Impact of nirsevimab immunization on pediatric RSV-related hospitalizations in Luxembourg

Respiratory syncytial virus (RSV) is one of the main reasons leading to hospitalization among young children worldwide and each year, an estimated 101,000 children below the age of 5 years die due to RSV infections. Since the end of 2022, the use of a long-acting monoclonal antibody (nirsevimab) in infants as means of passive immunization against RSV has been allowed in the European Union and European Economic Area (EU/EEA). However, only a few countries in the EU/EEA started using nirsevimab prior to the RSV season 2023/24.

In their rapid communication published in Eurosurveillance, Ernst et al. describe first experiences from Luxembourg where a one-dose nirsevimab prophylaxis has been recommended for all babies born in 2023 accompanied by a suggested catch-up immunization for children under the age of two years at high risk of serious illness. A national immunization campaign started in the autumn 2023 which reached an estimated 84% of new-borns (1,277 doses/1,524 births) in 2023.

Besides estimating coverage of nirsevimab immunization in Luxembourg among new-borns up to mid-December 2023, Ernst et al. looked at potential effects of this immunization in children under 5 years of age. To do this, they compared RSV-related pediatric hospitalization data from Luxembourg's national pediatric hospital in 2022 and 2023, i.e. before and after the national nirsevimab immunization recommendation.

Promising first signs: fewer hospitalizations, change in age structure and less severe illness

The authors could show a drop in hospitalizations related to RSV infections with a distinct decrease (69%) among infants younger than six months of age (232 confirmed cases in 2022 and 72 cases in 2023). In the wider age group of children under the age of 5 years, in total 241 children were hospitalized with a laboratory-confirmed RSV infection in 2023, compared with 389 cases in 2022 (38% decrease).

According to the available data in Luxembourg, the authors also noted a shift in the age structure among the children that were hospitalized due to RSV infection: the mean age of hospitalized children increased from slightly below 8 months in 2022 to more than 14 months in 2023, i.e. children were generally older when they were hospitalized. Ernst et al. attributed this to the "effect of the administration strategy shortly after birth. Most hospitalized children had not yet received a nirsevimab immunization […]".

In addition, data showed a reduced severity among the children that had to be hospitalized in 2023. Instead of around 5 days in 2022, children were hospitalized for around 3 days in 2023 and admissions of infants to the intensive-care unit fell from 28 in 2022 to 9 in 2023 in the age group of the most vulnerable (younger than six months).

The authors conclude that this "suggests that nirsevimab prophylaxis reduced severe RSV infections, particularly in infants < 6 months old, thereby alleviating healthcare strain" - while acknowledging that their comparison at this point in time only looks at two consecutive seasons and did not cover the full 2023/24 RSV season yet.