Nirsevimab protection against RSV hospitalization does not extend to second year

A French nationwide cohort study of over 250,000 infants shows that nirsevimab (Beyfortus) reduces RSV-related hospitalizations by 66% in 2023 and 72% in 2024 during the first year after administration. However, lead author Amélie Gabet, PhD, reports that this protective effect does not extend into the second year. Additionally, the study associates nirsevimab with slightly higher rates of hospitalizations for ENT bacterial infections and asthma, though researchers caution these findings are based on small numbers of events.

Nirsevimab first-year RSV hospitalization effectiveness

• ▪A French nationwide cohort study found that nirsevimab administration was associated with lower rates of hospitalization for respiratory syncytial virus lower respiratory tract infection (RSV-LRTI) during the first year after immunization.
• ▪The 2024 cohort findings demonstrated that nirsevimab's effectiveness was maintained despite a shift toward RSV B predominance and the emergence of RSV B variants carrying resistance-associated mutations.
• ▪In the 2023 cohort, nirsevimab's estimated effectiveness against RSV-LRTI hospitalization was 66% during the first year, with 299 immunized infants (0.8%) hospitalized compared to 899 unimmunized infants (2.4%).

Second-year follow-up outcomes

• ▪During the second year of follow-up for the 2023 cohort, researchers identified no protective association between nirsevimab administration and RSV-LRTI hospitalization.
• ▪During the second year of follow-up for the 2023 cohort, immunized infants experienced higher rates of hospitalization for asthma (0.9% versus 0.7%) compared to unimmunized infants.

Non-RSV hospitalization associations

• ▪During the second year of follow-up for the 2023 cohort, immunized infants experienced higher rates of hospitalization for asthma (0.9% versus 0.7%) compared to unimmunized infants.
• ▪Nirsevimab exposure was associated with higher rates of hospitalization for ear, nose, and throat (ENT) bacterial infections during the first year in both the 2023 and 2024 cohorts.
• ▪Nirsevimab was not associated with reduced hospitalizations due to non-RSV infections or asthma during either the first or second year of follow-up.

French nationwide cohort study methodology

• ▪The study included two matched cohorts of infants born before the 2023 and 2024 immunization campaigns, consisting of 37,366 matched pairs in 2023 and 87,763 matched pairs in 2024.
• ▪The study analyzed data from the French National Health Data System, matching infants who received a single dose of nirsevimab 1:1 with unimmunized infants.
• ▪The mean age of immunized infants at baseline was 4.5 months in the 2023 cohort and 4.8 months in the 2024 cohort, with both cohorts roughly evenly split between male and female infants.