Respiratory syncytial virus (RSV) is a negative-stranded RNA virus belonging to the Orthopneumovirus genus of the Pneumoviridae family and is a common virus that causes respiratory infections in the pediatric population [1]. In addition, RSV has been identified as an important causative virus of acute respiratory infections, lower respiratory tract infections (LRTI), clinical complications, and death in older adults [2].

The earlier the patient is infected after birth, the higher the rate of severe disease, which may progress to acute bronchiolitis or pneumonia and require oxygen supplementation and occasional ventilator management [1]. Premature infants and children with underlying diseases, such as congenital heart and chronic lung diseases, are at a higher risk of severe disease. Globally, including Japan, the RSV-specific monoclonal antibody palivizumab is administered to neonates, infants, and young children at risk of developing severe RSV infection [3]. Palivizumab can reduce hospitalization owing to RSV-associated LRTI in such patients [4]. However, the use of palivizumab is hindered by its high cost, making it less cost-effective, and its short half-life requires monthly intramuscular injections [5,6].

Recently, new preventive strategies, including maternal RSV prefusion F protein-based vaccine (RSVpreF) vaccination and the long-acting RSV monoclonal antibody nirsevimab, have been introduced to increase the options for RSV prevention. In the United States, starting from 2023, seasonal administration of the maternal RSVpreF vaccine for pregnant women as a one-time dose at 32–36 weeks of gestation has been recommended to prevent RSV-associated LRTI in neonates and infants aged less than 6 months [7,8]. In Japan, the vaccine has been available year-round since May 2024; however, vaccination is voluntary, requiring approximately US$150, and public awareness of this vaccine remains low [9]. In the United States, starting from 2023, administration of nirsevimab has been recommended for all neonates and infants under 8 months, depending on whether their mothers received the RSVpreF vaccine during pregnancy and the time of year (during the RSV epidemic in the United States, from October through March) [10]. In certain European countries, including Spain, Luxembourg, France, and Italy, nirsevimab has been administered universally to neonates and infants under 6–9 months during the RSV seasons since 2023 [11,12]. In contrast, in Japan, the health insurance system has limited nirsevimab administration to neonates, infants, and young children at high risk for severe RSV infection since its introduction in May 2024. In neonates and infants without underlying diseases, nirsevimab administration is performed out-of-pocket, incurring approximately 3000 US$ for the 50 mg formulation and 6000 US$ for the 100 mg formulation, as of April 10, 2025 (calculated as US$ = 146.56 yen).

Previously, RSV epidemics in Japan occurred from autumn to early winter, peaking in January or February. However, since 2017, the epidemic pattern has changed, starting in July and peaking in August-September [13]. Following the emergence of the coronavirus disease 2019 (COVID-19), the RSV epidemic barely occurred in 2020, and since 2021, epidemics began in April–June, peaked in July, and became less seasonal than before 2019 in Japan [3,14]. Therefore, predicting the beginning and peak of RSV epidemics has been challenging in recent years. Moreover, in 2023, our hospital, Niigata City General Hospital, documented a notable increase in cases of severe RSV infection requiring intensive care unit (ICU) management, which led to the current investigation.

The objectives of this study were 1) to elucidate the changes in the clinical characteristics of hospitalized RSV-infected pediatric patients in Japan before and during/after the COVID-19 pandemic, and 2) based on the results of the first objective, we aimed to identify the number of infants aged 0–5 months without underlying diseases who were hospitalized or admitted to the ICU, and who may have been potentially protected from severe RSV disease by maternal RSVpreF vaccination and/or nirsevimab, the latter of which is currently administered only to those at high risk for severe RSV infection in Japan.