FAQs For Mask Usage Skeptics

Written: 20 Apr 2020 by Jeremy Howard on fast.ai

A bit of skepticism is healthy, and it’s especially reasonable given how much the official guidance on masks has varied over time and across regions. But of course, a good skeptic reads the evidence, and makes an informed decision based on that. So here’s some frequently asked questions I’ve been seeing from curious skeptics, and answers (with citations).

Contents

Why should most people wear masks?

Wearing a mask decreases the number of people infected by an infectious mask wearer (“source control”), because it reduces by around 99% the number of droplets that are ejected during speech. It also probably somewhat decreases the likelihood of an uninfected wearer getting infected, although it’s less effective for this, since many of the droplets quickly evaporate into small droplet nuclei that are hard to block. Reducing the number of people infected has an exponential impact, because it decreases the effective reproduction rate, R.

About half of infections are from people that aren’t showing symptoms – so people that don’t know they’re sick are infecting others. Because masks are far more effective at blocking infection at the source, that means we need everyone to wear a mask in public, since otherwise unmasked people put those around them at risk.

Shouldn’t only sick people wear masks?

Patients without symptoms pose a risk of infecting others, so it’s not enough to wait until you have symptoms to wear a mask. Three recent studies show that nearly half of patients are infected by people who do not themselves have symptoms—thus they aren’t even coughing or sneezing yet, but they can spread the disease just by talking in close proximity to someone else.

Shouldn’t we just follow WHO’s guidelines?

88% of the world’s population lives in countries where the disease experts and government leaders recommend/mandate masks and disagree with the WHO.

Why does the world disagree with the World Health Organization about masks? 

The WHO “recommends those infected with the COVID-19 coronavirus to wear masks to prevent spreading it to someone else.”

And the WHO agrees with the U.S. CDC (and the rest of the world) that people who are non-symptomatic and infected can spread the virus.

The WHO also recommends public mask usage during high-severity influenza pandemics – just not during this Coronavirus pandemic.

So it follows very basic logic that if we don’t know who is infected (because of asymptomatic cases) and the WHO agrees that anyone who is infected needs to wear a mask, then we should all wear masks.

Hopefully, WHO will update their guidelines to be clearer in the future. Their most recent guidelines say that “WHO is collaborating with research and development partners to better understand the effectiveness and efficiency of nonmedical masks. WHO is also strongly encouraging countries that issue recommendations for the use of masks in healthy people in the community to conduct research on this critical topic. WHO will update its guidance when new evidence becomes available.”

Is there a randomized controlled trial (RCT) for the impact of masks on community transmission of respiratory infections in a pandemic?

A randomized controlled trial (RCT) is sometimes considered the “gold standard” for assessing evidence to see whether a medical intervention actually works. It’s mainly used for assessing new drugs. In an RCT, a representative sample is selected, and randomly split into two groups, one of which receives the medical intervention (e.g. the drug), and one which doesn’t (normally that one gets a placebo). This can, when things go well, show clearly whether the drug made a difference. Generally, a “p value” is calculated, which is the probability that effect seen in the data would be observed by chance. If that p value is less than some number (often 0.05) the RCT is considered to be “statistically significant”. Without an RCT, it can be harder to distinguish whether two groups differ because of the intervention, or because of some other difference between the groups.

There has never been, and will never be, an RCT for the impact of masks, or hand-washing, or social distancing on community transmission of respiratory infections in a pandemic. The reason is that the following steps would be needed:

  1. Select 100 or so communities that are representative, and do not have significant population interaction (i.e people don’t move from one region to another)
  2. Select at random 50 communities where everyone must all wear masks in public; the populations of the other 50 must never wear masks in public
  3. Wait a few months
  4. See how many people died in each set of communities

Because we have such a strong prior expectation that masks are likely to be effective, there are probably no jurisdictions where it would be considered ethical to run such a study. It would also be very challenging to ensure compliance. A smaller and simpler trial that does not look at whole communities, but only individuals, would face similar ethical problems, and would also not be able to actually answer the question of whether community transmission is impacted.

