Making sense of the research on COVID-19 and masks

Benjamin W. Abbott¹, Mitchell Greenhalgh¹, S. Isaac St. Clair¹, Jonas Bush²

1. Brigham Young University, Environmental Science; 2. Brigham Young University, Genetics, Genomics & Biotechnology

The interpretation and content of this report are those of the authors. (Published on 19 July 2020. Last updated on 21 August 2020). For the PDF version of this report, click here.

Scientific evidence can be difficult to interpret under the best of circumstances. During a global pandemic (and election year), it is no surprise that there is public confusion about what measures can effectively protect families and communities from COVID-19. Because the scientific and medical understanding of this disease is advancing so rapidly, we decided to put together a plain-language summary of the science on face coverings—a.k.a. masks. As lifelong residents of Utah County and scientists, we felt a responsibility to respond to the technical questions asked by friends and family. We did not receive any funding to carry out this work, nor do we plan on seeking financial support on this topic (our BYU lab mainly researches water and air pollution: benabbott.byu.edu).

Our four-person team compiled and read over 115 scientific studies on COVID-19. These studies were done by independent groups from all around the U.S. and the world. In the paragraphs below, we have done our best to accurately reflect the scientific evidence, pointing out where it is solid and where there is still uncertainty. There are four sections, with increasing levels of detail: 1. An executive summary, 2. A list of common questions, 3. A deep dive, and 4. FAQs about the study. We hope this summary is useful to you as you decide what is best for your family and as our community decides how best to face this threat together.

Executive summary:

1. In the first few months of the pandemic, there was scientific uncertainty about the usefulness of public masking. Conflicting guidance was given by several official sources^1–4.
2. There is now convincing evidence from multiple controlled experiments^5–7 and field observations^8–14 that wearing masks reduces the transmission of COVID-19 for healthcare workers and the public. Most of this evidence is COVID-19 specific and has emerged in the past few months^1,8,15,16.
3. Masks prevent infected people from spreading the virus to others by trapping the respiratory droplets (tiny moisture particles) that are produced when we cough, speak, and breathe^11,17,18. Cloth masks can stop 90% or more of the dispersal of droplets carrying the virus^6,17,19,20. There is some evidence that cloth masks also protect the wearer from infection^8,9, though this is less certain.
4. Masks are highly safe, with only minor and uncommon side effects^1,21,22. In addition to many medical studies, public masking has been proven safe among children, adults, and the elderly in cultures where this practice has long been common^11,23. However, some individuals should not wear masks, such as those with severely compromised respiratory systems²⁴ and individuals who cannot remove or adjust their own masks (children under 2 and people with severe disabilities)¹⁸.
5. Researchers from hospitals, universities, the private sector, and government agencies have concluded that masks could be one of the most powerful and cost-effective tools to stop COVID-19 and accelerate the economic recovery^{1,5,10,11,16,23,25}. There is universal agreement, however, that masking alone will not be enough to stop the pandemic. Masking is most effective when combined with physical distancing, frequent handwashing, rapid testing, and coordinated contact tracing^1,8,26.

Common Questions:

What is COVID-19?
Coronavirus disease 2019 (COVID-19) is caused by a respiratory virus named “severe acute respiratory syndrome coronavirus 2” (SARS-CoV-2). It is in the same family as the virus that caused the SARS outbreak of 2003, and both of the viruses appear to have originally come from bats^27,28 . The “severe” part of the virus’s name comes from its extreme contagiousness and serious complications. Thankfully, most people who have COVID-19 only experience flu-like symptoms such as fever, cough, difficulty breathing, and fatigue. However, COVID-19 is much more contagious than the flu, and it has a much higher death rate: 0.3 to 5.7% based on the most reliable estimates^29–34 . For reference, the common flu has a death rate of 0.1%. Serious COVID-19 complications include damage to the lungs, liver, and heart, permanent loss of taste and smell, blood clots, stroke, and death³¹ . The rate of complications including death is much higher in individuals with preexisting conditions, particularly the elderly and those with cardiovascular disease, diabetes, respiratory conditions, high blood pressure, cancer, an organ transplant, sickle cell disease, immune disorders, and those living in areas with more air pollution^33,35,36 .

