Effectiveness of nirsevimab in infants against respiratory syncytial virus and related events

Background

Nirsevimab is Centers for Disease Control and Prevention–recommended for infants to prevent respiratory syncytial virus (RSV) disease, the most common cause of lower respiratory tract disease (LRTD). Previously published nirsevimab effectiveness has been limited to the hospital setting. This study assessed nirsevimab's effectiveness against RSV, LRTD, and associated medical encounters across all settings in an integrated healthcare delivery system between 2023 and 2024.

Methods

Nirsevimab administration started on October 19, 2023. We included healthy-term infants born in April 2023 or later and excluded infants with high-risk conditions or whose mothers were RSV-vaccinated. The primary endpoint was a polymerase chain reaction positive RSV LRTD diagnosis by the International Classification of Diseases, 10th Revision code in any setting. Nirsevimab effectiveness against RSV LRTD was assessed by Cox regression and estimated as (1-hazard ratio). We estimated the association of nirsevimab treatment with the mean number of encounters per episode by linear regression and the odds of hospitalization by logistic regression.

Results

The study included 31,900 infants; 15,647 of whom received nirsevimab. There were 35 RSV LRTD episodes (6.1/1000 person-years) among nirsevimab-treated infants compared with 462 episodes (58.5/1000 person-years) among untreated infants. The adjusted nirsevimab effectiveness estimate against RSV LRTD was 87.2% (CI: 81.7%-91.1%). Nirsevimab-treated infants with RSV LRTD had a lower mean number of encounters (2.1 vs 2.7, P = .001) and lower hospitalization odds (odds ratio = 0.11, CI: 0.01-0.85).

Conclusion

In this large study, nirsevimab reduced the risk of RSV LRTD by 87% across all healthcare settings. Compared with untreated infants, those who had RSV LRTD after nirsevimab had significantly fewer medical encounters and were less likely to be hospitalized.