What does the actual scientific literature say about wearing a face mask to prevent the spread of COVID-19? It's less clear than a non-scientist like me might prefer. Indeed, a lot of the discussion seems to be happening in real time in working papers that have not yet been peer-reviewed or published in a journal. Ultimately, the case for wearing face-masks lacks a clear-cut scientific base--but it may still be a good idea. Let's stroll through some of the studies.
Several recent reviews of the existing literature on studies of face masks that use random controlled trial methods do not find a reason to wear a mask. For example, Julii Brainard, Natalia Jones, Iain Lake, Lee Hooper, and Paul R Hunter have published "Facemasks and similar barriers to prevent respiratory illness such as COVID-19: A rapid systematic review" (posted April 6, 2020). It's at the medRxiv (pronounced "med-archive"), a "preprint" system for sharing early drafts run by Cold Spring Harbor Laboratory, Yale University, and BMJ, where it says at the website: "Preprints are preliminary reports of work that have not been certified by peer review. They should not be relied on to guide clinical practice or health-related behavior and should not be reported in news media as established information." The Brainerd et al. paper does a systematic analysis of 31 studies across different masks, different settings, and with different methods. They write:
Based on the RCTs [randomized control trials]we would conclude that wearing facemasks can be very slightly protective against primary infection from casual community contact, and modestly protective against household infections when both infected and uninfected members wear facemasks. However, the RCTs often suffered from poor compliance and controls using facemasks. Across observational studies the evidence in favour of wearing facemasks was stronger. We expect RCTs to under-estimate the protective effect and observational studies to exaggerate it. The evidence is not sufficiently strong to support widespread use of facemasks as a protective measure against COVID19. However, there is enough evidence to support the use of facemasks for short periods of time by particularly vulnerable individuals when in transient higher risk situations. Further high quality trials are needed to assess when wearing a facemask in the community is most likely to be protective.
Another group of authors has their own literature review up on medRxiv, with similar findings. T. Jefferson et al. have written, "Physical interventions to interrupt or reduce the spread of respiratory viruses. Part 1 - Face masks, eye protection and person distancing: systematic review and meta-analysis" (April 7, 2020). They look back at evidence from a previous coronavirus outbreak--the SARS episode in 2003, and then also at 15 randomized control trial studies since then. They write:
Most included trials had poor design, reporting and sparse events. There was insufficient evidence to provide a recommendation on the use of facial barriers without other measures. We found insufficient evidence for a difference between surgical masks and N95 respirators and limited evidence to support effectiveness of quarantine ... Despite the lack of evidence, we would still recommend using facial barriers in the setting of epidemic and pandemic viral respiratory infections, but there does not appear to be a difference between surgical and full respirator wear.On the pro-mask side, a widely mentioned study is from Jeremy Howard et al. "Face Masks Against COVID-19: An Evidence Review" (most recent version July 12, 2020). This one is at preprint.org, a server run by the Multidisciplinary Digital Publishing Institute, and it again is not a peer-reviewed paper. These authors "synthesize the relevant literature," which is a way of saying that they are writing a persuasive essay, not summing up the results of earlier studies.
They point out: "A primary route of transmission of SARS-CoV-2 is likely via small droplets that are ejected when speaking, coughing or sneezing." They focus on masks not as a method of protecting the wearer, but as a method of protecting others. They write: "Although no randomized controlled trials (RCT) on the use of masks as source control for SARS-CoV-2 has been published, a number of studies have attempted to indirectly estimate the efficacy of masks." They point to one study with 10 people, another study with 4 people, a study of health care workers in Chinese hospitals, a case study of someone who flew on a plane from China to Toronto, and other pieces of evidence. Some studies looked at combinations of measures like masks, hand-washing, disinfecting and social distancing in earlier outbreaks of flu, and found that the combination helped to reduce the spread of disease. Finally, they argue that the costs of wearing masks are low, so the "precautionary principle" suggests that it is worth doing even given the imperfect evidence. They conclude: "When used in conjunction with widespread testing, contact tracing, quarantining of anyone that may be infected, hand washing, and physical distancing, face masks are a valuable tool to reduce community transmission."
For a skeptical view of this argument, Graham Martin, Esmée Hanna, and Robert Dingwall have their own preprint paper, "Face masks for the public during Covid-19: an appeal for caution in policy" (April 24, 2020), available at the SocArXiv website. They suggest several concerns (footnotes with citations omitted):
First, the very weak evidence for face masks should be reiterated. Although some important studies followed the outbreak of SARS-Cov-1 in the 2000s, by and large the quality and clarity of the evidence base for face masks as a means of reducing transmission is disappointing. Few studies examine use of face masks in community settings; those that do find no evidence of reduced transmission compared with no face masks. ... Of course, absence of evidence is not evidence of absence ... But existing research also provides little information on potential harms, such as “discomfort, dehydration, facial dermatitis, distress, headaches, exhaustion.” Here, too, absence of evidence should not be taken as evidence of absence.
