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I’ve bemoaned the increasing politicization of school vaccine mandates, in which support for such mandates is starting to fracture, with the right increasingly allying itself with antivaxxers, having been attracted by antivaxxers’ clever use of rhetoric associated with “freedom” and “parental rights” to oppose strengthening vaccine mandates and support weakening them with very permissive “personal belief exemptions.” Although antivaxxers have been co-opting right wing rhetoric for a long time, I was particularly alarmed in 2015 when several candidates for the GOP nomination actively pandered to antivaccine views in the second Republican presidential debate. Little did I know that this year a pandemic would reveal that it’s not just school vaccine mandates that have become politicized, but public health measures in general. We see this in many forms, from the protests against lockdowns to the way that wearing or not wearing masks, of all things, has become a symbol of political identity, with antipathy towards them largely associated with the political right. (I suppose that, had I known more history about previous pandemics, especially the 1918-19 influenza pandemic and the rise of the “Anti-Mask League of San Francisco,” I would have been less surprised at the resistance to masks.) It also didn’t help that the initial messaging and science on masks was confusing and sometimes contradictory. Fortunately, there’s a new meta-analysis of what we know about masks and COVID-19 transmission published in The Lancet last week that helps clarify things and gives me an excuse to update Steve Novella’s excellent post “Do Masks Work?”

Disinformation, information, and confusion over masks

Before I get to the study, though, I want to relate a brief story regarding news articles published a week and a half ago that show a bit of the disinformation about masks that is flying around social media. In brief, The New York Post published a story entitled “Healthy people should wear masks only if caring for coronavirus patients, WHO says“. It was a republication of a Fox News story entitled “WHO guidance: Healthy people should wear masks only when ‘taking care of’ coronavirus patients“. Here’s an excerpt from the NY Post story:

The World Health Organization is recommending healthy people, including those who don’t exhibit COVID-19 symptoms, only wear masks when taking care of someone infected with the contagion, a sharp contrast from the advice given by American public health officials who recommend everyone wear a mask in public.

“If you do not have any [respiratory] symptoms such as fever, cough or runny nose, you do not need to wear a mask,” Dr. April Baller, a public health specialist for the WHO, says in a video on the world health body’s website. “Masks should only be used by health care workers, caretakers or by people who are sick with symptoms of fever and cough.”

The recommendation differs from the Centers for Disease Control and Prevention (CDC), which urges individuals to wear a mask or face covering in public settings, regardless of infection, to limit the spread of the virus.

Now here’s the Fox News version:

The World Health Organization is recommending healthy people, including those who don’t exhibit COVID-19 symptoms, only wear masks when taking care of someone infected with the contagion, a sharp contrast from the advice given by American public health officials who recommend everyone wear a mask in public.

“If you do not have any respiratory symptoms such as fever, cough or runny nose, you do not need to wear a mask,” Dr. April Baller, a public health specialist for the WHO, says in a video on the world health body’s website posted in March. “Masks should only be used by health care workers, caretakers or by people who are sick with symptoms of fever and cough.”

The recommendation has not changed and differs from the Centers for Disease Control and Prevention (CDC), which urges individuals to wear a mask or face-covering in public settings, regardless of infection or not, to limit the spread of the virus.

Notice any differences? First, both articles are framed as though this is a new recommendation, particularly the NY Post article, given its deceptive editing. Notice how the NY Post article removes the part about the videos being posted in March and the mention of how the WHO’s recommendations have not changed. Basically both are presenting this information to cast doubt on the CDC recommendation to wear masks, although the Fox News story is marginally less deceptive in that it admits that these are old recommendations that have not been updated, while the NY Post edited out the phrases pointing that out. At the time, I suspected that this bit of disinformation was the reaction to a story that the WHO was about to do an about-face on its mask recommendations:

Also, unlike what the two articles claim, the WHO was not telling people not to wear masks, as Stephan Neidenbach pointed out:

This is cherry picking what the WHO is actually saying. The WHO’s advice has not changed since the beginning of April. There is a global concern over a major shortage of medical masks. This is what the WHO is asking healthy people not to wear, unless they are in close proximity with those who are sick. They need to be saved for the medical community and sick patients.

