We estimate the impact of indoor face mask mandates and other non-pharmaceutical interventions (NPI) on COVID-19 case growth in Canada. Mask mandate introduction was staggered from mid-June to mid-August 2020 in the 34 public health regions in Ontario, Canada’s largest province by population. Using this variation, we find that mask mandates are associated with a 22 percent weekly reduction in new COVID-19 cases, relative to the trend in absence of mandate. Province-level data provide corroborating evidence. We control for mobility behaviour using Google geo-location data and for lagged case totals and case growth as information variables. Our analysis of additional survey data shows that mask mandates led to an increase of about 27 percentage points in self-reported mask wearing in public. Counterfactual policy simulations suggest that adopting a nationwide mask mandate in June could have reduced the total number of diagnosed COVID-19 cases in Canada by over 50,000 over the period July–November 2020. Jointly, our results indicate that mandating mask wearing in indoor public places can be a powerful policy tool to slow the spread of COVID-19.

我们评估了室内佩戴口罩规定和其他非药物干预措施 (NPI) 对加拿大 COVID-19 病例增长的影响。 2020 年 6 月中旬至 8 月中旬，在加拿大人口最大的省份安大略省的 34 个公共卫生地区错开实施了口罩强制令。利用这种变化，我们发现，相对于不强制佩戴口罩的趋势，强制佩戴口罩与每周新增 COVID-19 病例减少 22% 相关。省级数据提供了佐证。我们使用谷歌地理位置数据控制移动行为，并将滞后病例总数和病例增长作为信息变量。我们对其他调查数据的分析表明，强制佩戴口罩导致自我报告的在公共场合戴口罩的人数增加了约 27 个百分点。反事实政策模拟表明，6 月份在全国范围内强制佩戴口罩可能会使 2020 年 7 月至 11 月期间加拿大确诊的 COVID-19 病例总数减少 50,000 多例。我们的结果共同表明，强制在室内公共场所佩戴口罩可以成为减缓 COVID-19 传播的强大政策工具。