‘Not finding causal effect’ is not ‘finding no causal effect’ of school closure on COVID-19

Abstract In a paper recently published in Nature Medicine, Fukumoto et al. tried to assess the government-led school closure policy during the early phase of the COVID-19 pandemic in Japan. They compared the reported incidence rates between municipalities that had and had not implemented school closure in selected periods from March–May 2020, where they rigorously matched for potential confounders, and claimed that they found no causal effect on the incidence rates of COVID-19. However, the effective sample size (ESS) of their dataset had been substantially reduced in the process of matching due to imbalanced covariates between the treatment (i.e. with closure) and control (without) municipalities, which led to the wide uncertainty in the estimates. That said, the study title "No causal effect…" is a rather strong statement because the results are also consistent with a strong mitigating effect of school closure on incidence of COVID-19.