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The following is a curated review of key information and literature about this topic. It is not comprehensive of all data related to this subject.
The use of masks to prevent and/or reduce transmission of SARS-CoV-2 has been a common mitigation strategy in countries throughout the world. While high-quality randomized controlled trials on the use of masking to prevent transmission of SARS-CoV-2 are limited, epidemiologic studies have provided support for the use of masks as a low-risk nonpharmaceutical intervention with significant potential benefits.
The rate of asymptomatic COVID-19 disease is high, and asymptomatic and presymptomatic transmission is known to occur (Moriarty, March 2020; Kimball, April 2020; Wei, April 2020; He, April 2020). Therefore, symptom monitoring alone is not adequate to prevent transmission. CDC recommendations on mask wearing vary based on COVID-19 vaccination status, degree of immunocompromise and setting. In July 2021, CDC recommended masking by fully vaccinated people in indoor settings given the spread of the highly transmissible Delta variant in areas of the U.S. with substantial or high COVID-19 transmission levels (Brown, August 2021).
The precise mechanisms by which masking prevents COVID-19 range from mechanical filtration of droplets or aerosols containing viral particles (Adenaiye, September 2021) to promotion of associated healthy behavioral and hygiene habits (Doung-Ngern, November 2020). Some have hypothesized masking may reduce viral inoculum and lead to milder disease (Gandhi, July 2020), though supporting evidence for this mechanism is limited (Brosseau, October 2021).
As noted above, there are a limited number of randomized controlled trials and several published observational studies on the use of masks to prevent SARS-CoV-2 transmission (Ginther, June 2021; Guy, March 2021). These studies have a high risk of bias, variation in outcome measures and, often, low adherence to interventions (Jefferson, November 2020; Talic, November 2021), which is a barrier to drawing conclusions from the data. In health care settings, there does not seem to be a difference between use of medical/surgical masks compared with N95/P2 respirators when used in routine care to reduce respiratory viral infection (Jefferson, November 2020). Disposable surgical masks seem to be more effective at preventing infection than cloth masks (MacIntyre, April 2015; Abaluck, September 2021).
Here we describe data on the use of masks to prevent transmission of SARS-CoV-2 in the community; for information on transmission prevention in the health care setting, including a discussion of the potential routes of SARS-CoV-2 transmission, please see the Personal Protective Equipment in Medical Settings section. For more on types of masks and respirators, see CDC’s page on Types of Masks and Respirators.
In summary: Overall, based on observational and animal studies, masks appear to be part of an effective strategy to reduce transmission of SARS-CoV-2. Existing study data are limited by variations in design and mask adherence, as well as challenges in isolating the effect of masking versus concomitant pharmaceutical and nonpharmaceutical interventions in real-world settings.
The Impact of Community Masking on COVID-19: A Cluster-Randomized Trial in Bangladesh (Abaluck, August 2021 – preprint, not peer-reviewed).
Overall, this real-world study in Bangladesh, which used a cluster randomized design, found that proper mask usage increased after mask promotion in both the control and intervention arms and symptomatic seroprevalence of SARS-CoV-2 infection was reduced in the intervention arms. Surgical masking seemed to have a greater effect in reducing seroprevalence of SARS-CoV-2 infection in this study compared to cloth masks. This study’s results suggest that surgical masks may help decrease transmission of SARS-CoV-2.
- 342,126 individuals in 600 villages in rural Bangladesh from November 2020 to April 2021.
- Symptomatic SARS-CoV-2 seroprevalence.
- This was a cluster-randomized trial in Bangladesh in which mask promotion strategies were used as the intervention.
- This included free masks, education about mask-wearing, role-modeling by community leaders and reminders.
- The study also randomized cloth versus surgical masks.
- The control group did not have any interventions.
