A study estimates the impact of non-pharmaceutical interventions (NPIs) on COVID-19 in 11 European countries from February 2020 to May 2020. Using a Bayesian model, researchers infer transmission rates (Rt) by working backward from observed deaths, accounting for time lags between infection and death. The model pools data across countries to improve estimates, despite data limitations such as incomplete reporting and varying testing policies. The study finds that NPIs, including lockdowns, have significantly reduced Rt, with Rt below 1 in all countries, indicating epidemic control. It estimates that 12–15 million people were infected in the 11 countries by May 2020, representing 3.2–4.0% of the population. Lockdowns had the largest effect, reducing Rt by 81%. The study also estimates that interventions averted 3.1 million deaths. However, the model has limitations, including the inability to fully capture fine-scale variation and potential underreporting of infections. The results suggest that continued NPIs are necessary to maintain control of the epidemic. The study highlights the importance of timely and accurate data in understanding the effectiveness of public health measures.A study estimates the impact of non-pharmaceutical interventions (NPIs) on COVID-19 in 11 European countries from February 2020 to May 2020. Using a Bayesian model, researchers infer transmission rates (Rt) by working backward from observed deaths, accounting for time lags between infection and death. The model pools data across countries to improve estimates, despite data limitations such as incomplete reporting and varying testing policies. The study finds that NPIs, including lockdowns, have significantly reduced Rt, with Rt below 1 in all countries, indicating epidemic control. It estimates that 12–15 million people were infected in the 11 countries by May 2020, representing 3.2–4.0% of the population. Lockdowns had the largest effect, reducing Rt by 81%. The study also estimates that interventions averted 3.1 million deaths. However, the model has limitations, including the inability to fully capture fine-scale variation and potential underreporting of infections. The results suggest that continued NPIs are necessary to maintain control of the epidemic. The study highlights the importance of timely and accurate data in understanding the effectiveness of public health measures.