COVID-19 Double Annual Epidemic Peaks in Summer and in Winter from 2022, Irrespective of the Rate of Mask Wearing and Vaccination.

Although vaccination for COVID-19 and mask wearing were two of the main preventive measures against infection, their impact is unclear. In the present study, by using national surveillance data in Japan, we compared the incidence rate and weekly case increase ratios of COVID-19 with the domestic stocks of masks and vaccination coverage. The trajectory of epidemic growth increased rapidly in the summer of 2021, concomitant with the launch of the mass national vaccination program. The most rapid spread of the epidemic was found in 2022, approximately 6 months after the national mass vaccination started, with the emergence of the Omicron variant. From 2022, two annual epidemic peaks occurred with seasonal changes. Whilst the winter peak follows the expected seasonal trend in respiratory infections, the summer peak may reflect a combination of short-term herd immunity and behavioral patterns. Nevertheless, these epidemic peaks continued irrespective of vaccine coverage and mask use. Further analysis into the duration of protective efficacy of the vaccines and mask use is required.