This aggregated population study of 219 regions in 11 European countries investigated the effect of the seemingly quasi-randomly assigned school winter holiday week on excess mortality associated with the COVID-19 pandemic during spring 2020. A secondary aim was to evaluate the impact of stringency and timing of the government responses to the early inflow of infected cases. Methods Regional data on mortality week 14-23 in 2020 compared with the same period 2015-2019 were retrieved from

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... and national statistical agencies. Data on initial government responses were obtained from the Oxford COVID-19 Government Response Tracker. Variance-weighted least square regression was used with further adjustment for population density and age distribution. Results Being a region with winter holiday exclusively in week 9 was in the adjusted analysis associated with 16 weekly excess deaths (95% confidence interval 13 to 20) per million inhabitants, which corresponds to 38% of the excess mortality during the study period in these regions. A more stringent response implemented in week 11, corresponding to 10 additional units on the 0-100 ordinal scale, was associated with 20 fewer weekly deaths (95% confidence interval 18 to 22) per million inhabitants. Conclusions Travelling during winter holiday in week 9 was an amplifying event that contributed importantly to the excess mortality observed in the study area during the spring 2020. Timely government responses to the resulting early inflow of cases was associated with lower excess mortality.