Limiting gatherings to fewer than 10 people and closing educational institutions were among the most effective nonpharmaceutical interventions at reducing transmission of SARS-CoV-2, a new modeling study finds. The results, informed by data from 34 European and several non-European countries, may guide policy decisions on nonpharmaceutical interventions (NPIs) to implement in current and future waves of infections. As the COVID-19 pandemic continues, governments are attempting to control it with NPIs. However, the effectiveness of different NPIs is poorly understood; better understanding which are most capable of reducing transmission could help governments more efficiently control the epidemic, while minimizing the social and economic costs. Jan Brauner and colleagues gathered chronological data on the implementation of NPIs for 41 European and non-European countries between January and the end of May 2020, each of which implemented different sets of NPIs, at different times, in different orders. Using a model that linked NPI implementation dates to national case and death counts, they estimated the effectiveness of a range of intervention types (excluding testing, tracing, and case isolation, for which they did not have data). Following a series of sensitivity analyses that varied conditions of their modeling approach, they report that closing both schools and universities (excluding preschools and nurseries) routinely contributed considerably to reducing transmission, as did limiting gatherings to 10 people or fewer. Closing businesses was effective but closing most nonessential businesses had limited benefit over targeted closures of high-risk businesses. "Issuing a stay-at-home order had a small effect when a country had already closed educational institutions, closed nonessential businesses, and banned gatherings," they say. The authors note several limitations of their study, including that their results cannot be used without qualification to predict the effect of lifting NPIs. "[C]losing schools and universities in conjunction seems to have greatly reduced transmission, but this does not mean that re-opening them will necessarily cause infections to soar." They conclude, "Our work offers insights into which areas of public life are most in need of virus containment measures so that activities can continue as the pandemic develops; however, our estimates should not be taken as the final word on NPI effectiveness." Readers can interactively explore the effects of sets of NPIs with the study's online mitigation calculator: http://epidemicforecasting.