[Temporal relationships between a state of emergency declaration or request and trends in the incidence of unlinked COVID-19 cases in Osaka from October 2020 to May 2021].

Objective　We examined the effectiveness of governmental declarations on the changing temporal trends in the incidence of COVID-19 cases with unknown transmission routes (unlinked cases) before SARS-CoV-2 vaccines became available in Osaka.Methods　Seven-day moving averages of the incidence of unlinked COVID-19 cases were calculated using daily reports posted on the official website of the Osaka Prefectural Government for the third (October 10, 2020-February 28, 2021) and fourth COVID-19 waves and about one week before and after (February 23, 2021-June 27, 2021). Then, we calculated daily percentage changes and identified dates of significant change ("Joinpoint") with a Joinpoint regression analysis. The date of a major change in the behavior of prefectural residents associated with the risk of infection (the date of a sudden change in behavior) was defined as the date counting backward from each Joinpoint date and considering the incubation period and interval between the date of onset and disclosure of the daily report. Subsequently, we examined the temporal relationships between the declaration date and defined date of the sudden change in behavior associated with the risk of infection.Results　Five Joinpoint dates contributing to a significant downward trend were identified: November 23, 2020, and in 2021, January 7, January 18, April 12, and April 30. We defined dates of sudden changes in behavior from each Joinpoint date from the corresponding time lag (8 to 9.9 days): in 2020, November 13 and December 30; in 2021, January 9, April 4, and April 22. Regarding the five estimated dates, the second emergency declaration was issued on January 9, 2021. Further, the first introduction of priority preventive measures was given on April 4, 2021. April 22 fell between the date that the third emergency declaration was requested and the date of its issuance.Conclusion　These descriptive epidemiological findings suggest that the issuance of these declarations could be triggers that reinforced infection avoidance behavior among Osaka prefectural residents, which resulted in the downward trends in unlinked COVID-19 cases.