WHO stresses that it is critical that medical masks and respirators be prioritized for health care workers.

The WHO is not making any global statement on non-medical masks at all. For that they are leaving it up to individual governments based on their own needs and considerations.

As you can see, there is no inherent contradiction in the WHO’s and CDC’s recommendations, just cherry picking of a passage by sources not known for their honesty or accuracy. That being said, though, it is true that the messaging on masks has been confusing and that the science has been less clear than we would like, as Steve himself pointed out. (My favorite line? “Wear the mask, but act as if the mask doesn’t work.”) So let’s see what this new study brings to the table. Will it help bring more clarity? My guess is: Yes, but not very much more. The reason is simple: GIGO (garbage in, garbage out). The quality of the studies that the meta-analysis relies on isn’t ideal.

Masks versus coronavirus: The evidence

This meta-analysis was funded by the WHO and carried out by the COVID-19 Systematic Urgent Review Group Effort (SURGE). The meta-analysis covered 172 observational studies and 44 comparative studies on SARS-CoV-2 (the coronavirus that causes COVID-19) and the related coronaviruses MERS-CoV (which causes Middle East Respiratory Syndrome) and SARS-CoV (the virus that causes the original SARS, first identified in 2002), doing a systematic review of the observational studies and a meta-analysis of the comparative studies. One could argue that not limiting the study to COVID-19 is a weakness, and perhaps it is, but these coronaviruses are highly related and COVID-19 is so new that there aren’t a lot of studies to look at.

In the introduction, the authors note:

SARS-CoV-2 spreads person-to-person through close contact and causes COVID-19. It has not been solved if SARS-CoV-2 might spread through aerosols from respiratory droplets; so far, air sampling has found virus RNA in some studies2, 3, 4 but not in others.5, 6, 7, 8 However, finding RNA virus is not necessarily indicative of replication-competent and infection-competent (viable) virus that could be transmissible. The distance from a patient that the virus is infective, and the optimum person-to-person physical distance, is uncertain. For the currently foreseeable future (ie, until a safe and effective vaccine or treatment becomes available), COVID-19 prevention will continue to rely on non-pharmaceutical interventions, including pandemic mitigation in community settings.9 Thus, quantitative assessment of physical distancing is relevant to inform safe interaction and care of patients with SARS-CoV-2 in both health-care and non-health-care settings. The definition of close contact or potentially exposed helps to risk stratify, contact trace, and develop guidance documents, but these definitions differ around the globe.

To contain widespread infection and to reduce morbidity and mortality among health-care workers and others in contact with potentially infected people, jurisdictions have issued conflicting advice about physical or social distancing. Use of face masks with or without eye protection to achieve additional protection is debated in the mainstream media and by public health authorities, in particular the use of face masks for the general population;10 moreover, optimum use of face masks in health-care settings, which have been used for decades for infection prevention, is facing challenges amid personal protective equipment (PPE) shortages.11

You might have noted lots of studies being publicized that are presented in alarmist terms because viral RNA is detected in the air or on surfaces. That’s why it’s really important to note, as the authors do, that detection of viral RNA alone does not mean that there’s infectious virus there. It’s worrisome, to be sure, but you have to remember that the virus could be inactivated and thus not infectious in those locations. That being said, the authors also note that, oddly enough, before their article, there were no comprehensive reviews of information on SARS-CoV-2 or related betacoronaviruses that have caused epidemics; so they decided to systematically review the effect of physical distance, face masks, and eye protection on transmission of SARS-CoV-2, SARS-CoV, and MERS-CoV.

In their review, the outcomes of interest were: risk of transmission (WHO-defined confirmed or probable) to people in health care or non-health-care settings by those infected; hospitalization; intensive care unit admission; death; time to recover; adverse effects of interventions; and “contextual factors such as acceptability, feasibility, effect on equity, and resource considerations related to the interventions of interest.” Confirmed cases were defined as requiring laboratory confirmation (with or without symptoms), while probable cases had clinical evidence of infection but in whom laboratory testing hadn’t been done to confirm infection or was inconclusive.