The American Statistical Association (ASA) has released a “Statement on Statistical Significance and P-Values” with six principles underlying the proper use and interpretation of the p-value. In particular, note the following principles:

  • P-values do not measure the probability that the studied hypothesis is true, or the probability that the data were produced by random chance alone.
  • Scientific conclusions and business or policy decisions should not be based only on whether a p-value passes a specific threshold.
  • A p-value, or statistical significance, does not measure the size of an effect or the importance of a result.

So what should policy decisions be based on? They should be based on an assessment of the potential upsides and downside of an intervention along with their probabilities, versus the potential cost, in order to come up with an approximate expected value (ideally, a probability distribution) if the intervention is used, versus not used.

Shouldn’t we wait for an RCT before doing something?

No. Even if we ignore the impossibility of running such an RCT, a British Medical Journal paper points out that “there is a moral argument that the public should be given the opportunity to change their behavior in line with the precautionary principle, even when direct experimental evidence for benefit is not clear cut”. The precautionary principle is (from Wikipedia) “a strategy for approaching issues of potential harm when extensive scientific knowledge on the matter is lacking.” Most nations have agreed, via UNICEF, to act in compliance with this principle.

No jurisdiction has waited for an RCT before recommending hand washing or physical distancing. Many jurisdictions have enforced extreme physical distancing through mandated lockdowns or shelter-in-place orders, despite the lack of an RCT showing their effectiveness at reducing community transmission of COVID-19, and despite this being a far more expensive intervention.

Aren’t there RCTs that show no effect of mask usage?

For an RCT to show no effect, we would need to observe two groups that are very similar, with enough data to make us confident that the effect is small enough to be not practically useful. There are no RCTs that have found this for any use of masks for any type of coronavirus.

The closest thing we have, perhaps, to a relevant RCT is the paper The First Randomized, Controlled Clinical Trial of Mask Use in Households to Prevent Respiratory Virus Transmission: This was an Australian study for influenza control in the community, but not during a pandemic, and without any enforcement of compliance (such as would be provided by a mask mandate). It stated that “observational epidemiologic data suggest that transmission of viral respiratory infection was significantly reduced during the SARS epidemic with the use of face masks as well as other infection control measures” and “in an adjusted analysis of compliant subjects, masks as a group had protective efficacy in excess of 80% against clinical influenza-like illness.” However, the authors noted that “we found compliance to be low, but compliance is affected by perception of risk. In a pandemic, we would expect compliance to improve. In compliant users, masks were highly efficacious.”

There is some evidence that basic masks are more effective for coronavirus than influenza. There is a lot of evidence that compliance with mask wearing can be high during the COVID-19 pandemic, since many communities are already at well over 80% compliance (including many at close to 100%, due to mask mandates).

Some other RCTs that are frequently discussed with regards to public mask use, none of which study impact on community transmission, include:

Doesn’t a mask need to be 100% effective to be useful?

No. No mask is 100% effective. However, fewer virus particles mean a better chance of avoiding infection, and research shows that, if you are infected, the lower your viral exposure load, the better your chance of only a mild illness.

Do we really know if the virus is transmitted through the air?

I claimed earlier that a mask works “because it reduces by around 99% the amount of droplets that are ejected during speech”. That’s only useful if we know that the virus is actually transmitted through the air. We have to be careful here, because many people (including the WHO) use a definition of “airborne” that does not include transmission caused by droplets traveling through the air, but only tiny particles that float freely in the air for hours. We don’t know for sure whether the virus is “airborne” under this definition. However, it does seem clear that a key transmission route of COVID-19 is via droplets that fly out of our mouths. It has been known since 1934 (and studied in hundreds of papers since) that respiratory infections are transmitted through these droplets, and that the smaller ones quickly evaporate. We’ve known since 1946 (and studied in hundreds of papers since) that this creates tiny particles that are extremely hard to stop. This mode of respiratory infection is well understood, and consistent with the transmission of SARS.

