The Effects of Non-pharmaceutical Interventions on COVID-19 Mortality: A Generalized Synthetic Control Approach Across 169 Countries

Importance: Governments have introduced non-pharmaceutical interventions (NPIs) in response to the pandemic outbreak of COVID-19. While NPIs aim at preventing fatalities related to COVID-19, previous literature on their efficacy has focused on infections and on data of the first half of 2020, which may be subject to underreporting and reporting delays. Low variation in treatment timing during the first wave further complicates identification of robust treatment effects. Objective: We enhance the literature on the effectiveness of NPIs with respect to the period, the number of countries, and the analytical approach. Design, Setting, and Participants: We analyze daily confirmed COVID-19-related deaths per capita from Our World in Data and 10 NPIs from the Oxford COVID-19 Government Response Tracker (OxCGRT) for 169 countries from July 1, 2020 to September 1, 2021. To identify the causal effects of introducing NPIs on COVID-19-related fatalities, we apply the generalized synthetic control (GSC) method to each NPI, while controlling for the remaining NPIs, weather conditions, vaccinations, and NPI-residualized COVID-19 cases. This mitigates selection into treatment and allows modeling flexible post-treatment trajectories. Results: We do not find substantial and consistent COVID-19-related fatality-reducing effects of any NPI under investigation. We observe a tentative change in the trend of COVID-19-related deaths around 30 days after strict stay-at-home rules and to a lesser extent after workplace closings, but effects are not statistically significant. As a proof of concept, the model identifies a significant fatality-reducing effect of COVID-19 vaccinations. Results are robust to multiple sensitivity checks. Conclusion: Many implemented NPIs may not have exerted a significant COVID-19-related fatality-reducing effect, though they might have mitigated fatalities by preventing exponential growth. Vaccinations were effective in reducing COVID-19-related deaths.