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SARS-CoV-2 Transmission to Masked and Unmasked Close Contacts of University Students with COVID-19 — St. Louis, Missouri, January–May 2021

Our take —

More than a year and a half into the pandemic, evidence regarding mask effectiveness is scarce, though the balance of evidence suggests a protective effect. In this study of university students in St. Louis, Missouri with COVID-19 and their close contacts, unmasked exposures (in which either the infected person or the contact was unmasked) were about 5 times more likely to result in SARS-CoV-2 transmission than exposures in which both parties were masked. Unfortunately, the analysis did not distinguish between mask use by the infected person and the contact. Other caveats apply: there were only a small number of fully masked exposures, vaccination status was not accounted for (although only 18 contacts were fully vaccinated), the nature of exposure may have varied considerably, and self-reported mask use may have been subject to error. However, these data provide more evidence that masking lowers the probability of SARS-CoV-2 transmission.

Study design


Study population and setting

This study, conducted in St. Louis, MO from January to May 2021, compared rates of SARS-CoV-2 infection by masking status among 378 close contacts during exposure to 265 St. Louis University (SLU) students with confirmed COVID-19. Cases were identified through SLU symptomatic and surveillance testing, and contacts were identified via the SLU contact tracing program. A close contact was defined as any single encounter during a 24-hour period during which individuals spent at least 15 minutes within 6 feet of one another, and mask use was recorded for both case and contact. The exposure was classified as unmasked if either the infected case or the contact was not wearing a mask. The outcome of SARS-CoV-2 infection was assessed via saliva-based RT-PCR testing 5-7 days after exposure. Demographic characteristics and vaccination status (unvaccinated, partial, or full) of contacts were collected. Logistic regression was used to estimate odds ratios of infection following unmasked vs. masked exposures, adjusting for the number of exposures experienced by the contact.

Summary of Main Findings

Of the 378 close contacts identified, 116 (31%) tested positive for SARS-CoV-2 infection. Among all contacts, 26 (7%) reported only masked exposure to the index case, while the remainder reported unmasked exposure (i.e., either the case or the contact was unmasked). Among contacts with only masked exposure, there were 2/26 (7.7%) positive tests; among contacts with any unmasked exposure, there were 114/352 (32.4%) positive tests. The odds ratio for SARS-CoV-2 infection, comparing unmasked to masked exposure, was 4.9 (95% CI: 1.5 to 36.5) after adjustment for the number of exposures. The odds ratio for infection with each additional exposure was 1.4 (1.2 to 1.6). There were 5 positive tests among partially vaccinated contacts (n=24) and no positive tests among fully vaccinated contacts (n=18). No information was provided on vaccination status by masking group.

Study Strengths

Close contacts were identified in an existing contact tracing program, The testing window for close contacts was narrow (5-7 days after exposure).


The authors did not distinguish between mask use by the infected case and mask use by the close contact, preventing any inference about source control vs. wearer protection. There was no adjustment for vaccination status in the analysis, nor were fully vaccinated contacts excluded in any sensitivity analysis. If fully or partially vaccinated individuals felt less inclined to wear masks, any protective effect of mask use may have been underestimated. Alternatively, if mask-wearing behavior and vaccine-seeking behavior were positively correlated, the reverse could be true. Mask use was self-reported and thus subject to recall bias (e.g., contacts may have misremembered) and social desirability bias (e.g., contacts may have misrepresented their masking behavior to be viewed more favorably). It is conceivable that some infected contacts acquired their infections from a case other than the one under investigation. There were no data reported on the severity of COVID-19 symptoms among infected contacts, and there were no data on type of masks used. Finally, despite the use of a standard definition of close contact, the nature and duration of exposures may have been highly heterogeneous, which would have an unpredictable effect on estimates.

Value added

Despite the prominence of masks as an object of debate in COVID-19 policy, this is one of the very few studies to have estimated differences in risks of exposure by masking status.

This review was posted on: 21 September 2021