The effectiveness of eight nonpharmaceutical interventions against COVID-19 in 41 countries

Abstract Background Governments are attempting to control the COVID-19 pandemic with nonpharmaceutical interventions (NPIs). However, it is still largely unknown how effective different NPIs are at reducing transmission. Data-driven studies can estimate the effectiveness of NPIs while minimizing assumptions, but existing analyses lack sufficient data and validation to robustly distinguish the effects of individual NPIs. Methods We collect chronological data on NPIs in 41 countries between January and May 2020, using independent double entry by researchers to ensure high data quality. We estimate NPI effectiveness with a Bayesian hierarchical model, by linking NPI implementation dates to national case and death counts. To our knowledge, this is the largest and most thoroughly validated data-driven study of NPI effectiveness to date. Results We model each NPI’s effect as a multiplicative (percentage) reduction in the reproduction number R . We estimate the mean reduction in R across the countries in our data for eight NPIs: mandating mask-wearing in (some) public spaces (2%; 95% CI: −14%–16%), limiting gatherings to 1000 people or less (2%; −20%–22%), to 100 people or less (21%; 1%–39%), to 10 people or less (36%; 16%–53%), closing some high-risk businesses (31%; 13%–46%), closing most nonessential businesses (40%; 22%–55%), closing schools and universities (39%; 21%–55%), and issuing stay-at-home orders (18%; 4%–31%). These results are supported by extensive empirical validation, including 15 sensitivity analyses. Conclusions Our results suggest that, by implementing effective NPIs, many countries can reduce R below 1 without issuing a stay-at-home order. We find a surprisingly large role for school and university closures in reducing COVID-19 transmission, a contribution to the ongoing debate about the relevance of asymptomatic carriers in disease spread. Banning gatherings and closing high-risk businesses can be highly effective in reducing transmission, but closing most businesses only has limited additional benefit.