Do We Have High-Level Evidence that Masks Work to Reduce COVID Spread? Grading the Evidence…

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[Please note that later this next week we will be moving COVID content from the Regenexx website to its own home on the web …]

As a physician who publishes a few scientific papers a year and runs both clinical and lab-based research teams in orthobiologics, it’s been interesting to observe the science behind the spread of COVID-19 and mask-wearing. This topic itself is enough to get many people incensed and angry on both sides of the ongoing debate. However, most on either side have no experience in how to grade the quality of the scientific evidence. So today I will teach you to think like a scientist. Let’s dive in.

First a Word on What I Do Everyday

Before I begin, here’s what I do personally:

  • As a healthcare worker, I wear an N-95 respirator at all times in the clinic and during procedures.
  • All of my staff is also required to wear N-95 respirators in the clinic-NO EXCEPTIONS.
  • All of my patients MUST be masked in our clinic. If they don’t have a mask we will provide them a high-quality medical-grade surgical mask (the same type that surgeons wear during a procedure)-NO EXCEPTIONS.
  • When I’m off duty, I personally wear a high-quality cloth surgical mask every place they tell me to indoors and outdoors.

Types of Masks

There are several different types of masks out there:

  • Cloth masks-This spans the gamut from a bandana to a scarf to now purpose-made fashion masks.
  • Surgical masks-These are disposable masks worn by surgeons in surgery.
  • KN95 Masks-The Chinese version of a mask that fits snugly to the face and filters 95% of all particles.
  • N95 Masks-The US and European version of a mask that fits tightly to the face and filters 95% of all particles.

My review here is on mask types that the public has easy access to right now which is surgical and cloth masks and NOT N-95 respirators. Note that the research on N-95 respirators preventing the spread of respiratory viruses is much more robust. 

What I Have Already Published on the Science Behind Mask Use

I’ve reviewed a few dozen existing and new studies on mask use and COVID-19. Hence, I won’t rehash all of those studies here in detail. Hence, if you want a good primer in what’s going on with this research, please see these selected posts:

Learning About Levels of Evidence in Medicine

The key take-home message of this blog is that all medical research can be “graded”. If it earns high grades, you can usually take the conclusions to the bank. So when terms like “proven” are used, they can only be used when we have multiple high-level studies supporting the treatment or intervention.

This is not a new concept as medical evidence grading scales have been around for decades (2).  Meaning that every researcher who publishes anything in medicine is required to understand these grades. Every physician who graduates medical school is also very familiar with these concepts.

In the table below, I applied a standard grading scale for medical evidence to the common types of research studies out there on mask use and COVID-19 transmission. In order to fit it all in and do justice to every type of research design on mask use, I stretched the scale out to 10 grades rather than the usual 5. I kept “Level-1” the highest grade. I then gave details on the type of study and an example for each. Please take a few minutes to review the table so the chart below makes sense.

First, it’s critical to note that only studies that meet level 1 or 2 on this chart get to use the term “proven”. In fact, real sticklers in the realm of evidence-based medicine would argue that only studies that meet level 1 can use that term. Everything else is “unproven”. Having said that, studies in level 3 would be pretty good evidence. Level 4 here would be “OK” evidence that could at least be passable to start a discussion. Level 5 and below is all interesting stuff and needed to start building an evidence base, but regrettably isn’t sufficient to prove that masks reduce the spread of COVID-19.

Charting All of the COVID Mask Studies

I performed a deep dive into all of the COVID mask studies out there. The goal here was to include only those studies on the SARS-CoV2 virus where transmission of the virus was measured directly or indirectly or general lab studies or computer models focused on COVID-19.

I initially was hesitant to include studies that were not yet published or case reports cited in news articles, but I included them because a “web writer” at UCSF wrote an article on mask use and these were her “references” (which is very poor research methodology) (1). I only included those very sketchy pieces of data here because patients have cited this UCSF article often.

There are many studies performed on various masks and water droplets in the lab. However, almost all are pre-COVID, so here I focused on those that were published post-COVID and covered in the media as “proof” that masks work.

On the chart below I have the “Proof Zone” in green as level 1 and 2 studies. The next level down I called the “Reasonable Zone” with level 3 and 4 studies and that’s in yellow. All of the study citations are listed at the bottom of the page using the same names as on the chart. The horizontal axis lists the “nicknames” I gave to each study and the vertical axis represents the levels of evidence shown above in the table. Each data point is labeled with a number 1-10 denoting how that study graded out.