How is COVID-19 transmitted?
While much is still being learned about this disease, it appears that the main way the virus is spread is through small droplets of moisture that are produced when a person exhales, talks, coughs, or sneezes^5,34,37. These tiny droplets can travel many feet through the air and linger in the air or on surfaces for several minutes to hours after the infected person has left the area^{17,19,35,38,39}. The virus can enter a new person’s body directly by being breathed in or landing in the eyes, nose, or other mucus membranes^40–42. The virus can also spread indirectly through objects and surfaces.

After infection, most people experience an incubation period where there are no symptoms—typically lasting 5 days, but sometimes lasting 15 days^1,5,43. Infected individuals are most contagious during the day before first symptoms and in the few days after onset, meaning that people who feel completely healthy can spread the virus to loved ones and strangers^10,37,44. Up to one in three COVID-19 carriers show no symptoms at all (asymptomatic)^12,45, though they can still spread the virus^1,4,46. It is estimated that half of all transmissions occur from individuals not showing symptoms^1,47. We have compiled information on the pathology of COVID-19 in children, adolescents, and young adults in a new report that is freely accessible here⁴⁸.

Are masks effective for individuals and communities?
There is clear evidence that face coverings reduce the spray of droplets produced during speaking, coughing, and sneezing^{6,17,19,49–52}. This is why masks have long been recommended for individuals with respiratory infections^{7,7,40,53–55}. Furthermore, home-made cloth masks are similar in effectiveness to surgical masks in diffusing the “jets” of droplets that could spread COVID-19 (for details on mask types, see Figure 1 and the “deep dive”)^6,19. For example, one of the most definitive studies on viruses and face masks found a 90% reduction in influenza viruses after breathing through a mask and a complete removal of coronaviruses⁵⁰. However, not all face coverings are equally effective. Multi-layer synthetic and cotton masks provide the most filtration, while bandanas and fleeces (neck gaiters) provide very little or no protection^6,20. Because only fitted respirators (the finest-meshed masks) provide reliable protection from external droplets and viral particles^41,53,56, public masking works through “source control,”¹⁵ where “my mask protects you, and your mask protects me.”⁵⁰

While it is always difficult to establish cause and effect at the community level, there is now strong evidence that public masking can slow or even stop the spread COVID-19 in states and countries^{4,7,11,13,16,44,57}. One of the most conclusive studies on the subject compared the effectiveness of universal masking in the health-care system of Massachusetts⁸. This large study involved 75,000 participants, including healthcare workers and patients. Of all the public health interventions, universal masking appears to be the most associated with stopping the exponential growth of COVID-19 cases, first flattening and then decreasing the infection rate⁸. Another study from Missouri demonstrated the effectiveness of universal masking in commercial settings⁹. Two hair stylists contracted COVID-19 but continued to work for 10 days, servicing 139 clients. Because the salon had a universal masking policy, all stylists and 98% of clients were wearing cloth, surgical, or N95 masks during the encounters. None of the clients or other stylists in the salon developed symptoms^9,16. Similar accounts are being reported from around the U.S. and the world^13,16,58,59, supported by quantitative simulations⁶⁰ and randomized controlled trials from other diseases⁷.

There is also recent evidence that masking is effective at state and national levels. In countries where public masking was common before the pandemic (where it was culturally normal for sick people to wear masks in public), COVID-19 had an initial daily growth rate of 10%, versus 18% in countries without such norms^10,61,62. Likewise, the COVID-19 growth rate and mortality rate are lower in countries and states that required public masking^12,13,63. The relationship between public masking and lower COVID-19 mortality could be due to lower viral loading (how many particles you are exposed to) decreasing the severity of infection⁶³. This suggests that even when masks do not completely stop transmission, they could still save lives. Universal masking was also pivotal to the success of South Korea and Hong Kong, which were able to contain COVID-19 without lockdowns^11,15,64.