Unfortunately, WHO makes things rather confusing, through their publication Modes of transmission of virus causing COVID-19. This document claims that “According to current evidence, COVID-19 virus is primarily transmitted between people through respiratory droplets and contact routes.” Five references provided to support this assertion (the WHO page currently lists 6, but it appears that the reference numbers are off by one, since the following reference does not relate to the correct document). However, a review of the references shows that none of them provide evidence supporting any particular transmission route. These are the references provided:

  • Community Transmission of Severe Acute Respiratory Syndrome Coronavirus 2, Shenzhen, China, 2020: “We suspect that community transmission and intrafamily transmission have potentially become the new transmission modes in the city. Also, nosocomial infection and transmission might pose a potential risk for COVID-19 control.” No evidence of whether transmission was through droplets, Aerosol, or fomites is presented.
  • A familial cluster of pneumonia associated with the 2019 novel coronavirus indicating person-to-person transmission: The focus of this study was to identify whether the key issue to analyze and control is super-spreader events, or whether other types of transmission could be a problem. “Our study showed that person-to-person transmission in family homes or hospitals, and intercity spread of this novel coronavirus are possible”. No analysis of transmission methods was done, however there is some evidence here that wearing a mask is effective, because the only person that was not infected in the family in this case was the only one to wear a mask.
  • Early Transmission Dynamics in Wuhan, China, of Novel Coronavirus–Infected Pneumonia: This study only looked at “infected persons, relatives, close contacts, and health care workers”. It did not attempt to identify the method of transmission or find out whether transmission to more remote people occurred.
  • Clinical features of patients infected with 2019 novel coronavirus in Wuhan, China: this study looks at “the epidemiological, clinical, laboratory, and radiological characteristics and treatment and clinical outcomes” of patients. It does not look at or comment on transmission method in any way.
  • Active Monitoring of Persons Exposed to Patients with Confirmed COVID-19: “despite intensive follow-up, no sustained person-to-person transmission of symptomatic SARS-CoV-2 was observed in the United States among the close contacts of the first 10 persons with diagnosed travel-related COVID-19. Analyses of timing of exposure during each patient’s illness as well as the type and duration of exposures will provide information on potential risk factors for transmission.” This analysis was not done in this paper, so therefore it does not provide any information about transmission methods.

However, there are cases reported in the literature which support the conclusion that the virus is transmitted through the air. In particular, the paper COVID-19 Outbreak Associated with Air Conditioning in Restaurant, Guangzhou, China, 2020, notes that “Virus transmission in this outbreak cannot be explained by droplet transmission alone. Larger respiratory droplets (>5 μm) remain in the air for only a short time and travel only short distances, generally <1 m”. Indirect Virus Transmission in Cluster of COVID-19 Cases, Wenzhou, China, 2020 notes that “the rapid spread of SARS-CoV-2 in our study could have resulted from spread via fomites (e.g., elevator buttons or restroom taps) or virus aerosolization in a confined public space (e.g., restrooms or elevators). All case-patients other than those on floor 7 were female, including a restroom cleaner, so common restroom use could have been the infection source. For case-patients who were customers in the shopping mall but did not report using the restroom, the source of infection could have been the elevators. The Guangzhou Center for Disease Control and Prevention detected the nucleic acid of SARS-CoV-2 on a doorknob at a patient’s house (5), but Wenzhou Center for Disease Control and Prevention test results for an environmental sample from the surface of a mall elevator wall and button were negative”. Other cases not yet reported in the scientific literature include a Seattle choir rehearsal where 45 have been diagnosed with COVID-19 or ill with the symptoms, despite all attendees required to use hand sanitizer, and a conference in Boston where 77 people became infected. Locations that had sporting events and festivals, where there tends to be singing and shouting, which eject larger and more droplets further distances, have had large COVID-19 outbreaks.

Perhaps the most interesting case for understanding transmission routes is a call center building in Guro-gu, Seoul, There are a total of 163 confirmed cases since 8 March. Of the 163 confirmed cases, 97 are persons who worked in the building (11th floor = 94; 10th floor = 2; 9th floor = 1), and 66 are their contacts.” So nearly all the cases in the building were on a single floor. This strongly suggests that transmission must be mainly through the air, otherwise, if transmission was primarily through touching surfaces, elevator buttons and front doors etc would have caused substantial transmission to other floors.