- In the intervention arm, the proportion of individuals with COVID-like symptoms was 7.62% (n=13,273) and in the control arm 8.62% (n=13,893). For symptomatic individuals who consented (n=10,952), blood samples were collected. Authors report that the intervention reduced symptomatic seroprevalence by 9.3% (adjusted prevalence ratio aPR, 0.91 [0.82, 1.00]; control prevalence, 0.76%; treatment prevalence, 0.68%).
- Notably, in the villages randomized to surgical masks (n=200), the relative reduction was overall 11.2% (aPR, 0.89 [0.78,1.00]) and 34.7% in individuals older than 60 (aPR, 0.65 [0.46, 0.85]).
- The study found that proper mask usage increased in the control villages (13.3%) and tripled in intervention villages (42.3%). This was sustained during the intervention period and two weeks after.
- However, after five months, the impact of the intervention waned — although mask-wearing was still higher in the intervention group.
- Social distancing also increased in control (24.1%) and treatment villages (29.1%).
- Given the real-world design of this study, staff were not blinded to study arm assignment, which can lead to potential selection bias.
- This study was conducted when the predominant circulating SARS-CoV-2 strain was B.1.1.7 (alpha).
Maximizing Fit for Cloth and Medical Procedure Masks to Improve Performance and Reduce SARS-CoV-2 Transmission and Exposure, 2021 (Brooks, February 2021).
Overall, improving the fit of a mask can improve effectiveness and should be combined with other infection prevention measures such as social distancing, hand hygiene and avoiding crowds and poorly ventilated indoor spaces until vaccine-induced population immunity is achieved.
- Laboratory-based experiments on two pliable elastomeric headforms.
- Whether improving the fit of masks by double masking (wearing a cloth mask over a medical procedure mask) or knotting and tucking masks (knotting the ear loops of a medical procedure mask where they attach to the mask’s edges and then tucking in and flattening the extra material close to the face) reduce the receiver’s exposure to an aerosol of simulated respiratory droplet particles of the size considered most important for transmitting SARS-CoV-2.
- An unknotted medical procedure mask blocked 56.1% of the particles from a simulated cough (SD = 5.8), and a cloth mask blocked 51.4% (SD = 7.1).
- A cloth mask over a medical procedure mask (double mask) blocked 85.4% of the cough particles (SD = 2.4), and the knotted and tucked medical procedure mask blocked 77.0% (SD = 3.1).
- Adding a cloth mask over the medical procedure mask or knotting and tucking the medical procedure mask reduced the cumulative exposure of the unmasked receiver by 82.2% (SD = 0.16) and 62.9% (SD = 0.08), respectively.
- This work was done in a lab and is not necessarily representative of real-world mask use.
- These experiments did not include other combinations of masks, such as cloth over cloth, medical procedure mask over medical procedure mask or medical procedure mask over cloth.
- Knotting and tucking can change the shape of the mask such that it no longer covers fully both the nose and the mouth of persons with larger faces, which would not be optimal for mask use in these individuals.
Effectiveness of Adding a Mask Recommendation to Other Public Health Measures to Prevent SARS-CoV-2 Infection in Danish Mask Wearers (Bundgaard, November 2020).
Overall, in this large population-based randomized controlled trial, recommending persons to wear masks in addition to social distancing was not associated with reduction in SARS-CoV-2 acquisition for mask wearers. The study is limited by a significant amount of mask nonadherence in participants recommended to wear them and by the fact that community caseload was low during the study. The results also cannot be extrapolated to determine the effectiveness of masks at reducing transmission of SARS-CoV-2, as the study was designed to assess protection of wearers, not transmission.
- Adults spending more than 3 hours per day outside the home without occupational mask use.
- During the study period, Danish authorities were not recommending use of masks in the community and mask use was uncommon (<5%) outside of hospitals.
- All participants were encouraged to practice social distancing. Participants were randomly assigned to receive a recommendation to wear disposable surgical face masks or not to wear a face mask.
- 3,030 participants were assigned to receive a recommendation to wear masks, and 2,994 were assigned to the control group.
- 4,862 (80.7%) of participants completed the study.