Here’s how the studies I was able to find fared:

As you can see, not a single study or case report used in the media as “proof” that mask use prevents the spread of COVID-19 rose to high-level or moderate level grades of clinical evidence. To get there, we would need a COVID-19 study that tested transmission with and without masks in a controlled fashion. That study doesn’t exist right now.

How About Other Studies?

There are a myriad of studies that were published pre-COVID and are not about the SARS-CoV2 virus. Their results run the gamut, but as I have reviewed before, they mostly show that cloth and surgical masks don’t reduce the transmission rate of respiratory viruses. A few show some efficacy of surgical masks only in certain circumstances. However, few of these studies reach into the “Proof Zone” shown above.  I will cover those and chart them in a later blog.

What Can We Say About the Level of Evidence for COVID Mask Studies?

We definitely can’t say that we have any research that proves that masks help stop or reduce the spread of COVID-19. That’s just not supported by the data right now. We could say something like: “Based on lower-level evidence, we believe mask-wearing may help reduce the spread of COVID-19, but higher-level evidence has yet to be published”. So why won’t the public health experts and scientists just level with the public? Good question.

Why is my conclusion so much different than the “experts”? I’m the only one so far willing to grade out the quality of the research we have and show you how it stacks up. With the torrent of media and social media pressure creating a tsunami of convergent opinions that it’s self-evident that mask use helps, that’s a tough thing to report.

“You can’t handle the truth…”

You may remember the science from “A Few Good Men” when Jack Nickelson is being cross-examined by Tom Cruise:

Tom is a Navy lawyer cross-examining a decorated marine colonel and is veering very close to insubordination by pressing for answers. The famous line that Jack screams back at Tom is “You can’t handle the truth…”

I’ve had quite a few commenters basically say the same thing. Even though they recognize that there may be gaps in our COVID mask use data, we shouldn’t level with the public on that topic. Why? Because telling them the truth would reduce mask use which could put lives at risk. Hence a little white lie never hurt anybody.

I can certainly see the logic of those commenters, but I also disagree. I think we can tell the public the truth in such a way as to inspire them to protect others. That often begins with the truth and works backward.

More Studies to Come

As more studies are published, I’ll place them on our grading scale above and let you decide. I personally hope that we see higher-level evidence published. As I’ve said, I already obey all of the mask recommendations, so it won’t change my life one bit if we have better research showing that masks work.

The upshot? At the end of the day, the research that we have on mask use and COVID-19 is of poor quality. That “is what that is”, meaning it objectively grades out a certain way. I sincerely hope we see better studies. In the meantime, I’ll continue wearing my masks until they tell me not to.


Chart References:

Jena Germany Comparison: IZA Institute of Labor Economics. Face Masks Considerably Reduce COVID-19 Cases in Germany: A Synthetic Control Method Approach. IZA DP No. 13319, June 2020

Kai Computer Model: Kai et al. Universal Masking is Urgent in the COVID-19 Pandemic: SEIR and Agent Based Models, Empirical Validation,
Policy Recommendations. Arxiv {Pre-print Server].

LASER Droplet Study-Anfinrud P, Stadnytskyi V, Bax CE, Bax A. Visualizing Speech-Generated Oral Fluid Droplets with Laser Light Scattering. N Engl J Med. 2020;382(21):2061-2063. doi:10.1056/NEJMc2007800

Health Affairs US Study-Lyu W, Welby G. Community Use Of Face Masks And COVID-19: Evidence From A Natural Experiment Of State Mandates In The US. Health Affairs.

Lefler Country Mask Use-Leffler, Christopher & Ing, Edsel & Lykins, Joseph & Hogan, Matthew & McKeown, Craig & Grzybowski, Andrzej. (2020). Association of country-wide coronavirus mortality with demographics, testing, lockdowns, and public wearing of masks (Update June 15, 2020). Accessed 7/11/20.

Airplane Case Report-Schwartz K, et al. Lack of COVID-19 transmission on an international flight. CMAJ April 14, 2020 192 (15) E410; DOI:

Missouri Hairstylist-Washington Post. The Outbreak that Didn’t Happen: Masks Credited with Preventing Coronavirus Spread Inside a Missouri Hair Salon. Accessed 7/11/20.

IHME Mask Model-University of Washington. The Institute for Health Metrics and Evaluation. New IHME COVID-19 Model Projects Nearly 180,000 US Deaths. Accessed 7/11/20.

