this non-RCT evidence is crucially important for the design of RCTs. Because respiratory viruses float in the air, to be optimally effective a mask must be made of high-filtration material and must be fit closely. It should not be removed while indoors or the person will immediately be exposed to infectious particles in the air.

It follows that we should not expect RCTs of badly designed masks, masks that don’t fit or masks that are worn only some of the time, to show an effect. Neither should we expect mere advice to wear masks to have any effect unless it is followed.

Finally, when comparing respirators with masks in places where there is a high risk of infection, such as a hospital, the respirator needs to be worn continuously until the person leaves the building, not just popped on occasionally when doing so-called “aerosol-generating procedures” – such as intubating a patient.

If we take these crucial details of RCT design into account, rather than just comparing any masks-on, masks-off experiments, we find that masks are effective, and respirators even more effective, in reducing the spread of respiratory disease. We can also explain why some previous reviews appeared to show that this wasn’t the case.

Most RCTs of mask-wearing by the public were actually trials of advice to wear masks. In both RCTs and observational studies (such as real-world experiments), there was a dose-response effect: the more people wore their masks, the more effective the masks turned out to be. And when there’s a looming pandemic, people do tend to wear their masks.

The bottom line

When we looked at RCTs, we found that masks do protect in the community, and N95 respirators (masks made using higher-grade filtration material and designed to fit closely around the face to protect against airborne contaminants) are superior to masks in healthcare workers, especially when respirators were worn continuously at work. Non-RCT evidence also shows that masks work and respirators work better.

Let’s hope we’re not heading for another pandemic. But as we contemplate that possibility, the bottom line from our recent review is masks work. Along with improving indoor air quality and avoiding crowded, underventilated places, they provide the best way to avoid catching a respiratory infection. And our findings support previous advice to not just wear any mask but wear the best mask available.

In relation to the research reported here, there was no direct funding. Trish Greenhalgh has received funding for related work from Wellcome Trust and National Institute for Health and Care Research. She is a member of Independent SAGE (unpaid role to engage directly with the public on COVID-19 related science).

C Raina MacIntyre receives funding from NHMRC (L3 Investigator grant and Centre for Research Excellence) and MRFF (Aerosol transmission of SARS-CoV-2 experimentally and in an intensive care setting) currently. She currently receives funding from Sanofi for research on influenza and pertussis. She is the director of EPIWATCH®️, which is a UNSW, Kirby Institute initiative.

David Fisman receives funding from the Canadian Institutes for Health Research. Over the past 5 years he has served on paid advisory boards for companies that manufacture vaccines against respiratory pathogens, including SARS-CoV-2, influenza and Streptococcus pneumoniae, including Sequirus, Pfizer, AstraZeneca, Merck and Sanofi-Pasteur. During the SARS-CoV-2 pandemic he served as a paid legal expert for the Ontario Nurses Association and the Elementary Teachers Federation of Ontario on issues related to workplace safety in the face of an airborne virus like SARS-CoV-2.