An atypical surge in RSV infections among children in Saitama, Japan in 2021

Background: In 2021, atypical seasonal respiratory syncytial virus (RSV) outbreaks were observed in several countries. However, the peak, duration, and severity of these outbreaks have not been assessed. Methods: Data were collected from almost all facilities with pediatric wards in Saitama Prefecture, Japan. Variables included the weekly number of patients admitted with RSV infection, their age, and the number of patients who required intubation. The average weekly admission rates (number of patients admitted divided by the number of hospitals) were compared by pre-pandemic year (2018 and 2019) and 2021, using analysis of variance. Results: In 2021, 1354 patients were admitted with RSV infection. The median age of the patients was < 12 months. The admission rate peaked around week 30. The slope of the peak in 2021 was markedly steeper than in the previous years. The mean weekly admission rate did not differ significantly between 2018, 2019, and 2021 (p = 0.77). The proportions of intubated patients did not differ significantly over the 4 years (2018­2021, p = 0.68). Conclusion: The overall number of RSV admissions and intubation rate in 2021 were similar to those in pre-pandemic years.