Trends in the incidence of symptomatic COVID-19 during the first endemic wave in the Japanese population

Abstract

Object: First coronavirus disease 2019 (COVID-19) endemic wave in Japan was almost stabilized in late May in 2020, without strong physical distancing interventions. Our objective was to analyze temporal trends in the incidence of symptomatic COVID-19 during this phase in Osaka Prefecture (population of 8.8 million).

Methods: We calculated 7-day moving averages of the date-of-onset-based number of symptomatic COVID-19 cases using anonymous data posted on the official website of the Osaka Prefectural Government between February 27 and May 23 in subjects whose route of transmission was known (linked case) or unknown (unlinked case). Joinpoint regression analysis was performed. Daily percent change (DPC) in the incidence and dates of significant change (“joinpoint”) were identified in the Joinpoint regression analysis.

Results: The maximum daily number of symptomatic COVID-19 cases was 72 on April 3. From March 12 to April 2, the incidence of unlinked cases significantly increased (DPC: +14.8%). Then, the incidence rapidly decreased until late May with accelerating downward trend between April 12 and 17 (DPC%: -15.8%). The temporal change in linked cases was almost synchronized to that of unlinked cases with 6-7 days’ delay.

Conclusions: The peak incidence of unlinked COVID-19 cases in the first endemic wave in Osaka was observed at April 2, 2020. The temporal trend was synchronically followed by that of linked cases with 6-7 days’ delay.