Irregular seasonality of respiratory syncytial virus infection persists in 2023 in Osaka, Japan.

OBJECTIVES

Owing to the nonpharmaceutical interventions against COVID-19, respiratory syncytial virus (RSV) infection was nearly absent in 2020. An unusual epidemic size and irregular seasonal pattern were observed worldwide in 2021. In Osaka, Japan, after disrupting the regular pattern of RSV infection dynamics (before the COVID-19 pandemic, RSV epidemics typically start in summer and peak around fall), the epidemic size of RSV infection returned to normal in 2022. However, the epidemic onset timing remained irregular in 2022 and 2023. This study investigated whether the onset of the RSV infection epidemic in 2023 was predictable using previous seasonal patterns.

METHODS

The weekly number of RSV infection cases obtained from sentinel pediatric sites between 2007 and the 15th week of 2023 was modeled using the time series susceptible-infected-recovered model. Forecasting of the remainder of 2023 was conducted based on estimated transmission parameters.

RESULTS

None of the estimated transmission rates from previous years successfully forecast the epidemic onset in 2023. Only the transmission rate estimated in the early part of 2023 captured the trend for that year, indicating irregular seasonal transmission rates.

CONCLUSIONS

It is still hard to forecast RSV epidemics because of the changed landscape due to the COVID-19 pandemic. The seasonality of RSV infection dynamics has not returned to pre-pandemic level in 2023. Cautious attention to future RSV dynamics in Japan is warranted because further changes may occur in the near future.