Zhang China Study-Renyi Zhang, Yixin Li, Annie L. Zhang, Yuan Wang, Mario J. Molina. Identifying airborne transmission as the dominant route for the spread of COVID-19. Proceedings of the National Academy of Sciences, 2020; 202009637 DOI: 10.1073/pnas.2009637117

Stutt Model-Stutt R, et al. A modelling framework to assess the likely effectiveness of facemasks in combination with ‘lock-down’ in managing the COVID-19 pandemic. Proceedings of the Royal Society A. 10 June 2020

Chinese Healthcare Workers-Ng K, Poon BH, Kiat Puar TH, et al. COVID-19 and the Risk to Health Care Workers: A Case Report. Ann Intern Med. 2020;172(11):766-767. doi:10.7326/L20-0175

Cloth Facemasks Respiratory Jets-Verma S, Dhanak M, Frankenfield J. Visualizing the effectiveness of face masks in obstructing respiratory jets. Phys Fluids (1994). 2020;32(6):061708. doi:10.1063/5.0016018

Household Facemask Use in China-Wang Y, Tian H, Zhang L, et al. Reduction of secondary transmission of SARS-CoV-2 in households by face mask use, disinfection and social distancing: a cohort study in Beijing, ChinaBMJ Global Health 2020;5:e002794.


General References:

(1) University of California San Francisco. Still Confused About Masks? Here’s the Science Behind How Face Masks Prevent Coronavirus. Accessed 7/11/20.

(2) The University of Oxford. Oxford Centre for Evidence-based Medicine – Levels of Evidence (March 2009). Accessed 7/11/20.

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18 thoughts on “Do We Have High-Level Evidence that Masks Work to Reduce COVID Spread? Grading the Evidence…

  1. Frank Romeo

    Jeez, telling the truth as we know it is a problem for the CDC and political powers to be. In general the “Experts” have been all over the map with recommendations. From Covid 19 is nothing to worry about ( Jan- early Feb.) to this is the worst pandemic mankind has faced ( Fauci says where may never be a cure or vaccine and the death rate will exceed any corona virus known to man). Yes wearing a mask is advisable particularly in your field of medicine. We must all make our work places as safe against virus propagation. But life must go on, as you’ve stated and I believe the numbers bare out the facts that the collateral deaths from, depression, suicide, drug overdose, and the abandonment of our previous levels of rigorous, testing and treatments of other diseases, like cancer not to mention the loss of quality of life issues associated with optional surgeries and treatments is high and largely unquantifiable. We can’t even trust the new case numbers, they commingle antigen and virus test results making a mess of the daily statistics. Ultimately we have to achieve herd immunity through vaccine or exposure, ipso facto. Sequestering everyone for a lifetime is unachievable, doing it for a year will crush the economy causing further death and destruction. Please keep writing reasoned scientifically based pieces.

  2. Vicki

    What about the adverse effects of mask wearing, such as breathing in your own CO2, accumulated germs on the mask and the wearer touching the mask then touching their eyes or other objects, sweating while wearing the mask, light-headedness from wearing the mask, headaches from wearing the mask, etc.

    If the masks have not been proven to be effective against minute virus particles, and are worn loosely and air is inhaled and exhaled around the sides, and they cause a plethora of problems with prolonged wearing, how can cities/counties mandate that people wear them inside of buildings?

    1. Chris Centeno, MD Post author

      Vicki, we also don’t have much data that mask-wearing is outright harmful. However, reviewing what we know should make a good blog at some point!

  3. Randy Ice PT, CCS

    OSHA has rules that employees who wear masks are to stop working if the O2 concentration inside the mask drops to 19.5% or less. Virtually all masks do that as seen in any number of YouTube videos demonstrating levels between 16 and 19.5%, Clearly they believe this is harmful to health or they would not have this rule. Empirically if you are older or have a chronic disease such as COPD, restrictive lung disease, pulmonary fibrosis or heart disease (CAD, CHF) the risk of breathing a reduced oxygen concentration as well as potential hypercapnia is clearly a danger. Add in people who exercise with a mask on via running, cycling or in a gym as I see all the time and I personally believe you have a recipe for a disaster.

    Several studies also show an increase risk of respiratory infections, the very disease masks are supposed to prevent.

    Here’s an article published today by a physicist who also concludes masks have no benefits in preventing disease transmission.

    Frankly the data you and others have presented indicates that probably THE most effective way of preventing this or any flu is to supplement with enough vitamin D3 to achieve a blood 25 hydroxyvitamin D level of 40 – 60 ng/ml at a minimum along other usual good health practices ( get enough sleep, exercise, eat real foods, reducing stress (ha!) and maintain a good attitude to avoid anxiety and depression).

