Respiratory syncytial virus (RSV) causes acute respiratory tract illness in individuals of all ages, although its severity depends on a variety of factors, including age, underlying disease and history of infection. Risks of serious symptoms, such as lower respiratory tract infection include children under 6 months,¹ children with chronic lung disease (CLD),¹ preterm infants,¹ infants with congenital heart disease (CHD),¹ children with Down syndrome² and children who are immunocompromised.³ In particular, RSV infections often cause respiratory failure in infants younger than 3 months, even without an underlying disease.

RSV causes seasonal outbreaks worldwide. In tropical and semi-tropical climates, seasonal outbreaks are usually associated with the rainy season.⁴ In temperate regions, many bronchiolitis epidemics from late autumn to winter are caused by RSV.⁴ In Japan, these epidemics usually occur from November to April, with a peak in January or February. Recently, the epidemic started in early August, a month earlier, based on the Infectious Disease Weekly Report for the 6 seasons from 2012 to 2017.⁵

In Japan, RSV rapid diagnosis kits are widely used; however, insurance is only applied to inpatients, outpatient infants less than 1 year of age and children who are eligible for palivizumab, a humanized monoclonal antibody. For this reason, RSV rapid antigen detection tests (RADTs) tend to be performed during the period when bronchiolitis is prevalent; thus, it is difficult to confirm whether other studies refer to actual seasonal variations. In our hospital, RSV RADTs are performed for almost all hospitalized children with suspected infectious diseases by means of control measures to prevent nosocomial infections during all seasons. Therefore, it has been possible to confirm RSV morbidity in children requiring hospitalization, even when RSV does not occur. The purpose of this study was to evaluate the seasonal variation in hospitalized patients with RSV infection over the past decade. Furthermore, we elucidated how different seasons influenced severity in children with RSV.