Inpatient Burden of COVID-19 in Japan: A Retrospective Cohort Study

ABSTRACT Background Changing disease dynamics and access to COVID-19 vaccines in Japan warrant a timely description of the burden of severe disease. Here we report inpatient healthcare resource utilization of COVID-19 in Japan and contextualize results with influenza. Methods We selected persons hospitalized for COVID-19 (ICD-10 code U07.1) from April 1, 2020 - January 31, 2024 or influenza (ICD-10 code J09.X – J11.x) from November 1, 2017 - October 31, 2019 from Medical Data Vision, a large hospital-based database in Japan. Outcomes of interest were length of stay, intensive care unit (ICU) admission, receipt of invasive mechanical ventilation (IMV), and inpatient mortality, assessed overall, as well as stratified by age groups and calendar time. Findings Among 5,684 hospitalized COVID-19 cases, persons were older (median age 80 vs 64) and had a longer length of stay (median 21 vs 5 days) than the comparator 18,584 influenza cases. The proportions of patients admitted to ICU (3% vs 1%), received IMV (6% vs 3%) and died in hospital (12% vs 3%) were higher in COVID-19 patients than influenza patients. Burden was higher in adult COVID-19 patients than pediatric COVID-19 patients, although for both COVID-19 burden surpassed influenza. Inpatient burden of COVID-19 between May 2023 and January 2024 remained greater than influenza, with 5-times longer length of stay, more frequent need for ICU care (3-times higher), IMV support (2-times higher) and in-hospital deaths (4-times higher). Interpretation These findings underscore the need for continued prevention and treatment of COVID-19 to prevent severe disease. Funding Pfizer Inc. RESEARCH IN CONTEXT Evidence before this study Since COVID-19 began in March 2020, differences by region have been noted. While evidence exists regarding burden of severe COVID-19 in countries such as the United States and the United Kingdom, it was not known whether similar patterns of length of stay, admissions to the intensive care unit, receipt of invasive mechanical ventilation or in-hospital mortality were observed in Japan. Added value of this study These results are important, as this is the first study to describe severe COVID-19 in comparison with influenza of older adults in the inpatient setting in Japan. These data fill an evidence gap using local data. Admission to the intensive care unit, receipt of invasive mechanical ventilation and death in the hospital were more frequent in patients with COVID-19 than patients with influenza. Inpatient burden increased with age and varied over calendar time. We observed notable differences in the burden of disease and care patterns in Japan as compared to other countries. Implications of all the available evidence Contrary to the perception that the omicron variant is less severe, hospitalizations for COVID-19 have continued to accrue and cause severe disease in all ages in Japan. Once hospitalized, individuals with COVID-19 are under medical care for a substantial amount of time. Thus, preventive measures such as vaccination or early treatment to prevent severe disease are important, even in the late Omicron period.