A Comparison of Clinical Presentations in Coronavirus Disease 2019 Caused by Different Omicron Variants in Japan: A Retrospective Study

OBJECTIVE: We evaluated the clinical differences in coronavirus disease 2019 (COVID-19) patients between the sixth wave with the Omicron BA.1/BA.2 dominant variant (from January to April 2022) and seventh wave with the Omicron BA.5 dominant variant (from July to August 2022). METHODS: This retrospective, single-center, observational study included COVID-19 patients admitted to our institution in the sixth wave (sixth-wave group) and the seventh wave (seventh-wave group). Inter-group comparisons of clinical presentations, the prognosis, and proportion of nosocomial infections were performed. RESULTS: A total of 190 patients were included (93 and 97 patients in the sixth- and seventh-wave groups, respectively). While there were no significant differences in severity, significantly more patients developed pneumonia caused by COVID-19 in the sixth-wave group than in the seventh-wave group. Although there was no marked difference in in-hospital deaths, more patients died from COVID-19 in the sixth-wave group than in the seventh-wave group. There were significantly more COVID-19 inpatients with nosocomial infections in the seventh-wave group than in the sixth-wave group. Pneumonia from COVID-19 was significantly more severe in the sixth-wave group than in the seventh-wave group. CONCLUSION: COVID-19 patients in the seventh wave are at a lower risk of pneumonia than those in the sixth wave. However, even in the seventh wave, patients with underlying diseases have a risk of death because of the exacerbation of underlying diseases triggered by COVID-19.