Effectiveness of face masks worn in community settings at reducing the transmission of SARS-CoV-2: A rapid review

Data sources and searches

Systematic electronic database searches were conducted in PubMed, Embase, NHS Evidence and Europe PubMed Central (PMC). The search terms and detailed search strategy are available as Extended data¹⁷. Searches were conducted from 1 January 2020 to 27 August 2020. The reference lists and citations of included studies were searched to identify other potentially relevant studies.

Study selection

Studies were eligible for inclusion if they reported on the effectiveness of wearing face masks of any type in community settings, including households, compared with not wearing face masks at reducing the risk of transmission of SARS-CoV-2. Clinical trials, cohort, case control and cross-sectional studies were eligible for inclusion. Case reports and studies conducted in healthcare settings or involving other respiratory viruses were excluded. As this review focused on direct evidence only, experimental laboratory studies and mathematical modelling studies were excluded. Language restrictions were not applied. We included both published studies and preprints.

All records identified from the database searches were exported to EndNote^® (Version X8) for reference management. Duplicates were removed. One reviewer reviewed the titles and abstracts of the remaining citations to identify records for full-text review. The full texts of potentially eligible studies were obtained and evaluated by two reviewers applying the predefined eligibility criteria. Figure 1 shows a PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) flow diagram of the study selection process.

Figure 1. PRISMA flow chart of studies included in this review.

Data extraction and quality assessment

Data extraction was conducted using a data extraction form developed for this review, and included study design, setting, participants, type of mask, adherence to masks, and effect estimates (OR, 95% CI) for associations between mask wearing and SARS-CoV-2 transmission. One reviewer extracted data, which was cross-checked by a second reviewer. Where discrepancies were identified, these were resolved through discussion between the reviewers. The Joanna Briggs Institute Critical Appraisal Checklists were used for quality appraisal¹⁸.

Data synthesis and analysis

Given the small number of studies and heterogeneous nature of the study designs, settings and presented data, a narrative synthesis was conducted.

Case-control and cohort studies

A case-control study conducted in Thailand included 1,050 contacts of symptomatic COVID-19 patients from nightclub, boxing stadium or enterprise office clusters identified through contact tracing²⁵. In adjusted analyses, the risk of infection was significantly lower for contacts who self-reported wearing any type of mask all the time, compared with contacts who did not wear a mask during the contact period (adjusted OR 0.23, 95% CI 0.09 to 0.60). Wearing a mask sometimes was not associated with a lower risk of infection compared with not wearing a mask (adjusted OR 0.87, 95% CI 0.41 to 1.84), although it is not clear what other factors were adjusted for. A US case-control study included 293 individuals without known close contact with a COVID-19 case recruited through an online patient portal in Ohio and Florida²⁴. There was no statistically significant difference between COVID-19 cases and controls in terms of mask wearing, with 64% of cases and 55% of controls self-reporting that they always wore a mask (unadjusted OR 1.40, 95% CI 0.80, 2.45).

A retrospective analysis of 127 SARS-CoV-2 patients in Taizhou, China included 41 presymptomatic patients who had recently travelled from Wuhan²³. Prior to developing symptoms, the 41 patients had close contact with 197 local residents, of whom 24 later developed COVID-19. Ten close contacts (8.1%) of presymptomatic cases who reported wearing masks were subsequently infected with SARS-CoV-2, compared to 14 (19.0%) close contacts of presymptomatic cases who reported not wearing a mask (unadjusted OR 0.38, 95% CI 0.16, 0.91). The type of mask worn by the presymptomatic index cases, and whether the close contacts also wore masks, was not reported, although the authors note that at the time of contact, mask wearing among local residents was not routine.

Wang et al.²¹ included 124 families in China with at least one laboratory-confirmed COVID-19 case. Secondary household transmission occurred in 41 families. In unadjusted analyses, household transmission was significantly reduced when all family members (compared with no family members) wore any type of mask (respirator, medical or cloth) all the time at home after the primary case’s illness onset date (unadjusted OR 0.20, 95% CI 0.07, 0.60), but not if only some family members wore masks (unadjusted OR 0.72, 95% CI 0.30 to 1.73). Transmission within families was also less likely when the primary case wore a mask at all times (unadjusted OR 0.30, 95% CI 0.11 to 0.82) after illness onset, but not when the primary case and/or family members wore a mask some of the time (unadjusted OR 1.15, 95% CI 0.46 to 2.87). In multivariable analysis, face mask use before the primary case’s illness onset date by one or more persons in the household significantly reduced transmission compared with no mask use (adjusted OR 0.21, 95% CI 0.06 to 0.79), although it is not clear if masks were worn all the time or sometimes.

