By Edward Chen
Masks are essential for stymying the transmission of COVID-19 because they block the SARS-CoV-2 virus, which causes the disease, from passing through. Whether it’s by reducing the spread of SARS-CoV-2 particles from infected individuals into their surrounding environment, or by restricting the passage of SARS-CoV-2 from the environment into the mouth and nose of a susceptible individual, the most common mode of transmission, masks are helpful for protecting both the wearer and their contacts.
Peer-reviewed studies have again and again demonstrated that wearing masks helps prevent the spread of COVID-19, including from asymptomatic and pre-symptomatic individuals who are unaware that they are infected, yet can spread the disease. This is especially important because such “silent transmission” from individuals who are asymptomatic and pre-symptomatic accounts for over 50% of new COVID-19 cases.
From a June 2020 study published in the Proceedings of the National Academy of Sciences before effective COVID-19 vaccines were available, “wearing of face masks in public corresponds to the most effective means to prevent interhuman transmission, and this inexpensive practice, in conjunction with simultaneous social distancing, quarantine, and contact tracing, represents the most likely fighting opportunity to stop the COVID-19 pandemic (emphasis added).” A joint US Centers for Disease Control and Prevention (CDC) and US Navy study of a COVID-19 outbreak onboard an aircraft carrier found that, even in the confined setting of a ship, service members who wore a face covering had a lower infection rate of 55.8% vs. the 80.8% infection rate of service members who didn’t use face coverings.
These differences are reflected on a regional level too. A widely quoted June 2020 study published in Health Affairs analyzed county-level data between March 31 and May 22, 2020 from Washington D.C. plus 15 US states that mandated community face mask use sometime between April 8 and May 15. The study found that mandating face mask use in public, which was defined differently depending on the state, reduced the growth of new COVID-19 cases by 0.9% within just 5 days and 2% after 3 weeks. The study also estimated that mandating face mask use averted between 230,000 and 450,000 new cases in the 53 days from March 31 to May 22, though the authors cautioned that these were estimates dependent on assumptions. The relatively short timespan studied did not allow for evaluating differences in deaths from COVID-19. A very recent March 5 study from the CDC corroborated these results and also additionally examined COVID-19 mortality (see footnote).
It’s no surprise, then, that the World Health Organization (WHO) and the CDC both recommend wearing masks. In fact, masks that fully cover the nose and mouth have been indefinitely required by the CDC since February 2, 2021 for traveling on all forms of public transportation. The CDC instructs transportation crewmembers to “[board] only those persons who wear masks,” including for international travel from foreign locations into the US, and to only allow people who wear masks to enter transportation hubs.
So masks are effective. What about "double masking"?
A new study first released by the CDC on February 10, 2021 evaluated the effectiveness of double masking, or “wearing a cloth mask over a medical procedure mask.” The study, which evaluated the effectiveness of different mask fits and configurations, reflected increasing scientific and CDC emphasis on maximizing the fit of masks. In the first experiment, researchers looked at how different mask configurations affected the emission of potassium chloride particles (which, at between 0.1 and 7 μm in diameter, are representative of SARS-CoV-2 particles). A disposable medical mask blocked 56.1% of the potassium chloride particles emitted by a simulated cough. A three-ply cloth mask was slightly worse at filtering, with 51.4% of particles blocked. Finally, 77.0% of particles were blocked with “knotting and tucking” a disposable mask, and 85.4% of particles were blocked in the double masking scenario.
In the second experiment, the researchers looked at not only the emission of particles (from the “source”), but the exposure of another person (the “receiver”) to these particles. The researchers tried 10 different combinations of mask configurations (no mask, single mask, knotting and tucking, and double masking) on the source and receiver. In the configurations where the source did not wear a mask, the receiver reduced their exposure to the potassium chloride particles by a mere 7.5% by wearing a disposable mask, 64.5% by knotting and tucking that disposable mask, and 83.0% by wearing a cloth mask on top of that disposable mask. When the source and receiver both knotted and tucked their single disposable mask, the exposure of the receiver was reduced by 95.9%. Exposure was reduced by 96.4%, the highest reduction observed in the study, when both source and receiver double masked. The authors did not test wearing two cloth masks or wearing two disposable masks. Regardless, the data demonstrates that “good fit can increase overall mask efficiency.” Based on this study, the CDC updated their recommendations to include double masking.
Even earlier, schools and news outlets alike had been considering the topic, with many quoting Dr. Anthony Fauci, director of the US National Institute of Allergy and Infectious Diseases, after he appeared on a January 25 episode of NBC’s Today show. In response to an anchor’s question on whether double masking “makes a difference,” Dr. Fauci responded that it “likely does… So if you have a physical covering with one layer, you put another layer on, it just makes common sense that it likely would be more effective.” At this earlier time, scientific studies were not yet available.
The CDC recommends combining a disposable, surgical mask with a cloth mask on top for “better fit and extra protection,” though they do not recommend combining two disposable masks because disposable masks “are not designed to fit tightly and wearing more than one will not improve fit.” Other options for improving fit include using mask fitters, using masks with metal nose wires, and tying knots on the ear loops of the mask, followed by tucking in extra mask material at the edges. This procedure of “knotting and tucking” is demonstrated in a video from the University of North Carolina. A JAMA Internal Medicine study experimenting with modified masks found that simply tying the ear loops and tucking the edge of the mask inward increased the “fitted filtration efficiency” from 38.5% for an unmodified mask to 60.3%.
The CDC has continued to stress the importance of wearing a “well-fitting” mask and recommends continuing to wear masks even after getting two doses of a COVID vaccine. It’s too early to know for sure exactly how well COVID-19 vaccines prevent the transmission of SARS-CoV-2, so wearing masks that filter out (or in this case, keep in) exhaled virus particles, and thereby protect people around the wearer, remains important. This is similar to why doctors wear masks around immunocompromised patients, even in non-pandemic times. (Based on data from Phase III clinical trials, AstraZeneca claims their vaccine reduces disease transmission by up to 67%. However, some unexplained discrepancies have been criticized. Even so, there isn’t data suggesting COVID vaccines fully prevent transmission).
Remember: Wear masks that fit snugly against your face and completely cover your nose and mouth, even if you are vaccinated. This may mean wearing a cloth mask on top of a disposable mask or tying the ear loops for a tighter fit. Gaps allow virus particles to leak in and out: Avoid it like the plague!
A more recent study released by the CDC on March 5, 2021 analyzed county-level data on cases and deaths over a longer range from March 1 to December 31, 2020. The CDC researchers found that state-issued mask mandates reduced the growth of new COVID-19 cases by 0.5% in the first 20 days and 1.8% between 81 and 100 days. Deaths were reduced by 0.7% in the first 20 days and 1.9% between 81 and 100 days, further supporting policies for mandatory mask use.
The study also looked at associations between reopening restaurants and COVID-19 case and death growth rates. Allowing on-premises dining (including both indoor and outdoor dining) was associated with increased COVID-19 case rates after about 6 weeks and increased COVID-19 death rates after about 2 months. Case and death growth rates were increased by 1.1% and 3.0%, respectively, between 81 and 100 days after reopening restaurants.
Results from ongoing research and the current understanding of COVID-19 are constantly evolving. This post contains information that was last updated on March 9, 2021.
Edward Chen is a master's student studying immunology. He's also the national president of Students vs. Pandemics. @EdwrdChen