Palivizumab prophylaxis and risk of RSV infection among moderate-to-late preterm infants: A nationwide Korean cohort study from 2018 to 2022

Respiratory syncytial virus (RSV) is a leading cause of respiratory infections in infants, and preterm infants are at higher risk of severe illness. While RSV immunoprophylaxis is widely considered necessary for very preterm infants in most countries, its efficacy in moderate-to-late preterm infants (MLPI) remains controversial. This study aimed to evaluate the preventive and therapeutic effects of palivizumab in MLPI, using data from the National Health Insurance Service of Korea. We analyzed the National Health Insurance Service data for infants born during the RSV season (October-March) from 2018 to 2022. Infants with a gestational age of 32 weeks 0 days' to 36 weeks 6 days' gestation were categorized as MLPI and those born between 37 weeks 0 days' and 41 weeks 6 days' gestation as full-term infants. Infants with congenital anomalies or severe cardiac diseases were excluded. Both groups were divided according to palivizumab administration. We compared the RSV infection rates and hospitalization duration. Of the 593,419 infants, 137,452 were excluded. Among 440,163 full-term infants, 938 (0.23%) received palivizumab, compared with 2096 (13.26%) of 15,804 MLPI. In full-term infants, RSV infection occurred in 0.68% of the non-recipients and 0.21% of the recipients. Among the MLPI, the infection rates were 1.18% in non-recipients and 0.62% in recipients, indicating a statistically significant reduction. Logistic regression analysis revealed that palivizumab reduced the risk of RSV infection in MLPI by an approximately 53% lower odds (adjusted odds ratio, 0.47; P = .011). Palivizumab significantly reduced the incidence and severity of RSV infection in MLPI in Korea. This study provides crucial nationwide baseline data on the RSV disease burden in this vulnerable population immediately before the introduction of novel prophylactic strategies such as nirsevimab, and can serve as evidence for future health policy decisions and cost-effectiveness analyses.