The article is open-access; so you can look at the data too. There were some studies that covered COVID-19, 64 studies, of which only seven were comparative in design. Studies included came from all around the world, as well, although, unsurprisingly given that SARS and COVID-19 both originated in Asia, China and other Asian countries were heavily represented. Risk of bias was generally low, and the authors didn’t find any strong evidence of publication bias.

Here’s the flowchart showing how studies were chosen:

In terms of physical distancing, the authors found that a physical distance of more than 1 meter is associated with an 82% decrease in the risk of transmission (adjusted odds ratio [aOR] 0·18, 95% CI 0·09–0·38). Specifically, a subanalysis of 29 unadjusted and 9 adjusted studies found that the absolute risk of infection from an exposed individual was 12.8% at 1 m and 2.6% at 2 m. Reassuringly, this risk remained the same even when the six COVID-19 studies in the subanalysis were isolated. A little surprisingly (to me, anyway), the risk was also the same in health care or non–health-care settings. Finally, each meter of increased distance resulted in a doubling in the change in relative risk.

When it came to masks, an analysis of 29 unadjusted and 10 adjusted studies demonstrated that the use of masks was also associated with a large decrease in transmission, both for N95 masks and for disposable surgical masks or similar reusable 12- to 16-layer cotton masks. (N95 masks are called that because they filter out 95% of particles. They are thicker, more difficult to use correctly, and have to fit correctly to be effective.) Overall, the authors reported that masks and respirators reduce the risk of infection by 85% (aOR 0·15, 95% CI 0·07–0·34), with greater effectiveness in health care settings than in the community, a difference that the authors attribute to the greatly increased use of N95 masks in healthcare settings compared to in the community. Indeed, in a subanalysis they found that respirators were 96% effective (aOR 0·04, 95% CI 0·004–0·30), while other types of masks were which were 67% effective (aOR 0·33, 95% CI 0·17–0·61; pinteraction=0·090). Finally, the authors note that eye protection (e.g., visors, goggles, face shields) is similarly associated with a decreased risk of transmission. Specifically protection also was associated with less infection (n=3713; aOR 0·22, 95% CI 0·12 to 0·39, RD −10·6%, 95% CI −12·5 to −7·7; low certainty).

In the discussion, the authors note:

We found evidence of moderate certainty that current policies of at least 1 m physical distancing are probably associated with a large reduction in infection, and that distances of 2 m might be more effective, as implemented in some countries. We also provide estimates for 3 m. The main benefit of physical distancing measures is to prevent onward transmission and, thereby, reduce the adverse outcomes of SARS-CoV-2 infection. Hence, the results of our current review support the implementation of a policy of physical distancing of at least 1 m and, if feasible, 2 m or more. Our findings also provide robust estimates to inform models and contact tracing used to plan and strategise for pandemic response efforts at multiple levels.

The use of face masks was protective for both health-care workers and people in the community exposed to infection, with both the frequentist and Bayesian analyses lending support to face mask use irrespective of setting. Our unadjusted analyses might, at first impression, suggest use of face masks in the community setting to be less effective than in the health-care setting, but after accounting for differential N95 respirator use between health-care and non-health-care settings, we did not detect any striking differences in effectiveness of face mask use between settings.

The study does have some limitations, the most significant of which is that all the studies included in the systematic review and meta-analysis were not randomized and most were observational, which means that they weren’t always adjusted, and might suffer from recall and measurement bias (e.g., direct contact in some studies might not be measuring near distance). This is the “GIGO” problem that I mentioned at the beginning; the quality of a number of the studies was not very high, and eight of the included haven’t been peer-reviewed yet, currently existing only on preprint servers. It also didn’t assess the effect of duration of contact with persons infected with coronavirus because, as the authors point out, whether or not this variable was judged a risk factor considerably varied across studies, from any duration to a minimum of 1 h. Still, this is probably the best evidence existing thus far supporting social distancing, facemasks, and possibly eye protection to slow the spread of COVID-19.