  4. Peter Salvatori

    Hopefully this isn’t too far off topic. I thought this USA Today article was an interesting read. I always thought N95 respirators filtered 95% of particules of 0.3 microns (and assumed the efficacy got worse as the particle size got smaller). According to the article N95 has the worst filtration around 0.3 micron, but improves for smaller particles due to “Brownian motion” and secondly electrostatic absorption. There has been more discussion lately about potential spread due to smaller droplets (areosols) so I thought I’d put this information out there.

  5. Justin

    Hi, have you seen the Goldman Sachs research on masks? I figure the folks at Goldman are motivated by money and if they say “These calculations imply that a face mask mandate could potentially substitute for lockdowns that would otherwise subtract nearly 5% from GDP” then they mean it. :) I’m curious about your take.

    BTW: I definitely appreciate your blog posts. We need diverse opinions out there.

    1. Chris Centeno, MD Post author

      They just ripped off the Health Affairs study! I graded that as poor evidence. Hence, these guys should stick to financial stuff!

  6. Glena

    Thanks for another intelligent article! Hard to come by. The idea that it’s better to lie to the public than tell the truth is, unfortunately a Hobbesian and long term practice here and probably most everywhere. (WMDs anyone?) So thanks for continuing to present sober information on an increasingly politicized issue.

  7. Stacey Kaufman

    Scary and confusing. I’m flying in next week for a PICL, and we’ll be wearing KN95 masks with an “Easy Flow” hepa filter air flow device attached, (have you heard of or looked at these?. Also wearing face shields. No idea if it will help, but hoping 🤞🤦🏻‍♀️🤷🏻‍♀️.

    1. Chris Centeno, MD Post author

      N-95/KN-95 masks are much more effective, as would a HEPA filtered mask. I haven’t looked at the “Easy Flow”.

  8. Joe LaValle

    So let’s flip the script….are there any high level studies —- levels 1 through 4 — that look the opposite way and prove masks, specifically readily available 3 ply surgical masks — are completely ineffective at preventing the spread of COVID19? Your earlier blogs pointed to a number of studies that you referenced as proving masks don’t work – but it would seem that most, if not all of these studies are also of low quality (5 or below on your scale) and are thus unreliable. And the handful of these studies I reviewed seemed to suggest that masks, especially surgical masks, are actually moderately effective in preventing the spread of influenza like illnesses instead of being ineffective. So if there are no high level studies showing masks are categorically ineffective, wouldn’t it be prudent to base – or at least weight – decisions during a pandemic on the preponderance of lower level studies? For example, should doctors ignore the evidence that steroid use significantly reduces COVID19 mortality because it came from an observational study? Or skip blood thinners to prevent clots as no double blind randomized trial of blood thinners to prevent Covid clotting has been conducted (at least to my knowledge)? Same with remdesivir usage – the FDA approved its use based on a single trial that showed only marginal benefit. And should clinical trials of any level that show efficacy of surgical masks in preventing the spread of covid 19 not factor into decision making on masks because the influenza virus and not the specific coronavirus was tested – even though flu and coronavirus are of similar size? I am certainly no health care expert – but it seems that the weight of evidence suggests a moderate benefit in face coverings in reducing the spread of covid 19. If there are high level studies that show masks just don’t work at all, please post a link – as I’d like to review. Thanks for the info provided so far….

    1. Chris Centeno, MD Post author

      Joe, as I said in medicine, everything is ineffective until proven effective. For medical care in one person, physicians have leeway to try new things. For public health policy, we should use data-driven decisions. That data should be high level. Or if it’s poor quality, just level with the public that we really don’t know and appeal to their better natures. Personally, I think it’s the misinformation on this topic (masks don’t work, they definitely work) that has caused poor compliance.
      You’re also preaching to the choir as I am a mask wearer. Hence, I think ultimately we’ll have to agree to disagree on this one. Nice chatting with you via comments.

    1. Chris Centeno, MD Post author

      Not much scientific analysis there…

  9. Rondo

    This Dr. says masks reduce the viral load. I don’t think he is right, given what I’ve read on your blog

    1. Chris Centeno, MD Post author

      Regrettably, I don’t hear an ounce of actual review of the science.

  10. Rondo

    Thanks for your level headed input.

  11. Laura H

    As someone who has been changing the design of my own cloth masks for several months now, I am always interested in articles investigating the effectiveness of different types of construction. One problem with evaluating mask use is that different fabrications of cloth masks & tightness of fit makes studying other than only one type similar to comparing apples & oranges. This is my latest source for the effectiveness of fabrics just to demo variations:

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