Second, it is unclear how well equipped the general public is to make proper use of face masks, or how readily good practice might be disseminated and taken up. Appropriate use of face masks is challenging and is something healthcare workers themselves can struggle with; poor use (including poor fitting, adjustment, touching) can reduce effectiveness and pose an infection risk in itself. ... For non-disposable clothbased masks, the evidence base is slim, though one hospital-based three-arm trial found worse infection outcomes in wearers of cloth masks than in wearers of medical masks and in a control group (usual practice, which included much mask-wearing). Cloth masks will retain moisture, with indeterminate consequences for their efficacy and for the creation of a microbiological environment favourable to other bacterial or viral organisms. ... An evidence base for homemade masks is likely to be elusive. However, the existing research, coupled with the potential for great variation in materials, fit, adherence, touching and adjustment, doffing, disposal, frequency of laundering and so on, suggests the need for caution in advising widespread uptake, especially given the paucity of evidence for cloth face masks, their use, and their possible microbiological downsides.
Third, at the microsocial level, the argument might be made that encouraging uptake of face masks might lead to reduced compliance with other measures, due to the false sense of security presented by the mask. Such arguments rest on evidence around risk compensation in other areas of public health, for example seatbelts, cycle helmets, vaccination against sexually transmitted infections, and injury prevention in competitive sports. ... [T]here is a case that face masks might promote, if not active risk-taking, then at least a complacency that might reduce adherence to other measures, especially given the largely collective rather than individual benefits that the wearing of masks seeks to address. ... There is also an argument that universal mask-wearing might aggravate the climate of fear already documented for Covid-19, 17 adding to mental health concerns by providing a constant reminder of the threat posed by other humans.
Fourth, potential downsides of the promotion of face masks in community settings present themselves at the macrosocial level. ... As a highly visible symbol of virtuous behaviour, those who fail to comply—for example, because of respiratory ailments that make prolonged mask-wearing dangerous, 20 or because of religious preferences such as beards worn by Sikh men or hijabs worn by Muslim women that may make mask-wearing difficult—may be subject to stigmatisation or worse. ... Meanwhile, notwithstanding the weak evidence base for face masks as a standalone measure, businesses or states might see widespread or mandatory mask-wearing as a warrant for a premature return to ‘business as usual’, justifying unsafe workplaces or crowded commuting conditions in terms of the protection offered by masks.
This leads us to our final point. ... Face masks (and measures to secure their uptake) are a complex intervention in a complex system: the results of a change of this nature are emergent, unpredictable, and potentially counterintuitive.
This list of potential costs of mask-wearing is not dire, but neither is it illusory.
Where does this leave us? It's perhaps worth pausing a moment to be clear on what "science" is telling us here. If "science" means peer-reviewed studies, then none of these essays are telling us anything--because they have not been peer-reviewed. The advice to wear masks based on a combination of partial information and the precautionary principle may be seem sensible, and may in fact be sensible, but "it just makes sense" is not a proven scientific result.
On the other side, the pandemic is happening now. The overall US death rate continues to be elevated, and COVID-19 is the likely cause. Comprehensive studies done with ideal methodology take months or years. We need to make a decision now, based on imperfect evidence, and then follow up as best we can with evaluating the results of that decision. Of course, the current wave of rule-making seems to emphasize mask-wearing in indoor or crowded settings.
On yet another side, I'll point out that the argument for experimenting with imperfect steps now, based on imperfect scientific evidence, applies to a lot more than just wearing masks. For example, it applies to the use of imperfect tests for COVID-19 as they are developed, to experimentation of imperfect treatments for COVID-19 as they are proposed, and maybe in the not-too-distant future the use of an imperfect vaccine. In all of these areas, the "science" is likely to be shakier than one might optimally prefer.
It seems to me that the public health experts have badly muddled the question of whether mask-wearing was needed. The general advice back in March and April was that masks were not needed; now in August and September, masks have become near-mandatory in various settings. I don't know whether the public health crowd was underreacting then or is overreacting now. I suppose one could even argue that the anti-mask recommendations early in the pandemic made sense because of the lack of clear-cut evidence, while the pro-mask recommendations now make sense because the pandemic is lasting longer than some of the early epidemiology models predicted. I have masks in my car and my office to follow the rules. But I don't automatically put on a mask when walking outside or standing six feet from someone and having a conversation. And if I see someone walking by without a mask when I happen to be wearing one, I don't snark at them--unless they are actually sneezing and coughing, or singing operatically, as they walk by.