As flu cases spike, is it time to start wearing masks again?

With flu season arriving early and NHS leaders encouraging people with symptoms to wear masks in public, a question arises: do masks actually work against the flu?

The short answer is that the evidence remains surprisingly weak. Studies conducted before the COVID pandemic generally found that masks made little to no difference in the spread of flu in everyday settings. There is little reason to think this has changed, although the COVID pandemic has taught us more about when masks can be helpful in reducing the spread of respiratory diseases.

This matters because flu cases began rising earlier than usual this year and are higher than we would normally expect at this point in the season.

While the number of people being admitted to hospital with a diagnosis of flu is still at moderate levels, the number of daily admissions is increasing. There are real concerns that we are heading towards an especially bad winter. This year, Australia suffered its worst flu season in at least 20 years.

The main flu strain currently circulating in the UK is a type of influenza A known as H3N2 – subclade K. This strain probably appeared first in the US, from where it has spread globally, extending the flu season in Australia and New Zealand and causing the early start of the flu season in Europe.

Crucially, this strain is quite different from the one used in this year’s vaccine. This means the vaccine may be less effective at preventing infection, although it should still help protect against severe illness.

Against this backdrop, Daniel Elkeles, the chief executive of NHS Providers, told Times Radio that if you’re coughing and sneezing “then you must wear a mask when you’re in public spaces, including on public transport, to stop the chances of you giving your virus to somebody else”.

The government guidance is less forthright, with a government spokesperson stating that people should consider wearing a mask in such circumstances, not that they must wear one.

Before the COVID pandemic, there had been several studies investigating the benefits of face coverings for respiratory viruses, including influenza. The most thorough of those reviews concluded that overall, masks made little or no difference to the spread of flu, either in the home or in public places.

They also concluded that N95 masks (tight-fitting, high-filtration masks) were no better than surgical masks in everyday settings. However, the authors were only able to identify a single low-quality study to support this finding.

In a review my colleagues and I conducted on the effectiveness of masks on the spread of respiratory infections prior to COVID, we found a similar poor effect overall. But individual studies in the review often gave very different results to each other.

Weaker studies were more likely to suggest masks work

Some studies suggested a strong protective effect, while others showed greater infection risk when people wore masks. Better-quality studies, such as randomised trials, generally found little or no benefit. In contrast, weaker study designs were more likely to suggest that masks worked.

The COVID pandemic added new evidence. The most robust recent review of masks and COVID in the community concluded that mask wearing was associated with a reduced risk of COVID transmission outside of healthcare settings. There was insufficient evidence to comment on the relative effectiveness of N95 respirator masks compared to standard surgical masks in public spaces, but in hospitals the balance of evidence was that there was little difference between the two types of mask.

These real-world studies contrast sharply with laboratory studies, which have generally found masks to be highly effective at reducing the amount of virus people release into the air and showing that properly fitted N95 masks are more effective than surgical masks for COVID and flu.

In the flu study, the researchers reported that a properly fitted N95 mask reduced the amount of flu virus released into the air by more than 94%. However, a poorly fitting N95 mask performed no better than an ordinary surgical mask – a crucial finding that suggests the gap between laboratory and real-world effectiveness may come down to how people actually wear masks.

The COVID lesson

Some of the most convincing evidence for the effectiveness of masks at preventing COVID was the UK’s Office for National Statistics (ONS) COVID infection survey. In this survey, up to 150,000 people were screened every two weeks for the virus whether or not they had symptoms. For part of the time, the survey also collected data on mask wearing.

My colleagues and I analysed data from the ONS survey and concluded that, in adults, always wearing a mask at work or in enclosed spaces – or both – was associated with about a 20% reduction in the risk of catching COVID during the time the delta variant was the dominant strain of the virus. But after the appearance of the omicron variant, there was little if any reduced risk in mask wearers.

In children, mask wearing was associated with less of a reduction in risk of testing positive for COVID, and during the omicron period there was even a small increased risk.

Evidence for the value of masks for flu remains less clear, suggesting little if any benefit. Nevertheless, I would still encourage people who are at risk of severe disease if they catch the flu to wear a mask in crowded indoor environments – especially if they have not yet received the vaccine.

If someone is ill with the flu, it is best that they should stay at home. If they have to go out into crowded indoor environments, then I would also encourage them to wear a mask. I would not encourage mask wearing in children, given the lack of clear benefit and potential for improper use.

For most people, the overall evidence does not support routine mask wearing. I would also not encourage the general public to wear N95 masks because these masks need to be properly fitted for them to be effective. Nevertheless, wearing a mask is a personal decision, and people should be free to decide on what makes them feel most comfortable.

Paul Hunter consults for the World Health Organization. He receives funding from National Institute for Health Research and has received funding from the World Health Organization and the European Regional Development Fund