There is some anecdotal evidence that masking is working in Utah. Since the mask mandate was passed for Salt Lake and Summit Counties on June 27^th, the number of new cases in those areas has decreased, while the number of new cases in the rest of the state has increased⁶⁵. Likewise, the July 10^th request by the Church of Jesus Christ of Latter-day Saints for all members to wear masks in public coincides with a flattening and then decrease of COVID-19 cases statewide⁶⁶.

Photo by Desai, A. N. & Aronoff, D. M. Masks and Coronavirus Disease 2019 (COVID-19). JAMA 323, 2103–2103 (2020).

Are masks dangerous?
Perhaps the most robust demonstration of the safety of public masking is that this practice has been widely used for decades in many countries, especially in Asia ^61,62,76 . In these regions, where it is common to wear a mask when sick or when there is an outbreak masking has proven safe among children, adults, and the elderly ^11,23 . Likewise, widespread use in the northeastern U.S. has been highly safe ^16,59 . This history of widespread use decreases the risk of unexpected side-effects ⁷⁶ .

Because masks have been used by medical professionals for centuries, there is a large body of scientific evidence about their performance and safety^{41,50,56,67–69}. Several mild and rare side effects have been identified, including skin irritation,  headaches, and general discomfort in a minority of users^1,21,22,70. However, certain individuals with acute, preexisting conditions, such as COPD and end-stage renal disease can have difficulty breathing when they wear a respirator (N95) for long periods^24,71–74. It is also recommended that individuals should not wear a mask if they cannot remove it on their own—for example, children under two, or people who are unconscious—for common sense, comfort, and safety reasons¹⁸.

Despite widespread concern on social media and in the news, there is no evidence that masking causes dangerous hypoxia (low oxygen) or hypercapnia (high carbon dioxide) in healthy individuals, even when multiple masks are layered on top of each other^22,70,74. In fact, wearing a surgical mask during exercise has been used as a treatment to reduce exercise-induced asthma⁶⁹.

There are two main concerns about public masking from a public health perspective, both based on studies of other diseases. First, there can be overcompensation or a “false sense of security,” where mask users are less vigilant in following other preventative measures such as physical distancing or hand washing^1,25,62,70. Second, if masks are worn improperly, there can be self-contamination, where bacterial or viral particles accumulated on the outside of the mask are transferred to the hands and then potentially to the mouth or eyes^41,54,75. In response to the first concern, observations from this pandemic suggest that wearers of masks are actually more likely to take additional precautions than those who don’t wear masks^1,5,11,25. To the second point, mask users should wash their hands after removing their mask and wash masks daily (Figure 1). However, the risk of self-contamination is most likely in clinical situations where masks are used to protect the wearers^41,75. This is less of a concern in a public masking scenario, where masks are used to contain infection sources. Additionally, it has been pointed out that “if a mask is contaminated at removal, it has (by definition) already protected the wearer from contagious droplets.”⁵⁹

Perhaps the most robust demonstration of the safety of public masking is that this practice has been widely used for decades in many countries, especially in Asia^61,62,76. In these regions, where it is common to wear a mask when sick or when there is an outbreak masking has proven safe among children, adults, and the elderly^11,23. Likewise, widespread use in the northeastern U.S. has been highly safe^16,59. This history of widespread use decreases the risk of unexpected side-effects⁷⁶.

What would the economic consequences be of public masking?
The most recent analysis by Goldman Sachs suggests that increasing masking by 25 percentage points from current levels would cut the COVID-19 growth rate by 3-fold and prevent the need for a second round of economic shutdowns ²³ . They predict this would result in an economic benefit of approximately $1 trillion. Another study found that 80% of the population wearing cloth masks when in public would be more effective at stopping the virus than a strict lockdown of the whole population ⁶⁰ . There is nearly universal consensus that masks could be one of the most powerful and cost-effective tools to stop COVID-19 and accelerate the economic recovery ^{1,5,10,11,16,23} .

Deep Dive:

What kinds of masks are we talking about?
There are three main types of masks each with different purposes. Experts have advised the general public to wear cloth face coverings to slow the spread of COVID-19, in conjunction with hand washing, physical distancing, and other protective measures (Figure 1).

