  We investigate the effect of school closure and subsequent reopening on the
transmission of COVID-19, by considering Denmark, Norway, Sweden, and German
states as case studies. By comparing the growth rates in daily hospitalisations
or confirmed cases under different interventions, we provide evidence that the
effect of school closure is visible as a reduction in the growth rate
approximately 9 days after implementation. Limited school attendance, such as
older students sitting exams or the partial return of younger year groups, does
not appear to significantly affect community transmission. A large-scale
reopening of schools while controlling or suppressing the epidemic appears
feasible in countries such as Denmark or Norway, where community transmission
is generally low. However, school reopening can contribute to significant
increases in the growth rate in countries like Germany, where community
transmission is relatively high. Our findings underscore the need for a
cautious evaluation of reopening strategies that ensure low classroom occupancy
and a solid infrastructure to quickly identify and isolate new infections.
