Title: Factors associated with time lag between symptom onset and reporting in the first epidemic wave of COVID-19 in Osaka, Japan.

Objectives: Longer reporting lags after symptom onset reportedly exert a substantial impact on onward transmission, increasing outbreak probability. Our study investigated the risk factors associated with reporting lag.

Methods: Using active epidemiological surveillance data for all symptomatic cases reported in Osaka Prefecture during the first wave of the coronavirus disease 2019 (COVID-19) epidemic (February 1-May 13, 2020), multivariable regression analyses were implemented to estimate the effects of exposure variables on reporting lag, by controlling for potential confounders.

Results: Cases in their 30s showed a longer reporting lag than cases ≥ 80 years old. Cases who lived in areas with a high COVID-19 incidence demonstrated a longer reporting lag. Cases with a history of visiting a nightlife district also showed longer reporting lag than cases without such a history. Healthcare workers and cases with immunodeficiency both displayed shorter reporting lags than others.

Conclusion: Identifying newly infected cases as soon as possible and increased testing capacity for all age groups, and for individuals with a history of visiting high infection-risk areas, represented important measures in shortening reporting lags in the first wave period. The evidence from this study may provide lessons for controlling future emerging diseases.