The relative effects of non-pharmaceutical interventions on early Covid-19 mortality: natural experiment in 130 countries

Background Concurrent non-pharmaceutical interventions have been implemented around the world to control Covid-19 transmission. Their general effect on reducing virus transmission is proven, but they can also be negative to mental health and economies, and transmission behaviours can also change in absence of mandated policies. Their relative impact on Covid-19 attributed mortality rates, enabling policy selection for maximal benefit with minimal disruption, is not well established. Methods We exploited variations in nine non-pharmaceutical interventions implemented in 130 countries (3250 observations) in two periods chosen to limit reverse causality: i) prior to first Covid-19 death (when policymakers could not possibly be reacting to deaths in their own country); and, ii) 14-days-post first Covid-19 death (when deaths were still low, on average). We examined associations with daily deaths per million in each subsequent 24-day period (the time between virus transmission and mortality) which could only be affected by the policy period. A mean score of strictness and timeliness was coded for each intervention. Days in each country were indexed in time by first reported Covid-19 death to proxy for virus transmission rate. Multivariable linear regression models of Covid-19 mortality rates on all concurrent interventions were adjusted for seasonality, potential confounders, and potential cross-country differences in their mortality definitions. Robustness was checked by removing countries with known data reporting issues and with non-linear, negative binomial, models. Results After adjusting for multiple concurrent interventions and confounders, and accounting for both timing and strictness of interventions, earlier and stricter school (-1.23 daily deaths per million, 95% CI -2.20 -0.27) and workplace closures (-0.26, 95% CI -0.46 -0.05) were associated with lower Covid-19 mortality rates. Only controlling for strictness international travel controls, and only controlling for timing later restrictions on gatherings, were also associated with lower Covid-19 mortality. Other interventions, such as stay-at-home orders or restrictions on public transport, were not significantly associated with differences in mortality rates across countries. Findings were robust across multiple statistical approaches. Conclusions Focusing on compulsory, particularly school closing, not voluntary reduction of social interactions with mandated policies appears to have been the most effective strategy to mitigate early Covid-19 mortality.