Impact of universal nirsevimab prophylaxis on clinical and ultrasound presentations of lower respiratory tract infections in hospitalized pediatric patients.

Lower respiratory tract infections (LRTIs) remain a major cause of morbidity and mortality in young children worldwide, with respiratory syncytial virus (RSV) as the leading pathogen. This study aims to evaluate the impact of implementing universal nirsevimab prophylaxis on the clinical and lung ultrasound (LUS) presentations of LRTIs in hospitalized pediatric patients. We conducted a prospective observational study at a tertiary pediatric hospital in southern Spain, including 197 children under 14 years of age hospitalized with LRTIs. Two periods were differentiated: the pre-nirsevimab period (October 2022-April 2023) and the post- nirsevimab period (October 2023-April 2024). Patients were classified by clinical diagnosis and microbiological results. LUS was performed upon admission, after 48 h, and upon discharge. Clinical severity scores, oxygen requirements, intensive care admissions, and length of stay were recorded. A total of 94 patients were included in the pre-nirsevimab period, and 103 in the post-nirsevimab period. During the second period, admissions due to bronchiolitis decreased (61.7% vs. 34.0%, p = 0.001), whereas those due to pneumonia and asthma exacerbations increased. RSV-related admissions dropped significantly (69.6% vs. 32.0%, p = 0.001), while there was a rise in metapneumovirus and Mycoplasma pneumoniae infections. Post-nirsevimab patients were older (13 vs. 5.5 months, p = 0.023) and required less oxygen (72.8% vs. 86.0%, p = 0.023) and fluid therapy (9.7% vs. 30.9%, p = 0.001). There were no differences in ultrasound scores, sizes, numbers or locations of consolidations.

conclusionsA shift in etiology and age distribution was observed, alongside an increase in viral pneumonias and asthma exacerbations in the post-nirsevimab period. However, ultrasound parameters remained similar across periods. These findings support the role of nirsevimab in modifying the burden and clinical spectrum of pediatric respiratory infections. WHAT IS KNOWN: • Universal nirsevimab prophylaxis reduces RSV-related hospitalizations and bronchiolitis incidence, modifying the clinical burden of pediatric LRTIs. However, its impact on LUS findings in hospitalized children has not been well characterized. WHAT IS NEW: • Despite clinical changes after universal nirsevimab prophylaxis (including lower RSV incidence and reduced oxygen needs), LUS profiles of LRTIs remained unchanged, suggesting stable imaging findings regardless of shifts in viral etiology.