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Mask Effectiveness for Preventing Secondary Cases of COVID-19, Johnson County, Iowa, USA

Our take —

Since the start of the pandemic, surprisingly little data has emerged on how the risks of secondary SARS-CoV-2 transmission may depend on the masking status of infected individuals and their close contacts. This study, conducted through a contact tracing program in one Iowa county from October 2020 to February 2021, collected information from 431 infected cases and 969 of their close contacts to estimate differences in the transmission risk by masking status. The authors found that when at least one person was unmasked during the exposure, the risk of secondary transmission was double compared to when both the infected person and their close contact were masked (26% vs. 13%). The risk of transmission was higher when only the contact was unmasked, compared to when only the infected case was unmasked, but the sample sizes were too small to draw a firm conclusion about this difference. Although this observational study is subject to several possible sources of bias, it provides rare individual-level data supporting the effectiveness of masks in preventing infection by the wearer.

Study design

Retrospective Cohort

Study population and setting

This study, conducted by the public health department of Johnson County, Iowa, compared SARS-CoV-2 secondary attack rates (SARs) by the masking status of index cases and their close contacts from October 23, 2020 to February 28, 2021. Through the Iowa state contact-tracing program, the authors identified 969 close contacts of 431 cases (people who had tested positive for SARS-CoV-2 infection) who met inclusion criteria and for whom data were available on masking status during exposure and subsequent SARS-CoV-2 testing results. Close contacts were defined as: people who spent more than 15 minutes within 6 feet of a case during that case’s infectious period; who spent 2 hours or more in the same enclosed space as a case; or who experienced “substantial direct exposure” to the case (this latter criterion was evaluated on a case-by-case basis). The masking status of both case and contact during exposure was assessed via interviews with cases. Cases were also asked about dates and durations of exposure, symptoms during exposure, and the setting of the exposure (indoors vs. outdoors). Contacts were interviewed and asked for demographic information, date of symptom onset, previous COVID-19 history, and vaccination history. Contacts were excluded from the study if exposure occurred in a household, health care, or long-term care setting; or if no SARS-CoV-2 testing results from 2-14 days after the exposure were available. Secondary attack rates were calculated for combinations of masking status of the case and contact.  Multivariable logistic regression was used to estimate the association between mask score (the number of persons masked during an exposure: 0, 1, or 2) and SARS-CoV-2 transmission, adjusting for age, exposure setting, whether the case was symptomatic during exposure, and exposure duration. The study was initiated in response to a change in guidance from the Iowa Department of Health on September 29, 2020 in which close contacts were advised to perform symptom monitoring for 14 days instead of home quarantine if both the initial case and the contact were fully masked during exposure.

Summary of Main Findings

The average number of contacts per case was 2.25, and the median age of contacts was 18 years (range: 0 to 90 years). The overall secondary attack rate (SAR) was 20.5% (95% CI: 18.1 to 23.2), with the following SARs by masking status of case and contact, respectively: unmasked/unmasked 26.4% (95% CI: 22.9 to 30.7), unmasked/masked 10.0% (95% CI: 4.0 to 25.3), masked/unmasked 29.1% (95% CI: 19.3 to 43.9), masked/masked 12.5% (95% CI: 9.6 to 16.3). Among the 590 contacts (61%) in which at least one person was unmasked, the SAR was 25.6% (95% CI: 22.3 to 29.4). In multivariable logistic regression, an increase in mask score of 1 unit (representing one additional person masked, from 0-2) was associated with 30% lower odds of a secondary case (odds ratio 0.70, 95% CI: 0.57 to 0.84). Longer exposure and older age were associated with higher odds of secondary infection. Results were similar when restricted to children aged 5-18 years. Only 16 contacts had received at least one vaccine dose prior to exposure; all 16 tested negative to SARS-CoV-2. Of the 3 contacts with a prior positive SARS-CoV-2 test result, one tested positive after exposure.

Study Strengths

This study was able to measure masking status among both cases and contacts, and calculated secondary attack rates for each combination of masking status. Nesting the study within a contact tracing program allowed for standardized collection of covariates.


The sample by definition did not include individuals who could not be contacted or who refused to participate in contact tracing investigation; the authors did not report the number of possible cases who were thus excluded from analysis. The resulting sample may not be representative of the wider population of cases and their close contacts. Additionally, an unknown number of contacts without testing results were excluded, which may have resulted in overestimation of the SARs. The sample size was too small to permit strong inference about differences in protection when the case vs. the contact was masked. Classification of mask use relied on self-report, which is subject to several possible biases including faulty recall, social desirability (i.e., individuals may have reported what they felt the interviewer wanted to hear), or a desire to protect contacts from the possibility of quarantine. Finally, this study took place before widespread vaccination; these results might or might not apply to a population with higher vaccination rates.

Value added

This is one of the very few studies to estimate and compare secondary attack rates among close contacts by the masking status of the initial case and the close contact during exposure.

This review was posted on: 9 November 2021