Alpha and Delta variants and vaccination effectiveness against severity in COVID-19 inpatients based on medical claims in Japan.

Some mutated strains of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) presumably have high infectivity and pathogenicity. Using Japanese medical claims data, we assessed the pathogenicity of Alpha and Delta variants and vaccine effectiveness by severity. Inpatient records from the Medical Information Analysis Databank for the National Hospital Organization were used. Severity was defined as the proportion of inpatients using respiratory ventilators among inpatients with oxygen administration. We regressed severity and fatality on the proportion of patients with Alpha or Delta variant and on vaccination coverage, while allowing for some lag to reflect development from infection to hospitalization. We also examined results obtained when using data for all new inpatients, instead of inpatients with oxygen administration, as the denominator for severity. Estimation results were better when using severity defined by inpatients with oxygen administration as the denominator than when using all new inpatients. Especially for severity measures for inpatients 65 years old or older with oxygen administration, we confirmed an association of vaccination with lower severity and an association of Delta variant infection with high severity. Vaccines were most effective for people 65 years old or older. The age distributions of inpatients and confirmed patients were greater than for people younger than 65 years old. Vaccination reduced severity and fatality and Alpha and Delta variants might increase severity and fatality among inpatients 65 years old or older receiving oxygen therapy.