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Monday effect on confirmed cases of COVID-19 in Japan

By Kuninori Nakagawa, Taro Kanatani

Posted 22 Jul 2021
medRxiv DOI: 10.1101/2021.07.20.21260858

We examined the phenomenon of fewer new confirmed cases of COVID-19 on Mondays in Japan, which we refer to as the Monday effect, and reveal the details of this effect. Specifically, we estimated the difference between the number of new positive cases that decreased over the weekend and the number of new confirmed cases that decreased at the beginning of the week. In Japan, prefectures aggregate and announce the number of confirmed daily cases. This analysis allows us to examine whether there is a Monday effect in each prefecture. We show that the Monday effect is due to the decreased number of inspections on the weekend appearing at the beginning of the week due to a time lag. Our results indicate that the administrative system causes delays in some prefectures, and that some prefectures are less likely to conduct screenings on holidays. Our results also suggest that delays generally occur in prefectures with a population of over 2 million.

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