The authors conclude:

Our comprehensive systematic review provides the best available information on three simple and common interventions to combat the immediate threat of COVID-19, while new evidence on pharmacological treatments, vaccines, and other personal protective strategies is being generated. Physical distancing of at least 1 m is strongly associated with protection, but distances of up to 2 m might be more effective. Although direct evidence is limited, the optimum use of face masks, in particular N95 or similar respirators in health-care settings and 12–16-layer cotton or surgical masks in the community, could depend on contextual factors; action is needed at all levels to address the paucity of better evidence. Eye protection might provide additional benefits. Globally collaborative and well conducted studies, including randomised trials, of different personal protective strategies are needed regardless of the challenges, but this systematic appraisal of currently best available evidence could be considered to inform interim guidance.

I’d agree that this is the best evidence we currently have, but I’d also caution that there is still a lot of uncertainty. Also, as Steve pointed out in his previous post, people stink at using masks properly. He suggested that that’s because they aren’t medical personnel and aren’t used to wearing masks, but even medical personnel tend to adjust their masks take them off, or even fail to cover their noses with them. Even I struggle with this sometimes, and I’m a surgeon; I’ve been wearing surgical masks, sometimes for many hours at a time, since I was a young man.

This study led C Raina MacIntyre and Quanyi Wang to conclude in an accompanying editorial:

For health-care workers on COVID-19 wards, a respirator should be the minimum standard of care. This study by Chu and colleagues should prompt a review of all guidelines that recommend a medical mask for health workers caring for COVID-19 patients. Although medical masks do protect, the occupational health and safety of health workers should be the highest priority and the precautionary principle should be applied. Preventable infections in health workers can result not only in deaths but also in large numbers of health workers being quarantined and nosocomial outbreaks. In the National Health Service trusts in the UK, up to one in five health workers have been infected with COVID-19,10 which is an unacceptable risk for front-line workers. To address global shortages of PPE, countries should take responsibility for scaling up production rather than expecting health workers to work in suboptimum PPE.11

Chu and colleagues also report that respirators and multilayer masks are more protective than are single layer masks. This finding is vital to inform the proliferation of home-made cloth mask designs, many of which are single-layered. A well designed cloth mask should have water-resistant fabric, multiple layers, and good facial fit.12 This study supports universal face mask use, because masks were equally effective in both health-care and community settings when adjusted for type of mask use. Growing evidence for presymptomatic and asymptomatic transmission of SARS-CoV-213 further supports universal face mask use and distancing. In regions with a high incidence of COVID-19, universal face mask use combined with physical distancing could reduce the rate of infection (flatten the curve), even with modestly effective masks.14 Universal face mask use might enable safe lifting of restrictions in communities seeking to resume normal activities and could protect people in crowded public settings and within households. Masks worn within households in Beijing, China, prevented secondary transmission of SARS-CoV-2 if worn before symptom onset of the index case.15 Finally, Chu and colleagues reiterate that no one intervention is completely protective and that combinations of physical distancing, face mask use, and other interventions are needed to mitigate the COVID-19 pandemic until we have an effective vaccine. Until randomised controlled trial data are available, this study provides the best specific evidence for COVID-19 prevention.

As Steve mentioned before, the issue is indeed complex, but I agree that on COVID-19 wards N95 respirators should be the minimum level of protection for healthcare workers. Indeed, in our hospital, they’re now required in the operating room as well, even though we test all of our patients for COVID-19 before surgery. The authors cite two randomized trials currently being carried out. One is a randomized controlled trial of N95 masks versus standard medical masks among nurses taking care of COVID-19 patients; the other is a Danish study in which participants are first screened for COVID-19 antibodies and then randomized to wear masks or not outside the hospital. At the end of the trial, if they haven’t developed symptoms necessitating swab testing for COVID-19 they will be retested for COVID-19 antibodies. Personally, in light of this systematic review and meta-analysis, I have to question whether these studies can be completed because, for the nurses taking care of COVID-19 patients at least, I don’t see how there can be clinical equipoise (genuine uncertainty about which group will do better). I know that, if I were taking care of COVID-19 patients all the time, I wouldn’t be too thrilled to be randomized to the regular medical facemask group.

