Study population and setting
This study was carried out in Germany and examined the effectiveness of compulsory public face mask requirements to reduce SARS-CoV-2 transmission. The authors first compared the experience of the city of Jena, which mandated mask use by the public on April 6, 2020, to a counterfactual scenario in which Jena did not implement a mask mandate. To construct the counterfactual, the authors employed a synthetic control approach, in which data from 401 regions in Germany were used to construct a “donor pool” of regions that were weighted to most closely estimate the pre-mandate cumulative case count in Jena and other regional characteristics. Observed cumulative cases after the mask mandate were compared to the modeled counterfactual. Second, the authors used the same approach for all regions in Germany that mandated mask use by April 22, 2020 (n=32). The authors also performed various sensitivity analyses and checks on how well the model assumptions were met.
Summary of Main Findings
There were 16 newly reported cases of COVID-19 in Jena between April 6 and April 26, 2020; in the counterfactual scenario of no mandate in Jena (the synthetic control), there were 62 new cases; this represents an estimated 74% reduction in new cases over the 20-day span. Placebo-in-space tests (estimating “effects” in other locations that did not actually have a mask mandate, and comparing the estimated effects in Jena to the distribution of these “placebo effects”) indicated statistically significant differences in Jena (p<0.10) beginning 13 days after the mask mandate. This indicates the results from Jena were unlikely to be due solely to chance. The lag was argued to be concordant with the incubation period of COVID-19, plus delays in testing and reporting. When considering all regions with mask mandates, the authors estimated that an average of 28 cases per region were prevented over a 20-day period after the intervention, corresponding to a 51% reduction in new cases during the post-intervention period.
The authors performed several analyses to check the robustness of their findings to alternative assumptions, considered alternative explanations for the observed reduction in new cases during the study period (including anticipation of the mandate), and used an SIR model to estimate the lag period after which any effects of a mask mandate might be observed.
Dates of mask mandates may be poor proxies for individual mask-wearing behavior. The study estimates the effectiveness of a mask mandate, not of mask wearing, and this difference was not always emphasized. There may be geographic heterogeneity in personal behavior that was associated with the timing of mandates. The authors considered cumulative case counts, demographic characteristics, and health care system characteristics as covariates, but did not consider any other non-pharmaceutical interventions that may have varied by region. The authors ruled out other interventions as an explanation for the results because other interventions in Jena were at least 10 days away from the mask mandate, but there may have been unmeasured confounding by other factors, including individual behavior change and/or epidemiologic characteristics of SARS-CoV-2 spread in Jena. There was little discussion of the makeup of the donor pool for the primary analysis, though the authors did perform some sensitivity analyses. Finally, results may not be generalizable to mask mandates during other time periods or locations, which may have different public responses to a mask mandate and different levels of general community transmission.
This study provides evidence of the effectiveness of mask mandates in reducing transmission of SARS-CoV-2 at the regional level in Germany.