N95 Respirator: An N95 mask (formally called a respirator) has an extremely small pore size that filters out most particles and aerosols. Given the shortage of these devices, it has been recommended that N95 masks should be reserved and set aside for healthcare workers in clinical settings. Because they are fitted to the face and have such a high filtration capacity, these respirators provide protection from outside droplets and particles.

Surgical Masks: Surgical masks provide basic protection for the wearer but are primarily meant to protect others from droplets and particles coming from the wearer (“source control”). They are sometimes referred to as ‘medical masks.’ Some studies show similar performance to N95 respirators for source control^67,75.

Cloth Face Coverings: These are the reusable “home-made” masks that we have been recommended to wear when in public. Surprisingly, they have proven as or more effective as surgical masks and even N95 respirators in some circumstances to contain droplets and disperse air^6,17,19,20. They should be washed and dried daily, and some sources recommend alternating days between masks (Figure 1). The level of filtration depends on the type of fabric and number of layers (Figure 2), with multi-layer masks of synthetic and natural fibers outperforming bandanas and neck gaiters, which provide very poor filtration^{6,20,49,52,53}. However, several research groups have pointed out that because the blocking of jets of air and trapping of droplets is the purpose of masks in this pandemic, the most important consideration is comfort, which will increase likelihood of wearing the mask in different environments and reduce adjustments and touching of the mask^5,10,59.

The most recent studies have shown that even a basic covering is quite effective at limiting the spread of infectious respiratory droplets from speaking or coughing (see Figure 3) ^6,17,78 . Research measuring the number of droplets from the mouth of someone speaking with and without a mask shows that although cloth coverings certainly aren’t foolproof, they prevent most particles from traveling long distances where they could potentially infect others ^6,78 . Multi-layer masks can prevent the spread of more than 90% of respiratory droplets—similar in source control to surgical and N95 masks (Figure 2) ^6,20 . Other research has also discovered serious differences in filtration of COVID-19 sized droplets between N95, surgical, and cloth masks. This research also found that the most important thing while wearing a mask is to have a proper fit. Even N95 respirators filtered less than 50% of particles when worn with gaps ⁴⁹ .

Because of the unparalleled nature of a pandemic in recent history, it was initially difficult to obtain research on the effectiveness of different mask types in public spaces. However, recent studies have shown decreased cases of COVID-19 in areas where masks and social distancing are mandated, and experts say that the efficacy of these mandates will only increase over time^13,14,23,77.  

The most recent studies have shown that even a basic covering is quite effective at limiting the spread of infectious respiratory droplets from speaking or coughing (see Figure 3)^6,17,78. Research measuring the number of droplets from the mouth of someone speaking with and without a mask shows that although cloth coverings certainly aren’t foolproof, they prevent most particles from traveling long distances where they could potentially infect others^6,78. Multi-layer masks can prevent the spread of more than 90% of respiratory droplets—similar in source control to surgical and N95 masks (Figure 2)^6,20. Other research has also discovered serious differences in filtration of COVID-19 sized droplets between N95, surgical, and cloth masks. This research also found that the most important thing while wearing a mask is to have a proper fit. Even N95 respirators filtered less than 50% of particles when worn with gaps⁴⁹.

Face coverings in past pandemics:
Since the late 19^th century, facemasks have been used in efforts to prevent the spread of infectious diseases. The first studies began using masks to create a sterile surgical environment. By 1920, surgical masks began to be implemented in many hospitals, although they were slow to catch on because surgeons found them to be annoying and generally preferred to not speak while operating⁷⁶. It wasn’t until the 1960s that masks became widespread in hospitals in the United States. Since then, multiple types of research have been conducted to evaluate how much masks can help, including laboratory tests, “natural experiments” where researchers compare effectiveness of different policies, and computer simulations based on what we know about transmission dynamics and the pathology of COVID-19^1,5,76.