In terms of cloth masks, the evidence is more mixed than it is for medical masks and N95 respirators. For instance, a recent review of the literature by Clase et al published two weeks ago notes that, even though there was no direct evidence (before the SURGE review) that cloth masks block transmission, it is still reasonable to recommend them based on their ability to decrease contamination of public spaces and reduce aerosol droplets:

When we apply the principles of evidence-based medicine to public policy, there is high-quality, consistent evidence that many (but not all) cloth masks reduce droplet and aerosol transmission and may be effective in reducing contamination of the environment by any virus, including SARS-CoV-2. No direct evidence indicates that public mask wearing protects either the wearer or others. Given the severity of this pandemic and the difficulty of control, we suggest that the possible benefit of a modest reduction in transmission likely outweighs the possibility of harm. Reduced outward transmission and reduced contamination of the environment are the major proposed mechanisms, and we suggest appealing to altruism and the need to protect others. We recognize the potential for unintended consequences, such as use of formal personal protective equipment by the general public, incorrect use of cloth masks, or reduced hand hygiene because of a false sense of security; these can be mitigated by controlling the distribution of personal protective equipment, clear messaging, public education, and social pressure.

It’s a complicated question, and I tend to agree, although personally when I’m out and about in a store I tend to wear one of my medical masks. Certainly, as the review/opinion piece says, there’s no good evidence that cloth masks increase the risk of transmission and prior plausibility combined with evidence that they decrease the emission of respiratory droplets suggest that they are worthwhile to wear. There is also newer evidence from Asia that East Asian countries’ early adoption of masks has played a role in their ability to control the transmission of COVID-19 and that Japan’s culture of wearing masks when ill could explain why it has escaped relatively unscathed compared to many nations.

A review published in Science a week and a half ago makes a similar argument.

Airborne spread from undiagnosed infections will continuously undermine the effectiveness of even the most vigorous testing, tracing, and social distancing programs. After evidence revealed that airborne transmission by asymptomatic individuals might be a key driver in the global spread of COVID-19, the CDC recommended the use of cloth face coverings. Masks provide a critical barrier, reducing the number of infectious viruses in exhaled breath, especially of asymptomatic people and those with mild symptoms (12) (see the figure). Surgical mask material reduces the likelihood and severity of COVID-19 by substantially reducing airborne viral concentrations (13). Masks can also protect uninfected individuals from SARS-CoV-2 aerosols and droplets (13, 14). Thus, it is particularly important to wear masks in locations with conditions that can accumulate high concentrations of viruses, such as health care settings, airplanes, restaurants, and other crowded places with reduced ventilation. The aerosol filtering efficiency of different materials, thicknesses, and layers used in properly fitted homemade masks was recently found to be similar to that of the medical masks that were tested (14). Thus, the option of universal masking is no longer held back by shortages.

From epidemiological data, places that have been most effective in reducing the spread of COVID-19 have implemented universal masking, including Taiwan, Japan, Hong Kong, Singapore, and South Korea. In the battle against COVID-19, Taiwan (population 24 million, first COVID-19 case 21 January 2020) did not implement a lockdown during the pandemic, yet maintained a low incidence of 441 cases and 7 deaths (as of 21 May 2020). By contrast, the state of New York (population ~20 million, first COVID case 1 March 2020), had a higher number of cases (353,000) and deaths (24,000). By quickly activating its epidemic response plan that was established after the SARS outbreak, the Taiwanese government enacted a set of proactive measures that successfully prevented the spread of SARS-CoV-2, including setting up a central epidemic command center in January, using technologies to detect and track infected patients and their close contacts, and perhaps most importantly, requesting people to wear masks in public places. The government also ensured the availability of medical masks by banning mask manufacturers from exporting them, implementing a system to ensure that every citizen could acquire masks at reasonable prices, and increasing the production of masks. In other countries, there have been widespread shortages of masks, resulting in most residents not having access to any form of medical mask (15). This striking difference in the availability and widespread adoption of wearing masks likely influenced the low number of COVID-19 cases.

