Severe RSV Infection Occurring at the End of Nirsevimab's Protection Window: A Case Report.

We present the case of a five-month-old late preterm infant who developed severe bronchiolitis caused by respiratory syncytial virus, requiring hospitalisation and high-flow nasal cannula support. This occurred 142 days after the infant received a single dose of nirsevimab as part of the regional immunisation campaign, administered on day three of life. The patient had no underlying conditions. PCR testing revealed RSV A and human rhinovirus co-infection. The infant improved with supportive care and was discharged in stable condition after 8 days. This case raises concerns about possible waning immunity near the end of the expected 150-day protection window of nirsevimab, particularly in infants immunised early in the RSV season. Additionally, co-infection with hRV may have contributed to disease severity. Although nirsevimab remains a highly effective preventive tool, this case highlights the potential for waning immunity near the end of the protection window and suggests that ongoing surveillance is essential to optimize immunization strategies, particularly in regions with prolonged RSV seasons.