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Mask wearing in pre-symptomatic patients prevents SARS-CoV-2 transmission: An epidemiological analysis

Our take —

Little evidence has emerged so far regarding the effectiveness of mask-wearing (on the part of infected individuals) in reducing transmission of SARS-CoV-2. In this small study from China, people with COVID-19 who reported wearing masks before they felt symptoms infected a smaller percentage of their close contacts than did people with COVID-19 who did not wear masks. This provides some support for the notion of mask-wearing by the public as a means of reducing asymptomatic or pre-symptomatic transmission.

Study design

Retrospective Cohort

Study population and setting

This study in Taizhou, China considered 127 patients (56% male, median age 46 years) with laboratory-confirmed SARS-CoV-2 infection, 63 of whom had recently traveled to Wuhan, China, from January 23 to March 1, 2020. Demographic data, exposure history, and behavioral data including mask-wearing were collected via questionnaire from confirmed cases and from close contacts. Additionally, the study considered a sequential community cluster of 21 COVID-19 cases (16 laboratory-confirmed, 5 clinically-confirmed) originating from a couple returning from travel to Wuhan.

Summary of Main Findings

The 41/63 patients returning from Wuhan who provided contact history and epidemiologic data had close contact with 197 people while pre-symptomatic. 28 cases reported wearing masks while pre-symptomatic, while 13 reported no mask use. The attack rate among contacts of pre-symptomatic mask-wearing cases was 8%, while the attack rate was 19% among contacts of non-mask-wearing cases (p<0.01). In the community cluster of 21 cases, only one case reported mask use during close contact with the presumed source of infection.

Study Strengths

The study drew on detailed contact data derived from epidemiologic investigation.

Limitations

It is not clear whether cases were truly free of any symptoms during the “pre-symptomatic” period, so these results may not be interpretable as pertaining specifically to asymptomatic or pre-symptomatic transmission. Mask use data were retrospectively acquired via self-report, and may be subject to either poor recall or overreporting due to perceived social acceptability. The sample size is fairly small. No mask use data are reported for the uninfected close contacts in the community cluster (n=36), precluding estimation of an effect size. It is also unclear what proportion of close contacts were named and successfully identified. Furthermore, PCR testing for SARS-CoV-2 likely underestimates the true attack rate among contacts and may have biased the results.

Value added

This study provides some limited evidence in support of the effectiveness of mask-wearing to prevent outward transmission of SARS-CoV-2; evidence on this question from epidemiologic data has been scarce.

This review was posted on: 10 July 2020