During the COVID-19 pandemic, masks were weaponized for partisan purposes. “The politicization of mask use,” says William Hanage, infectious disease epidemiologist at Harvard University, “makes as much sense as politicizing gravity.” Masks are simply a tool—a protective barrier—that can help to reduce the spread of respiratory infections, just as condoms are a barrier that can reduce the spread of sexually transmitted infections. And as we head into winter, with rising rates of multiple respiratory viruses, including flu, RSV, and new coronavirus variants, masks could help all Americans to avoid getting sick.
Mask use is on the decline in the United States. Recent public polling shows that nearly two thirds of Americans never or rarely wear a mask outside their homes, a sharp rise from just a quarter during the height of the Omicron wave in January 2022. There are many reasons for the decline in masking. These include pandemic fatigue, a justified perception that the worst of the COVID-19 pandemic is behind us (there has been a sustained decline in daily COVID-19 deaths), widespread COVID-19 vaccination (80% of Americans have now had at least one vaccine dose), reduced federal and state efforts to provide free high quality masks to the public, and the removal of mask mandates.
Despite these trends, it is important for the public to know that community masking can help prevent the spread of a range of respiratory infections. The Centers for Disease Control notes that flu hospitalization rates are higher than usual for the time of year, an additional impetus to promote mask use. A useful analogy is to think of masks like umbrellas, says Simon Nicholas Williams, a Lecturer in Psychology at the University of Swansea in Wales. When it’s raining or the forecast is for rain, we take an umbrella out with us. “But just as there’s no need to carry an umbrella with us when it’s sunny,” he says, “we needn’t be expected to wear masks all the time.”
Respiratory disease transmission can be divided broadly into spread by droplets, which are larger than 5 microns and fall quickly to the ground, and aerosols, which are smaller than 5 microns and can float in the air for hours as well as be inhaled. The science of such transmission suggests that all viruses and bacteria that travel by these routes should similarly be stopped by mitigation measures that broadly target these routes—including high quality masks such as N95 or KN95s that block both droplets and aerosols.
The data on the efficacy of masks to prevent respiratory infections dates back to well before COVID-19. During the SARS outbreak in 2003, studies showed that the use of face masks probably reduced transmission. A study on SARS transmission from five Hong Kong hospitals, for example, in which staff were exposed to infected patients found that staff who reported mask use were less likely to be infected. Even during SARS, though, there were signs that upgrading masks could be important. An outbreak from a Hong Kong hospital in which unidentified cases transmitted to healthcare workers who were only wearing surgical masks flagged the relevance of high quality masks such as N95s.
Data on tuberculosis transmission in healthcare settings is also informative. Irregular use of N95 respirators among Brazilian healthcare workers was associated with a higher risk of latent tuberculosis (whereby someone is harboring the bacteria, but it is in a dormant state with a risk of becoming active later on). Furthermore, other airborne pathogens, such as a bacterium called Coxiella burnetti, which passes from animals to humans and causes the illness Q fever, have caused outbreaks that were stopped directly with the use of N95s. During the COVID-19 pandemic, use of N95 masks had the added benefit of reducing transmission of other airborne pathogens, including tuberculosis.
The type of mask that is sufficient depends on the pathogen in question and whether it transmits more effectively by the airborne route, or by larger respiratory droplets. For instance, studies looking into RSV found that while detectable in aerosols, this route was unlikely to be efficient, meaning N95-caliber masks aren’t required. However, given we are now faced with a concurrent increase in multiple respiratory viruses at once—including flu, RSV, and COVID-19—masks that block both routes are preferable.
Currently, the CDC does not list masks under their guidance to Americans for how to prevent flu. And data has been mixed in the past with regard to what type of mask would truly be needed. A randomized trial examining whether use of N95 respirators versus medical masks in preventing influenza among healthcare workers found no significant difference. However, the study was primarily conducted in outpatient clinic settings, which are notably different than within a hospital, or within a crowded public setting for longer periods of time. In another randomized trial, use of N95 masks was associated with significantly lower rates of respiratory illness, and lower rates of bacterial colonization of the respiratory tract. Furthermore, COVID-19 mitigation efforts of the past two seasons very likely contributed to exceptionally low flu transmission, as noted by the CDC. Given that flu is less transmissible than COVID-19, the level of community masking needed to blunt transmission would likely be lower and more easily achievable. With limited yet mixed data, we would be wise to heed caution and err on the side of masking in high risk, indoor crowded public settings where these diseases spread most easily.
What the COVID-19 pandemic has made clear is that we need more robust data to understand many aspects of public masking, including how effective masks will be in preventing other respiratory viruses. While the efficacy of masking differs by what type of mask and material is used, as well as mask fit, it is also affected by when and where masks are used, and how consistently. In healthcare settings, we as physicians consistently use N95 respirators in rooms of patients with airborne diseases because we know they reduce inhalation of infectious aerosols, which reduces the chance that we get sick. This basic principle holds true whether in a hospital room, or in a crowded bus or grocery store.
Getting vaccinated against flu and COVID-19 is the most important step you can take to prevent severe illness from these conditions. Unfortunately, there is no licensed RSV vaccine, though there is a candidate vaccine for infants that is showing promising results. Even with the vaccines that we do have, while we will continue to be affected by respiratory viruses every year, the COVID-19 pandemic should remind us that we have a means to reduce spread through using high quality masks. Masks will likely help reduce the spread of multiple viruses and some bacteria—and while more data must be generated to understand how best to improve our community level efforts, we have enough already to tell us to mask up this season.