Nirsevimab highly effective against severe RSV disease in infants: JAMA Study

A new study published in the Journal of American Medical Association showed that nirsevimab offers protection against a variety of respiratory syncytial virus (RSV) outcomes, and this protection wanes with time highlighting the necessity of maintaining its uptake and optimizing its use.

The long-acting monoclonal antibody nisevimab has showed promise against lower respiratory tract infections (LRTI) linked to the respiratory syncytial virus. Thus, to determine the efficiency of nirsevimab against medically treated RSV infections in newborns and to evaluate the ways in which effectiveness varies by illness severity, dose, and time after immunization, Hanmeng Xu and colleagues carried out this investigation.

The Yale New Haven Health System's outpatient, inpatient, and emergency department data were used in this test-negative case-control research. Infants who tested positive for RSV using polymerase chain reaction between October 1, 2023, and May 9, 2024, and were eligible for nisevimab were included. Infants who tested positive for RSV were considered cases, while those who tested negative for the virus were considered controls.

The exposure criterion was nisevimab vaccination, which was confirmed by state immunization registries. Multivariable logistic regression was used to determine effectiveness after controlling for possible confounders, age, and calendar month. Different models looked at estimated efficacy by severity (defined as requiring high-flow oxygen or being admitted to an intensive care unit), time since vaccination, dose, and clinical situation. All-cause LRTI and hospitalization related to all-cause LRTI were among the broader outcomes that were also examined.

There were 2410 (78.0%) RSV-negative controls and 680 (22.0%) RSV-positive cases out of the 3090 babies in the analytic sample. The uptake of nirsevimab was 10.7% (330 patients), with 309 RSV-negative controls and 21 RSV-positive cases receiving vaccinations. The adjusted efficacy was 80.5% against hospitalizations, 61.6% against outpatient visits, and 68.4% against medically attended RSV infection.

When it came to severe RSV infections, the highest projected efficacy was noted. Despite a decrease from 79.3% at 2 weeks postimmunization to 54.8% at 14 weeks postimmunization, the predicted efficacy against RSV infections was still considerable. There was little variation in estimated efficacy by dose.

Nirsevimab seemed to be beneficial against all-cause LRTI and all-cause LRTI-associated hospitalizations during the height of the RSV season. The predicted efficacy against these larger outcomes was insignificant from February to May 2024, when the majority of LRTIs were caused by other infections. Overall, these results confirm US recommendations that prescribe nirsevimab for all infants starting their first RSV season and highlight the advantages of RSV immunoprophylaxis.

Reference:

Xu, H., Aparicio, C., Wats, A., Araujo, B. L., Pitzer, V. E., Warren, J. L., Shapiro, E. D., Niccolai, L. M., Weinberger, D. M., & Oliveira, C. R. (2025). Estimated effectiveness of nirsevimab against respiratory syncytial virus. JAMA Network Open, 8(3), e250380. https://doi.org/10.1001/jamanetworkopen.2025.0380