During the 1918 influenza pandemic, many western cities (including San Francisco, Phoenix, Seattle, and Juneau) tried to mandate facemasks by law. At that time, most Americans did not understand why they needed to wear masks, and they resisted what they perceived as an unnecessary irritant. “The primary purpose of wearing a mask is not to prevent a healthy person from getting sick, but rather to prevent people already infected from contaminating others through casual contact”⁷⁹.  Unfortunately, the 1918 pandemic took the lives of 675,000 Americans, and another 50 million worldwide⁸⁰. As the United States approaches 140,000 deaths from COVID-19, we should consider all the available evidence on this subject, including context from past pandemics.

COVID-19 and Back to School Efforts:
We have now completed a detailed report on school reopenings during the pandemic, which is accessible here⁴⁸. We invite you to consult the report directly, but for convenience, we have pasted the executive summary below.

1. Children (0-10 years old) and adolescents (11-18) are not immune to COVID-19, though the disease is usually less severe and less easily transmitted by children^81–87. Children may be one-third to one-half as susceptible to COVID-19 as middle-aged adults, though adolescents show intermediate to near-adult susceptibility and transmission^81,88,89. Transmission by children appears to account for a small minority of overall community and household cases (likely no more than 5 to 10%)^82,90–92.
2. Evidence on teachers’ risk of infection in school reopenings is limited. Risk of infection may vary based on the school type (lower in elementary, higher in junior high and high school), though this is not well established^93–96. Adult to adult contacts within schools may be the greatest risk for teachers⁹⁷. Protective measures are needed because 1 in 4 teachers is in a high-risk category for COVID-19⁹⁸.  
3. Many countries have reopened primary and secondary schools with safe outcomes for students and teachers^93,97,99,100. Some outbreaks have occurred, particularly associated with high schools^94–96. Factors increasing likelihood of success seem to include: low community spread, preventive measures at school (social distancing, masking, handwashing), rapid testing, contact-tracing, and cohorting^101,102.
4. No countries have attempted to reopen schools with the level of community spread that the U.S. is currently experiencing. Daily cases in countries that have reopened successfully are typically below 20 per million people¹⁰⁰. That number is currently 199 for the U.S. and 120 for Utah⁶⁵. Early reports from U.S. districts that have reopened suggest frequent exposures , highlighting how reducing community spread should be paramount.
5. There are substantial risks and costs associated with not reopening schools, especially for primary and secondary school children, including decreased psychological wellbeing, social development, educational progress, nutrition, and safety^103–111. While risks can be dependent on or mitigated by family circumstances for some, many children are highly vulnerable to harm from full school shutdowns^112,113.
6. Safe reopenings cannot be achieved by interventions at school alone. In-school protective measures must be implemented along with family actions and community support to lower student, teacher, and community infection rates^{99,101,102,114–116}.
7. Observational and modeling studies suggest that risk of outbreaks is very high in colleges and universities^117–120. This is due to residential and nonresidential student interactions and high transmission among young adults. Widespread and rapid testing, a large quarantine capacity, and a sustained level of vigilance will be needed before campuses can be safely reopened for in-person instruction, even with mask-wearing and social distancing practices in place for classrooms.

In-depth analysis of health risks associated with masking:
Certain health conditions and statuses preclude or increase the risk of wearing face coverings¹²¹. Young children under the age of 2 and those who have trouble breathing, due to chronic respiratory conditions or acute or end-stage illness should not wear masks^24,73,74,122. Masks are not appropriate for those who are unconscious or otherwise incapable of donning and doffing their own mask. Increased caution should be used when considering masking for individuals with physical, intellectual, or developmental disabilities, such as those of the deaf/hard of hearing community who rely on lip reading to communicate. Lastly, there is little research on the use of masks during exercise, though the World Health Organization recommends against it^69,123–125.

As with all medical interventions, there are several documented side effects of wearing a mask. Face masks can cause discomfort, and wearing a mask can direct exhaled breath into the eyes, leading to general irritation and potentially touching the face, which is a danger to transmission and infection⁷⁰. Wearing a mask in itself can be a reminder to not touch your face, but if they’re adjusted or removed frequently, this may negate the protective benefits against transmission and infection^126–128. One study showed that masks were associated with headaches, vague discomfort, and acne, although the headaches were significantly associated with pre-existing headaches²¹.