- SARS-CoV-2 infection in the mask wearer at 1 month by antibody testing, RT-PCR or hospital diagnosis.
- Participants reported having spent a median of 4.5 hours per day outside the home.
- In the mask group during follow-up, 46% of participants wore the mask as recommended, 47% predominantly as recommended and 7% not as recommended.
- SARS-CoV-2 infection occurred in 42 of 2,392 participants (1.8%) in the mask group and 53 of 2,470 (2.1%) in the control group.
- In an intention-to-treat analysis, the between-group difference was −0.3 percentage point (CI, −1.2 to 0.4 percentage point; p = 0.38) (OR 0.82; p = 0.33) in favor of the mask group.
- In a per-protocol analysis that excluded participants in the mask group who reported nonadherence (7%), SARS-CoV-2 infection occurred in 40 participants (1.8%) in the mask group and 53 (2.1%) in the control group (between-group difference, −0.4 percentage point; p = 0.40) (OR, 0.84; p = 0.40).
- A total of 52 participants in the mask group and 39 control participants reported COVID-19 in their household. Of these, 2 participants in the face mask group and 1 in the control group developed SARS-CoV-2 infection.
- 20% of the study population did not complete the study.
- Only 46% of the individuals in the intervention group wore masks as recommended.
- The sample size was insufficient to determine the statistical significance of a 20% reduction in infections.
- This study was performed in a setting with relatively low transmission. During the first week of May, the daily incidence of new confirmed COVID-19 cases in Denmark was one-quarter of daily incidence in the United States.
- The effect of a mask recommendation also depends on other factors including the prevalence of the virus, physical distancing adherence and the frequency and characteristics of gatherings.
- Of COVID-19 diagnoses in this study, 84% (80 of 95) were made using antibody tests, whose accuracy varies.
Trends in COVID-19 Incidence After Implementation of Mitigation Measures (Gallaway, October 2020).
- The study examined daily COVID-19 case numbers and 7-day moving averages in Arizona from Jan. 22–Aug. 7, 2020 compared with implementation of enhanced community mitigation measures to assess mitigation strategies’ effects.
- On June 17, local officials began implementing and enforcing mask wearing (via county and city mandates). Statewide mitigation measures included limitation of public events; closures of bars, gyms, movie theaters and water parks; reduced restaurant dine-in capacity encouraging residents to wear masks routinely, even when not mandated.
- The 7-day moving average of daily cases peaked during June 29–July 2 (range = 4,148–4,377), stabilized during July 3–12 (range = 3,609–4,160), and subsequently decreased 75% from July 13 (3,506) to August 7 (867).
- Mitigation measures put in place in June were extended through August to further limit transmission.
Incidence of Nosocomial COVID-19 in Patients Hospitalized at a Large US Academic Medical Center (Rhee, September 2020).
- Cohort study of 9149 patients admitted to a single academic medical center in the United States
- Examined the incidence of COVID-19 among patients 12 weeks after the first inpatient case was identified, and assessed all patients with COVID-19 for whom a first positive RT-PCR test result occurred on hospital day 3 or later, or within 14 days after hospital discharge.
- Infection control measures in place included airborne infection isolation rooms, personal protective equipment (PPE), PPE donning and doffing monitors, universal masking, restriction of visitors, and RT-PCR testing of symptomatic and asymptomatic patients.
- 697 COVID-19 cases were confirmed, translating into 8656 days of COVID-19–related care.
- The median age was 51 years [IQR 30-67 years]; 57.3% were female
- 12 of the 697 hospitalized patients with COVID-19 (1.7%) first tested positive on hospital day 3 or later (median, 4 days; range, 3-15 days).
- Of these, 1 case was considered hospital acquired, most likely from a presymptomatic spouse who was visiting daily and diagnosed with COVID-19 before visitor restrictions and masking were implemented.
- Among 8370 patients with non–COVID-19–related hospitalizations, 11 (0.1%) tested COVID-19 positive within 14 days
- 1 case was deemed likely to be hospital acquired, albeit with no known exposures.
