Molecular and Epidemiological Analysis of Respiratory Syncytial Virus Detected in Tokyo, Japan in 2021 Season.

During the COVID-19 pandemic in 2021, Japan experienced an outbreak of respiratory syncytial virus (RSV) infection. A total of 51 RSV cases were detected from the infant specimens, including 38 rhinorrhea and 13 nasopharyngeal swabs, collected at the Tokyo Metropolitan Institute of Public Health. Of these 51 cases, 12 belonged to RSV-A and 39 to RSV-B. G protein gene sequences of RSV-A belonged to the ON1 genotype, whereas RSV-B belonged to the BA9 genotype; thus, different types of RSV were detected during the same period, suggesting that the unusual 2021 RSV season was not due to a single strain or genotype. Of all RSV-positive cases, the proportion of cases aged ≥2 years was 56.8% in 2021, which was higher than 31.2% in the past 5 years. This indicates that infants aged <1 year who were originally susceptible to RSV infection were less likely to be infected with RSV because of the COVID-19 control measures. The 2021 epidemic peaked in the 28th week, which was 9 weeks earlier than the average from 2016 to 2020. It seems necessary to accumulate and analyze further data, such as factors that became an outbreak and the characteristics of the detected viruses in 2021.