Comparative analysis of acute respiratory infections of viral etiology in children under 6 months with and without nirsevimab in the Balearic Islands (2022-2023 and 2023-2024).

Introduction: Since 2023, a monoclonal antibody (nirsevimab) specifically directed against the preF form of RSV has been marketed in Spain. The impact of late immunization in the population <6 months as a cause of other respiratory infections requiring a hospital visit has been analyzed.

Material and methods: The viral etiology of acute respiratory infections (ARIs) diagnosed in the 2022-2023 and 2023-2024 seasons has been prospectively compared, and in this last season between recipients and non-recipients of nirsevimab. Global provisional coverage was 77% of the population.

Results: In the 2022-2023 season, 303 < 6 months with an ARI were detected, while in the 2023-2024 season there were 278 minors (19% less). The positivity in the first season was 79.9% compared to 70.5% in the current season. A significant difference has been observed in the detection of RSV between both groups and a decrease of 82.9% of cases in the current season. Of the 278 cases <6 months detected in the 2023-2024 season, 192 (69.1%) received immunization with nirsevimab and 86 (30.9%) did not receive it. The percentage of positivity in those immunized was 69.3%, compared to 73.3% in those not immunized. Significant differences have been observed in the detections of RSV and influenza between both groups.

Conclusions: Immunization at <6 months with nirsevimab has shown a significant reduction in RSV infections compared to the previous season. It does not seem, however, that it can reduce infections by other respiratory viruses.