Some of the main arguments against masks involve fear of hypoxia and hypercapnia—a lower availability of oxygen and an overabundance of carbon dioxide caused by wearing masks. Professionals, cited in media and press publications, and national and international public health organizations concur that it is highly unlikely for either hypoxia or hypercapnia to occur while wearing a mask^{15,71,72,122,123,129–131}. The only cases where this has been documented to occur are with the use of N95 respirators by medical patients with acute, preexisting conditions, such as AECOPD and end-stage renal disease^24,71–73. There are several studies that are widely cited on social media that misinterpret the medical evidence about masks. For example, a small trial of surgical masks among surgeons showed a slight decrease in blood oxygen¹³². The levels stayed within a healthy range and the authors conclude that the deoxygenation may have been due to the stress of performing surgery rather than the masks.¹³² Similarly, the studies we cite above about how serious disease can prevent some individuals from safely wearing a mask^24,71–73, are often referenced as evidence for why masks are not safe for healthy individuals. The current literature and industry practices indicate that there is very minimal risk of hypoxia and hypercapnia when wearing N95 respirators, and even when a surgical mask is layered on top there appears to be no physiological burden or significant change in oxygen or carbon dioxide levels^22,59. There is no evidence that cloth masks, which are looser fit and not sealed, would cause such conditions.

One significant risk of wearing masks lies in overcompensation, also described as the “football-helmet effect.” The concept is that the safety provided by a mask can give a false sense of security, leading to greater disregard of other protective measures. For example, cloth masks primarily provide source control rather than external protection (“my mask protects you, your mask protects me”), though some level of external protection appears likely depending on the fabric and way the mask is worn^{41,70,126,133}. The fear is that someone wearing a mask might be more likely to engage in risky behaviors and therefore put themselves and others at higher risk of infection¹³⁴. Furthermore, if masks are not worn or cleaned properly, they can result in “self-contamination”¹⁸, as discussed previously. While there is no evidence that masks cause fungal or bacterial infections¹³⁰, the more humid habitat created by a mask may allow SARS-CoV-2 to remain active and possibly lead to the defeat of innate immunity and subsequent infection and transmission⁷⁰.

In past situations with less virulent diseases and lower levels of community spread, experts expressed caution when extending the successes of mask-wearing to large interpersonal contexts such as schools or healthcare settings^41,54,135. They emphasized that other preventative measures such as physical distancing, washing hands, and staying home would be more effective than purely relying on masks¹³⁶. In the current pandemic, the consensus is growing that public masking should be used in combination with other efforts rather than not at all^{1,5,7,10,126,133,136}. Even the cautious and consensus-driven World Health Organization, which initially recommended against masks, now encourages their use in areas of widespread COVID-19 transmission¹³⁷, in light of new information on the disease and the results of large-scale comparative studies¹³⁵.

Frequently asked questions (FAQs):

1. How did you select the papers you included?

We generated an initial list of studies using Google Scholar and Web of Science. We focused this initial search on studies specifically dealing with COVID-19 and masks. As we read those papers, we added pertinent studies they cited to our list. Additionally, we considered about a dozen studies that concerned individuals sent us. These studies were mostly about the safety of masks. Our goal was to provide a non-technical summary for those interested in this topic. Because the medical literature on COVID-19 is so large, we cite many studies that are reviews, summaries, and commentaries on the state of the literature. While we also read many specific primary research articles, we favored citations of reviews and summaries to encourage readers to interact with the peer-reviewed holistic assessment of the state of scientific understanding. These synthesis papers are often more reliable and robust than any individual study upon which they draw¹³⁸. They also avoid the "single-study syndrome," where one study is interpreted without context from other work. Science is a joint venture and only reliable when the result is repeatable and falsifiable.