Association Between Universal Masking in a Health Care System and SARS-CoV-2 Positivity Among Health Care Workers (Wang, July 2020).
- This study assessed the association of hospital masking policies with the SARS-CoV-2 infection rate among healthcare workers
- Prior to the intervention period, the SARS-CoV-2 positivity rate increased exponentially from 0% to 21.32%, with a weighted mean increase of 1.16% per day and a case doubling time of 3.6 days (95% CI, 3.0-4.5 days).
- After a universal hospital masking policy for all healthcare workers and patients, the proportion of symptomatic healthcare workers with positive test results for SARS-CoV-2 declined from 14.7% to 11.5%, with a weighted mean decline of 0.49% per day and a net slope change of 1.65% (95% CI, 1.13%-2.15%) more decline per day compared with the preintervention period
- This decline occurred despite rising case numbers in the state.
Physical distancing, face masks, and eye protection to prevent person-to-person transmission of SARS-CoV-2 and COVID-19: a systematic review and meta-analysis (Chu, June 2020).
- In a systematic review and meta-analysis of 172 observational studies in healthcare and non-healthcare settings of > 20,000 patients with COVID-19, MERS, or SARS who did not use any mitigation measures, the risk of virus transmission was 17.4%.
- The addition of a mask or respirator decreased the transmission risk to 3.1%.
- There was a stronger association of protection with N95 or similar respirators as compared to disposable surgical masks or similar reusable 12–16-layer cotton masks.
- The primary studies on face masks in this meta-analysis had low-certainty evidence.
Community Use Of Face Masks And COVID-19: Evidence From A Natural Experiment Of State Mandates In The US (Lyu, June 2020).
- This study assessed the effect of statewide mask mandates’ effect in 15 states. The authors focused on state executive orders or directives signed by governors that mandated use.
- Mandating public face masks was associated with a decline in daily COVID-19 growth rates by 0.9, 1.1, 1.4, 1.7, and 2.0 percentage points in 1-5, 6-10, 11-15, 16-20, and 21 or more days after state face mask orders were signed
- Overall, an estimated 230,000-450,000 cases were averted by state mask mandates.
Absence of Apparent Transmission of SARS-CoV-2 from Two Stylists After Exposure at a Hair Salon with a Universal Face Covering Policy (Hendrix, May 2020).
- Two hair stylists in Missouri who wore masks throughout their time working served 139 clients prior to being diagnosed with COVID-19.
- Of 104 clients who were interviewed, 98% wore masks for the entire appointment.
- No symptoms of COVID-19 were identified among the exposed clients or their secondary contacts.
- Six close contacts of stylists A and B outside of salon A were identified: four of stylist A and two of stylist B. All four of stylist A’s contacts later developed symptoms and had positive PCR test results for SARS-CoV-2. All of the contacts either lived with the stylist together in another household. None of stylist B’s contacts became symptomatic.
Surgical Mask Partition Reduces the Risk of Noncontact Transmission in a Golden Syrian Hamster Model for Coronavirus Disease 2019 (COVID-19) (Chan, May 2020).
- The authors utilized a golden Syrian hamster SARS-CoV-2 model to examine the effect of a surgical mask partition on the transmission of SARS-CoV-2.
- SARS-CoV-2-challenged index hamsters and naive hamsters were placed into 2 different closed-system cages separated by a polyvinyl chloride air porous partition with unidirectional airflow within an isolator.
- Noncontact transmission was found in 66.7% (10/15) of exposed naive hamsters.
- Surgical mask partition for challenged index or naive hamsters reduced transmission to 25% (6/24, P = .018).
- Surgical mask partition for challenged index hamsters reduced transmission to 16.7% (2/12, P = .019) of exposed naive hamsters.
- Infected naive hamsters had lower clinical scores, milder histopathological changes, and lower viral nucleocapsid antigen expression in respiratory tract tissues.