Cross-sectional studies

Cheng et al.²⁰ reported that among 961 confirmed COVID-19 cases in Hong Kong until 8 April 2020, there were 11 clusters of 113 persons engaged in ‘mask-off’ activities such as dining or karaoke, compared with three clusters of 11 people in ‘mask-on’, workplace settings. Further individual level data are not reported. Clipman et al. included 1,030 participants recruited from an online panel in Maryland, US²². A total of 55 participants (5.3%) self-reported ever testing positive for SARS-CoV-2, with 18 testing positive in the previous two weeks. Mask use was self-reported by all participants for the previous two weeks only. In an analysis including only the 18 participants who had tested positive for SARS-CoV-2 within the last two weeks, consistent indoor mask use (unadjusted OR 0.21, 95% CI 0.06 to 0.76) in the last two weeks was associated with a significantly lower likelihood of infection, compared to no or occasional mask use. Consistent outdoor mask use did not appear to significantly reduce the likelihood of infection (unadjusted OR 0.41, 95% CI 0.13 to 1.27). The authors noted that almost 40% of respondents who reported wanting a test were not able to get one; it is therefore possible that the number of positive cases were underestimated in this study.

In a survey of 8,158 Chinese adults recruited through three social networks, 57 (0.7%) reported having been infected with SARS-CoV-2¹⁹. Of the respondents, 5,120 provided data on face mask use, with 97.9% reporting wearing a mask when going out. Compared with wearing a mask, not wearing a mask was associated with a significantly increased risk of SARS-CoV-2 infection (adjusted OR 7.20, 95% CI 2.24 to 23.11), controlling for socio-demographic variables, hand washing, coughing etiquette, and social distancing. Wearing a mask (compared with not wearing one) was associated with a significantly reduced risk of SARS-CoV-2 infection among those who practiced hand washing (RR 0.11, 95% CI 0.04 to 0.29), proper coughing etiquette (RR 0.18, 95% CI 0.05 to 0.57) and social distancing (RR 0.03, 95% CI 0.01 to 0.11).

Adherence to wearing face masks

Adherence to wearing face masks was poorly defined and was self-reported in five of the seven studies. One study did not report any adherence data²⁴. One study reported observed adherence rates among the general public in Hong Kong (defined as the percentage of individuals observed in public wearing a mask) that ranged from 95.7% to 97.2% across three days²⁰. In the other cross-sectional studies, 97.9% of participants in China self-reported wearing a mask when they went out¹⁹, while 53% of participants in the US reported always wearing a mask when visiting public indoor and outdoor locations²². In the study set in households in China, 37.1% of all included family units (primary cases and/or their household contacts) reported wearing masks all the time at home after illness onset, 29.8% reported wearing masks sometimes, and 33.1% of families reported never wearing masks²¹. Data for individual family members were not presented. In the case-control study set in Thailand, 14% of cases and 24% of controls reported wearing a mask all of the time, while 38% of cases and 15% of controls reported wearing a mask sometimes²⁵. In the study of presymptomatic index cases and their close contacts in China, 68.3% of index cases reported wearing a mask during the presymptomatic phase of COVID-19, although frequency of use was not reported²³.

Effectiveness of different face mask types

Two of the seven studies compared different mask types. In unadjusted analyses, one study reported that while medical masks appeared to reduce the risk of SARS-CoV-2 infection compared to not wearing a mask, wearing non-medical masks or alternating between medical and non-medical masks did not, although the type of mask worn was not associated with lower risk of SARS-CoV-2 infection in a multivariable model that also included adherence²⁵. Speaker et al. did not find any differences in the risk of infection between cloth, surgical, N95 or other (unspecified) mask types²⁴. None of the included studies mentioned the use of shields or visors.

Harms or adverse outcomes

Harms or adverse outcomes were not reported by any of the included studies. There were no data to indicate that wearing masks might reduce physical distancing. One study reported that individuals who reported wearing masks all the time were more likely than those who did not wear masks to report washing their hands regularly and practice social distancing. Those who reported wearing masks only sometimes were more likely to wash their hands, but were also more likely to have physical contact and a longer duration of contact than those who did not wear masks²⁵. There were no direct comparisons reported between those who always and those who sometimes wore a mask.

Risk of bias and study quality

The studies included in this review were of low quality with a high risk of bias. For the case-control studies, it is not clear how well cases and controls were matched, and one study noted the potential misclassification of cases and controls²². The wearing of face masks was based on self-report in six of the seven included studies. Observational studies are subject to selection and reporting biases as well as inadequate adjustment for potential confounding factors, and the studies included in this review were either based on unadjusted analyses or did not clearly state which confounders were included in multivariable analyses. The cross-sectional survey by Xu et al.¹⁹ was based on a non-random snowball sample, and while the authors reported that the web page hosting the survey was accessed over 21,000 times, it is not known how many people received the link, leading to potentially high reporting and selection biases. Speaker et al.²⁴ reported a response rate of 9%, leading to a potentially high response bias. It is not possible to establish causal links between wearing of face masks and SARS-CoV-2 infection based on these observational studies. Three of the seven included studies are published as pre-prints and have not been formally peer-reviewed^19,22,24. It is possible that the data could change prior to publication, and findings from these studies should be interpreted with caution.

