Monday effect on confirmed cases of COVID-19 in Japan

We examined the phenomenon of fewer new confirmed cases on Monday in Japan, which we refer to as the Monday effect. In Japan, prefectures aggregate and announce the number of daily confirmed cases. We analyzed the impact of this effect in each prefecture. The effect is mainly found in prefectures with populations of 2 million or more. This effect is also constantly observed in the three major metropolitan areas in Japan. However, the magnitude of the observed effect is uncorrelated with both the number of positives per 1,000 people and the population size. Our results suggest that the reporting delay occurs in prefectures above a specific size, but the magnitude of the delay differs among prefectures. We consider two possible explanations for this effect: 1) delays caused by the administrative system. 2) fewer tests are conducted on the previous day. Our results indicate that delays are caused by the administrative system in some prefectures and that some prefectures with larger populations are less likely to conduct screenings on holidays.