Japan experienced four major epidemic periods (i.e., the 3rd-6th waves) of COVID-19 from January 2020 to March 2022. A different strain predominated in each period: the wild-type strain, an Alpha variant, a Delta variant, and an Omicron variant were predominant in waves 3, 4, 5, and 6, respectively. In this study, we conducted a descriptive analysis to investigate the differences in symptoms and severity during each epidemic period. We analyzed a total of 31,037 patients with COVID-19 who were reported to the Okayama City Public Health Center between February 1, 2020 and March 31, 2022. We described and compared the proportion of symptoms, the number and proportion of severe COVID-19 cases, and the number and proportion of deaths for each epidemic period. We also compared the proportion of severe COVID-19 cases depending on the number of vaccinations for the 5th and 6th waves.
Differences in symptoms and severity were observed in each epidemic period. In the 5th and 6th waves, the results suggested that vaccination had a preventive effect against severe COVID-19.