I have only been able to find a single case that seems likely to be caused by infection through surface contact. In this case, a person in Singapore who sat in a church pew that was earlier used by an infected patient became infected themselves. It’s possible that the virus was transmitted through the pew, although it’s also possible that since the people went to the same church, they came into contact before or after the service.

If it’s spread through the air, can a cloth mask really stop it? Isn’t the virus too small?

Coronavirus particles are so small that they can fit through the weave of most household cloth materials. Medical masks, such as N95 respirators, use special materials that create difficult to navigate pathways in the fabric that make it very hard for these tiny particles to get through the material. They also are specially fitted to the face of each healthcare worker to minimize gaps that these particles can get through.

Many commentators have been distracted by this, not realizing that the droplets that are ejected from an infected mask wearer are far bigger than the virus particles, and are easily blocked with around 99% efficacy, as shown in this recent NEJM paper that used laser light scattering to explore the effect. (The paper includes videos that make it easy to see for yourself what’s going on.)

We don’t know for sure yet whether droplets ejected during breathing are also an important transmission path. These droplets are much smaller, and have a lower total viral load, compared to droplets from speech, but I haven’t found any studies that directly study the impact of this on COVID-19 transmission.

The good news, however, is that we do have a study that shows the impact of wearing an unfitted mask on seasonal coronavirus transmission, based on the amount of virus particles found in droplets ejected during breathing. In this study, the unfitted mask was 100% effective in blocking these for seasonal coronavirus.

There’s even a study that tested the efficacy of a cloth mask at blocking COVID-19. Unfortunately, there are some problems with the study:

  • Symptomatic people should stay home, so testing speech would have been more helpful than testing coughs
  • The test was done at 8 inches, which is much closer than people should be if following physical distancing guidelines
  • Only 4 patients were tested
  • The data analysis was not done correctly.

We can’t fix the first three problems, but we can fix the fourth. When we do, we find that over 95% of the viral load was blocked by the cloth masks.

To improve protection for the wearer, a coffee filter or paper towel can be easily inserted into the mask to improve filtering. For instance, the Hong Kong Consumer Council recommended design includes a paper towel, after scientists “scanned kitchen paper towel under electronic microscope and revealed that the fabric size, gaps and layout of kitchen paper towel are similar to that of the middle layer of surgical mask”.

Won’t wearing masks make people just be less careful about physical distancing?

There is no evidence that mask use reduces compliance with other recommended strategies, such as physical distancing. Anecdotal evidence suggests that wearing masks is a useful reminder of the gravity of the situation, and may remind others to keep their distance. Historically, public health initiatives such as seat belts, condoms, and motorbike helmets are generally associated with concerns about negative outcomes due to increasing risky behavior, but overall population results have not borne out these concerns in practice.

As our paper explains (section B.2; see paper for citation details):

“One concern around public health messaging promoting the use of face-covering has been that members of the public may use risk compensation behavior and neglect physical distancing based on overvaluing the protection a surgical mask may offer due to an exaggerated or false sense of security (49). Similar arguments have previously been made for HIV prevention strategies (50) (51) and other safety devices and mandates such as motorcycle helmet laws (52) and seat-belts (53). However, research on these topics finds no such increase in adverse outcomes at the population level but rather improvements in safety and well-being, suggesting that even if risk compensation occurs in some individuals, that effect is dwarfed by the increased safety at the population level (53, 54). Further, even for deliberately high-risk recreational activities such as alpine skiing and snowboarding, wearing a helmet was generally associated with risk reduction oriented behavior (55), suggesting safety devices are both compatible with and perhaps encourage safety-oriented behavior. Even for high-risk recreational activities like alpine skiing and snowboarding, helmet use has greatly reduced injury rates (56).