2. How did you ensure you weren’t getting a biased sample of studies?

We considered all the scientific studies on masks and COVID-19 that we could find, including about a dozen sent to us by individuals who do not support masking. We continue to consider all studies that are sent, so if we missed a particularly pertinent piece on COVID-19 and masks, please send it along (but please read FAQ #4 before sending unrelated studies). Likewise, if you see an error or point of confusion in the report, please let us know and we will update if we find there is an error. This has happened several times already, mainly with typos, but also with two important mistakes: 1. In the viral transmission section, we initially wrote “COVID-19 viruses” when the study in question was actually about coronaviruses generally, and 2. Our initial description of the most recent WHO recommendations were confusing. At the top of the report, we mention the “last updated on” date, so you can know when the last change was made.

3. Why do you cite some non-scientific sources such as news articles and government reports?

We cited a few of these articles for context to provide an idea of what the larger conversation is outside of science. In one section (the deep dive on mask safety), we cite some of these news reports because they have quotes from legitimate medical experts. We mention that we are citing media sources in that section to make sure people know what we are basing our interpretation on.

4. What about this or that study that was done on another disease?

There is a huge amount of research on other diseases, including several on the effectiveness of public masking in stopping those diseases (though fewer than we expected)^7,40,41,54. We cite some of this research in our report to provide context. For example, it’s helpful to know that public masking was widely considered ineffective before the COVID-19 pandemic based on studies done with other diseases in very different contexts. This explains why many medical experts and agencies recommend against mass masking initially—that was the state of the available science in December 2019 and January 2020.

For our report, we focus heavily on the COVID-19-specific studies for one very important reason: COVID-19 is the disease causing this pandemic. Findings from previous outbreaks and community studies are helpful as a starting place, especially in the absence of more directly pertinent evidence. However, the pathology of each disease and the societal dynamics of each outbreak are different. Naively applying findings about influenza and masks in normal times to COVID-19 during a global pandemic is problematic, to say the least. As we evaluated the evidence from multiple studies, we gave more weight to those that dealt specifically with the current outbreak, while still considering the other studies for context.

One specific study is worth mentioning here, because of how often it is brought up in discussions of masks: MacIntyre and others 2015, "Facemasks for the prevention of infection in healthcare and community settings."⁴⁰ This study is often brought up as evidence that masks (especially cloth masks) don't work, and that they might actually cause harm (increase risk of infection). On first read of the abstract, it is easy to get this impression. However, this is a completely incorrect interpretation of this research for several reasons. First, the study is on different diseases in different conditions (see last two paragraphs). Second, the study tested the effectiveness of masks at protecting the wearer (in this case healthcare workers), rather than source control, which is the primary purpose of masks in the COVID-19 pandemic. Third, the study did not have a "no mask" control, so they have no way of comparing cloth mask with no mask. The authors make this clear in the "Strengths and limitations" section of their paper, which appears adjacent to the abstract when you download the PDF. Importantly, the same research group did a study the following year on the effectiveness of masks at reducing infection when worn by sick individuals⁴¹. They found that masks likely reduced infection, though they acknowledge that they didn't have enough participants to strongly conclude anything. Since the COVID-19 pandemic, this research group has done a systematic review of the effectiveness of masks⁷, including their 2015 study. They conclude:

“community mask use by well people could be beneficial, particularly for COVID-19, where transmission may be pre-symptomatic. The studies of masks as source control also suggest a benefit, and may be important during the COVID-19 pandemic in universal community face mask use as well as in health care settings.”

5. Why don’t you cite this or that specific study?

There are thousands of papers on COVID-19, which is a scientific achievement on its own. I salute the researchers and organizations that have, often at great expense, changed their research programs to address this important challenge. However, this means that we cannot cite every study on COVID. See the previous FAQs for info on how we decided what to focus on and what to cite.

6. Why do you claim there are controlled experiments when there haven’t been any randomized controlled trials of public masking?

One of the most common arguments made against masking is that there aren’t any randomized controlled trials (RCTs) showing effectiveness of public masking. This is not true. A very recent systematic review by Macintyre and Chughtai⁷ of public masking studies (including many RCTs) concluded that masks are effective at slowing the spread of many respiratory diseases. Additional RCTs on this subject are coming out frequently⁵⁵. However, there are not, to our knowledge, any COVID-19 specific RCTs yet published. This deserves some context. 