Face masks in the context of COVID-19

A number of studies have estimated the impact of policies mandating face mask use in public on COVID-19 incidence. These studies were excluded from this review as they provide indirect evidence on population transmission only. Lyu and Wehby reported that mandated face mask use in 15 US states (plus the District of Columbia) was associated with a significant decline of 2 percentage points in the daily growth rate of COVID-19 after 21+ days³⁷. As noted by the authors, these findings describe the intention-to-treat effect of face mask mandates at a community or state level, and do not indicate the effect of wearing a face mask in public at an individual level³⁷. A number of studies have reported similar positive findings, suggesting that policies mandating face masks are associated with a subsequent decrease in COVID-19 cases^38,39. However, these policies are rarely introduced in isolation, and it can be difficult to disentangle the effects of multiple measures targeting reduced transmission. One non-peer reviewed quasi-experimental study of non-pharmaceutical interventions in Europe found that wearing face masks in public was not associated with any independent additional impact on population incidence of COVID-19, once other public health measures were accounted for⁴⁰. However, the authors noted that the specific settings in which face masks were made either mandatory or voluntary varied widely between countries.

A mathematical model for assessing the population-level impact of control and mitigation strategies in New York estimated that the use of medical masks in public could lead to the elimination of the pandemic if at least 70% of the residents used them, based on a high compliance rate⁴¹. The use of cloth masks was estimated to lead to a significant reduction, but not elimination, of the burden of COVID-19. Using US data in a compartmental model for assessing the community-wide impact of mask use, Eikenberry et al. reported that broad adoption of even relatively ineffective face masks may meaningfully reduce community transmission of COVID-19 in both healthy and asymptomatic persons⁴². Fisman et al. highlighted that the impact of mask use could be reduced with ‘assortative mixing’, whereby those who wear masks predominantly interact with other mask wearers, with infections becoming concentrated in non-masked populations⁴³. National and international public health guidance on the use of face masks is based on low certainty direct evidence of clinical effectiveness, indirect evidence that supports plausibility of effectiveness, as well as a consideration that SARS-CoV-2 appears to be more infectious than many other respiratory pathogens, including during the presymptomatic phase of the disease⁴⁴, highlighting the necessity of considering a range of infection prevention and control measures, including face masks, to reduce the spread of infection. In the absence of other data, it has been suggested that community wide wearing of masks should be promoted according to the precautionary principle^9,45.

Mask types

There is limited evidence that compares the effectiveness of different grades of face mask (respirators, medical and cloth) worn in the community. One previous RCT set in households compared medical masks with P2 respirator masks, and found no differences in ILI or laboratory-confirmed influenza infections between the two types of masks³¹.

Adherence to face masks

Two of the included studies reported high levels of adherence, with one study reporting that while wearing a mask all the time significantly reduced the risks of SARS-CoV-2 infection, wearing a mask sometimes did not²⁵. Studies of face masks to reduce transmission of influenza or ILI in households or schools have suggested that adherence with wearing face masks may play an important role in determining their effectiveness^31,46, although further research is needed.

Harms or adverse outcomes associated with wearing face masks

The World Health Organization previously suggested that face masks may introduce a false sense of security and lead wearers to neglect hand hygiene and physical distancing⁴⁷. While harms were not reported by the studies reviewed, one case-control study did report that while individuals who reported wearing masks all the time were more likely to wash their hands regularly and practice social distancing than those who did not wear masks, those who reported wearing masks only some of the time were more likely to have physical contact and longer duration of contact, although they were also more likely to wash their hands²⁵. There do not appear to be any studies currently published that have investigated or quantified risk compensation behaviour during mask use⁴⁸. A study of recorded video footage taken both before and during the COVID-19 pandemic in public outdoor spaces reported that mask wearing may be associated with a reduction in face touching behaviours during the current pandemic⁴⁹. Others have argued that, in the context of COVID-19, any risk compensation that may occur in some individuals would be dwarfed by the potential protective impact of face mask use at the population level⁵⁰.

Underlying data

All data underlying the results are available as part of the article and no additional source data are required.

Extended data

Figshare: Extended data. https://doi.org/10.6084/m9.figshare.13008449.v1¹⁷.

This project contains the following extended data:

Reporting guidelines

Figshare: PRISMA checklist for ‘Effectiveness of face masks worn in community settings at reducing the transmission of SARS-CoV-2: A rapid review’. https://doi.org/10.6084/m9.figshare.13008449¹⁷.

Data are available under the terms of the Creative Commons Attribution 4.0 International license (CC-BY 4.0).