In general, various forms of risk compensation theories have been proposed for many different safety innovations, but have not been found to have empirical support (57) at the population level. These findings strongly suggest that, instead of withholding a preventative tool, accompanying it with accurate messaging that combines different preventative measures would display trust in the general public’s ability to act responsibly and empower citizens, and risk compensation is unlikely to undo the positive benefits at the population level (58).”

Mightn’t people handle their masks wrong and make things worse?

As discussed, it appears that transmission of COVID-19 through surfaces is very rare. There are no reported cases that I’ve found that show transmission through an infected mask. Since there are now hundreds of millions of people around the world required to wear masks in public, we would expect to have seen examples of this by now.

The idea that wearing masks could increase risk due to touching it doesn’t stand up to scrutiny. If your mask has virus particles in it, there’s two possibilities for how they got there:

  • You’re already infected, in which case this isn’t really an issue, or
  • You’re not already infected, and the virus particles came from someone else, which means that your mask stopped them from going into your mouth.

COVID-19 is transmitted through the inside of the mouth, nose, or the eyes. If a mask stops virus particles from entering your mouth, then it’s done its job. People should be told to wash their mask when they get home, to minimize the chance that they get infected through an infected surface.

What if people touch their face more and infect themselves in the process?

As discussed, COVID-19 is transmitted through the inside of the mouth, nose, or the eyes. A mask covers the mouth and nose, making it much harder to accidentally touch them. People should be encouraged to avoid touching their mouth and nose regardless of whether or not they are wearing a mask.

Where am I going to get a mask anyway?

Masks can be made by cutting the ends off a sock, stapling rubber bands to a piece of kitchen towel, cutting the arms of a t-shirt, folding a handkerchief over hair-ties or rubber bands, by using a scarf or bandana, and so forth. There is no evidence to suggest that any of these masks are not effective at blocking droplets from an infected person.

Won’t this make people take masks away from healthcare workers?

Simple homemade cloth masks made of cut up cotton t shirts, paper towels, a handkerchief, etc are very effective for source control, so there’s no need to take medical masks away from healthcare workers. In regions that have mandated mask usage, most people are wearing DIY masks, not medical masks.

Isn’t wearing a mask a personal choice?

The Republican governor of Maryland, Larry Hogan, said “Some people have said that covering their faces infringes on their rights, but this isn’t just about your rights or protecting yourself; it’s about protecting your neighbors. And the best science that we have shows that people might not know that they’re carriers of the virus, through no fault of their own, and they could infect other people. Spreading this disease infringes on your neighbors’ rights.”

Making a “personal choice” to not wear a mask can put those around you at risk.

(Some people genuinely can’t safely wear a mask, of course, but that is a separate issue.)

Mightn’t wearing a mask cause people of color to get harassed?

Mandating universal mask wearing, rather than just recommending mask use, may have additional benefits such as reducing stigma. From our paper (section B.2; see paper for citation details):

For many infectious diseases, including, for example, tuberculosis, health authorities recommend masks only for those infected or people who are taking care of someone infected. However, research shows that many sick people are reluctant to wear a mask if it identifies them as sick, and thus end up not wearing them at all in an effort to avoid the stigma of illness (60, 61). Stigma is a powerful force in human societies, and many illnesses come with stigma for the sick as well as fear of them, and managing the stigma is an important part of the process of controlling epidemics as stigma also leads to people avoiding treatment as well as preventive measures that would “out” their illness (62). Many health authorities have recommended wearing masks for COVID-19 only if people are sick; however, reports of people wearing masks being attacked, shunned and stigmatized have already been observed (63). Having masks worn only by the suspected/confirmed infected also has led to employers in high-risk environments like grocery stores and prisons, and even hospitals, banning employees from wearing one sometimes with the idea that it would scare the customer or the patients (64, 65). Further, in many countries, minorities suffer additional stigma and assumptions of criminality (66). In that vein, black people in the United States have reported that they were reluctant to wear masks in public during this pandemic for fear of being mistaken as criminals (67, 68).

Isn’t wearing a mask something that only Asian cultures do?

Many Western regions have now mandated mask wearing in many public places, including many parts of the USA and many countries in Europe. There is no sign that Westerners are unable or unwilling to wear masks.