RCTs are powerful tools that can—when done correctly—quantify the effectiveness of a drug or other intervention. They are often described as the gold standard for establishing cause and effect and are usually required before high-risk changes in medical practice are implemented (for example introducing a new medication). RCTs also take time and ethically require that the trial doesn’t cause undue risk or harm to the participants. In the context of a global pandemic, it is not surprising that there are not yet RCTs on COVID-19 and public masking. Indeed, concern over the lack of RCTs sparked a fruitful discussion in the medical community (mainly in March and April), as it should have, about whether to change recommendations without RCTs. Check out this paper by Greenhalgh and colleagues¹ from April for a snapshot of what was being discussed at that point.

As this discussion played out in the medical community, other types of evidence started becoming available, including:

1. Longitudinal studies quantifying infection rates and death rates after implementation of masking in different settings
2. Comparative studies measuring outcomes in different regions and countries
3. Case studies of specific events where the virus was or was not transmitted
4. Large-scale studies on the pathology (transmission dynamics, contagiousness, symptoms, etc.) of COVID-19
5. Laboratory studies quantifying the effectiveness of masks in filtering, muffling, and containing particles and droplets 

The first two types of studies often cannot definitively establish the specific cause of the outcome; for example, how much was the masking itself versus the psychological reminder to respect greater physical distance that resulted in the decrease in death? However, they are extremely powerful (and often more robust because they can involve many more people than even the largest RCT) at assessing overall outcome of an intervention. The third kind of study can’t provide definitive evidence of the generality of an intervention (e.g. will masks always protect people getting their hair done by an infected hair dresser?), but in the absence of more controlled studies, these case studies are indispensable as metrics of real-world performance and outcomes. The fourth and fifth kinds of studies complement the others by establishing specifics of how the disease works and first-order estimates of potential effectiveness of interventions (in this case masking).

Given the mounting evidence, most medical experts and public health entities updated their recommendations early this year. Given the high likelihood of benefit and the extremely low risk of the intervention (there is abundant evidence that mask wearing is exceedingly safe for healthy individuals), recommending public masking was determined to be the only ethical course of action. While most papers we read continue to call for RCTs in their conclusions or discussion, they also recognized that such results could still be months or years out. Some papers have also pointed out that given the strength of the evidence, an RCT on public masking may no longer be ethical because asking people to not wear masks in areas of COVID-19 would knowingly endanger participants.

Because our report is meant for a general audience, we classified studies as controlled or observational. We used controlled to refer to any study with a manipulation (for example, implementation of healthcare worker and then patient masking in the Massachusetts study, or lab experiments on mask filtration of droplets and viral particles). We used observational to refer to studies that compare groups or outcomes without a targeted intervention. We recognize this division is imperfect, but after discussion, we decided it was a clearer distinction than observational and manipulative, the more common scientific terms.

7. Were you trying to prove something with this study? Aren't you biased?

Our only purpose in performing this research was to make more of the science available to the public. Here is the email Dr. Abbott sent to his lab group on July 14th, when he initiated the project:

Dear lab group, 

Just reaching out to see if any of you might be interested in contributing to a one or two page report on efficacy and risks of wearing masks? Maybe it's just my social media feed, which is dominated by moms and dads, but this appears to be a hot topic right now. As there is very little trust in currently available summaries, I think a succinct and carefully nonpoliticial mini-review could do a lot of good. I don't know if it would be publishable in a peer-reviewed journal (depends on how much we want to put into it), but we could easily self publish on the lab website and do a press release.

To the question of bias, yes, like all humans, we are. Our beliefs and values influence how we see the world around us. We discussed this as a group of authors and worked hard to provide a fair summary of the literature. Staying neutral was helped by the variety of political views and backgrounds among the co-authors. We take scientific integrity extremely seriously, plus I believe that we have a God-given responsibility to be honest. We recognize there are inevitable gaps and room for improvement, but we have done my best to accurately represent the scientific literature.

Especially during these politically charged times, all of us need to be extra vigilant and responsible about what we share and say. Whether it is this report or anything else, please read the article before reposting and interpreting it on social media. It’s a matter of basic honesty and integrity. Plus, what we say and share has serious consequences in the real world.

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