Combined effect of early diagnosis and treatment on the case fatality risk of COVID-19 in Japan, 2020.

Japanese government initially enforced restrictions on outpatient attendances among febrile individuals suspected of having COVID-19, asking everyone to remain at home for at least 4 days from the onset of fever. This restriction was cancelled on 8 May 2020, and a new antiviral, remdesivir, was approved from 7 May 2020. To investigate how this policy change influenced the prognosis of people with COVID-19, we estimated the case fatality risk as a function of the date of illness onset from April to June 2020. We used an interrupted time-series analysis model with an intervention date of 8 May 2020, and estimated time-dependent case fatality risk by age group. The case fatality risk showed a decreasing trend in all groups, and models were favored accounting for an abrupt causal effect, i.e., immediate decline in fatality risk. The trend was estimated at - 1.1% (95% CI [confidence interval]: - 3.9, 3.0) among people aged 60-69 years, - 7.2% (95% CI - 11.2, - 2.4) among those aged 70-79 years, - 7.4% (95% CI - 14.2, 0.2) among those aged 80-89 years, and - 10.3% (95% CI - 21.1, 2.7) among those aged 90 and over. Early diagnosis and treatment greatly contributed to reducing the case fatality risk.