Effects of Face Mask Mandates on COVID-19 Transmission in 51 Countries: Retrospective Event Study
Authors: Näher Anatol-Fiete , Schulte-Althoff Matthias , Kopka Marvin , Balzer Felix , Pozo-Martin Francisco 
Published: December 2023

Background
The question of the utility of face masks in preventing acute respiratory infections has received renewed attention during the COVID-19 pandemic. However, given the inconclusive evidence from existing randomized controlled trials, evidence based on real-world data with high external validity is missing.

Objective
To add real-world evidence, this study aims to examine whether mask mandates in 51 countries and mask recommendations in 10 countries increased self-reported face mask use and reduced SARS-CoV-2 reproduction numbers and COVID-19 case growth rates.

Methods
We applied an event study approach to data pooled from four sources: (1) country-level information on self-reported mask use was obtained from the COVID-19 Trends and Impact Survey, (2) data from the Oxford COVID-19 Government Response Tracker provided information on face mask mandates and recommendations and any other nonpharmacological interventions implemented, (3) mobility indicators from Google’s Community Mobility Reports were also included, and (4) SARS-CoV-2 reproduction numbers and COVID-19 case growth rates were retrieved from the Our World in Data—COVID-19 data set.

Results
Mandates increased mask use by 8.81 percentage points (P=.006) on average, and SARS-CoV-2 reproduction numbers declined on average by −0.31 units (P=.008). Although no significant average effect of mask mandates was observed for growth rates of COVID-19 cases (−0.98 percentage points; P=.56), the results indicate incremental effects on days 26 (−1.76 percentage points; P=.04), 27 (−1.89 percentage points; P=.05), 29 (−1.78 percentage points; P=.04), and 30 (−2.14 percentage points; P=.02) after mandate implementation. For self-reported face mask use and reproduction numbers, incremental effects are seen 6 and 13 days after mandate implementation. Both incremental effects persist for >30 days. Furthermore, mask recommendations increased self-reported mask use on average (5.84 percentage points; P<.001). However, there were no effects of recommendations on SARS-CoV-2 reproduction numbers or COVID-19 case growth rates (−0.06 units; P=.70 and −2.45 percentage points; P=.59). Single incremental effects on self-reported mask use were observed on days 11 (3.96 percentage points; P=.04), 13 (3.77 percentage points; P=.04) and 25 to 27 (4.20 percentage points; P=.048 and 5.91 percentage points; P=.01) after recommendation. Recommendations also affected reproduction numbers on days 0 (−0.07 units; P=.03) and 1 (−0.07 units; P=.03) and between days 21 (−0.09 units; P=.04) and 28 (−0.11 units; P=.05) and case growth rates between days 1 and 4 (−1.60 percentage points; P=.03 and −2.19 percentage points; P=.03) and on day 23 (−2.83 percentage points; P=.05) after publication.

Conclusions
Contrary to recommendations, mask mandates can be used as an effective measure to reduce SARS-CoV-2 reproduction numbers. However, mandates alone are not sufficient to reduce growth rates of COVID-19 cases. Our study adds external validity to the existing randomized controlled trials on the effectiveness of face masks to reduce the spread of SARS-CoV-2.