Case-Control Study of Use of Personal Protective Measures and Risk for SARS-CoV-2 Infection, Thailand (Doung-ngern, November 2020). This study with 211 cases of COVID-19 and 839 controls evaluated the effectiveness of personal protective measures against SARS-CoV-2. Wearing masks all the time during contact was independently associated with lower risk for SARS-CoV-2 infection compared with not wearing masks; wearing a mask sometimes during contact did not lower infection risk. The type of mask worn was not independently associated with infection. Maintaining >1 m distance from a person with COVID-19, having close contact for <15 minutes and frequent handwashing were independently associated with lower risk for infection. Contacts who always wore masks were also more likely to practice social distancing.
Global projections of potential lives saved from COVID-19 through universal mask use (Gakidou, October 2020). This study measures reduction in transmission associated with the use of cloth or paper masks in a general population setting using a Bayesian meta-regression of 40 studies. Use of simple masks can reduce transmission of COVID-19 by 40% (95% uncertainty interval [UI], 20% to 54%). Worldwide universal mask use would lead to a reduction of 815,600 deaths (95% UI, 430,600 to 1,491,000 deaths) between Aug. 26, 2020 and Jan. 1, 2021, the difference between the predicted 3 million deaths (95% UI, 2.20 to 4.52 million) in the reference and 2.18 million deaths (95% UI, 1.71 to 3.14 million) in the universal mask scenario over this time period.
Comparing Associations of State Reopening Strategies with COVID-19 Burden (Kaufman, October 2020). This interrupted time series quasi-experimental study estimated excess COVID-19 cases and deaths after reopening compared with trends prior to reopening for two groups of states: (1) states with an evidence-based reopening strategy, defined as reopening indoor dining after implementing a statewide mask mandate, and (2) states reopening indoor dining rooms before implementing a statewide mask mandate. On average, the number of excess cases per 100,000 residents in states reopening without masks was 10 times the number in states reopening with masks after 8 weeks (643.1 cases; 95% confidence interval [CI], 406.9, 879.2 and 62.9 cases; CI = 12.6, 113.1, respectively). Over 50,000 excess deaths were prevented within 6 weeks in 13 states that implemented mask mandates prior to reopening.
The Impact of Mask-Wearing and Shelter-in-Place on COVID-19 Outbreaks in the United States (Zhang, August 2020). This agent-based disease transmission model parameterized with estimates of COVID-19 characteristics and U.S. population demographics attempts to quantify the impact of several public health measures, including non-medical mask-wearing, shelter-in-place and detection of asymptomatic infections. Non-medical mask-wearing by 75% of the population (IQR, 36.1-39.4%), 44.2% (IQR, 42.9-45.8%) and 47.2% (IQR, 45.5-48.7%), respectively, in the absence of a shelter-in-place strategy. , decreasing attack rate, hospitalizations and deaths by over 82% when combined with mask-wearing.
Association of country-wide coronavirus mortality with demographics, testing, lockdowns, and public wearing of masks (Leffler, Aug 2020): In this preprint epidemiologic study examining predictors of per-capita SARS-CoV-2-related mortality in 200 countries up until May 2020, in multivariable analysis of 196 countries, the duration of mask-wearing by the public was negatively associated with mortality (p<0.001). In countries with cultural norms or policies supporting public mask-wearing, per-capita coronavirus mortality increased on average by 15.8% each week, as compared with 62.1% each week in remaining countries.
Face Masks Against COVID-19: An Evidence Review (Howard, July 2020): In this epidemiologic study the authors examined the results of this narrative review examining mask usage suggest mask wearing reduces the transmissibility per contact by reducing transmission of infected droplets in both laboratory and clinical contexts. The authors note public mask wearing is most effective mitigation of transmission when compliance is high.
COVID-19 and the Social Distancing Paradox: dangers and solutions (Marchiori, May 2020): In this epidemiologic study the authors examined how wearing personal protective equipment in the community may influence social distanicn gpractices. The study findings suggest when people wear face masks they and those around them may be more likely to socially distance.