As does Greenhalgh et al in The BMJ:

Substantial indirect evidence exists to support the argument for the public wearing masks in the covid-19 pandemic. The virus has been shown to remain viable in the air for several hours when released in an aerosol under experimental conditions,18 and such aerosols seem to be blocked by surgical masks in laboratory experiments.19 Individuals have been shown to be infectious up to 2.5 days before symptom onset,20 and as many as 50% of infections seem to occur from presymptomatic individuals.21 Community prevalence of covid-19 in many countries is likely to be high.22 Modelling studies suggest that even a small reduction in community transmission could make a major difference to demand elsewhere in the system (eg, for hospital bed space and ventilators).23

Again, the evidence is not as clear as we would like, but, on balance, it is plausible that masks prevent transmission, and there is good evidence that they decrease levels of aerosols, which is particularly important given that we know that asymptomatic people can transmit the virus. Now, with this new systematic review and meta-analysis, there is moderate quality evidence that they do prevent COVID-19 transmission, both in healthcare settings and in community settings.

So what now?

As you might have gathered, even this “best evidence” so far is only good enough to lead the authors to say that they are “moderately” certain that social distancing and the wearing of masks are likely to produce a large decrease in COVID-19 transmission. On the other hand, in a pandemic involving a virus that was only identified and sequenced less than five months ago, what that means is that a lot of the science is still uncertain and will change. Worse, because everyone is working so fast and papers are being rushed through, peer review sometimes suffers. Retraction Watch maintains a list of COVID-19 papers that have been retracted thus far that continues to grow, and two very high profile COVID-19 papers were retracted by The Lancet and The New England Journal of Medicine last week because no one bothered to look into whether a tiny company could produce the sort of database whose data were analyzed in the paper and that company refused to let an independent third party audit of its data. On the mask front, a study purporting to show that masks don’t stop COVID-19 spread was retracted because it was pointed out that the authors didn’t recognize the concept of limit of detection (LOD) of its in-house test for COVID-19 and didn’t express their findings below the LOD as “< LOD." Given how many of their measurements were below the LOD, their results were basically uninterpretable. It was a widely cited study, too, referenced by dozens of news stories, nearly 10,000 Twitter users, and the World Health Organization. These sorts of retractions add to the feeling of mistrust of science among too many members of the public.

The bottom line remains, though, that we have to learn to go with what we know, even in the face of uncertainty, even in the face of crappy retracted science papers, and even in this age of political division, and, right here, right now, there is more than enough evidence to suggest that mask wearing significantly decreases the risk of COVID-19 transmission. As Steve wrote last week, this is a marathon, not a sprint, and it is science that will eventually win the day. As I’ve said many times before, science as process can be messy, and in a pandemic every misstep, each of which would probably have made little stir if it were in a different area of science under different circumstances, will be magnified and politicized. We need to keep that in mind and view with caution each new discovery breathtakingly announced to the public.

And, yes, we need to wear our masks but, as Steve put it, continue to behave as though they don’t work very well. Eventually science will settle the issue, but until it does the balance of evidence supports routine wearing of masks. It’s also important to remember that masks are not the be-all and end-all. Social distancing, masks, and face protection all work together to reduce the risk of coronavirus transmission as low as feasible.

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Posted by David Gorski

Dr. Gorski's full information can be found here, along with information for patients. David H. Gorski, MD, PhD, FACS is a surgical oncologist at the Barbara Ann Karmanos Cancer Institute specializing in breast cancer surgery, where he also serves as the American College of Surgeons Committee on Cancer Liaison Physician as well as an Associate Professor of Surgery and member of the faculty of the Graduate Program in Cancer Biology at Wayne State University. If you are a potential patient and found this page through a Google search, please check out Dr. Gorski's biographical information, disclaimers regarding his writings, and notice to patients here.