Fewer RSV Hospital Admissions Among Infants Given Nirsevimab

CATEGORY: IMMUNE HEALTH

Infants who received nirsevimab—a long-acting antibody injection against respiratory syncytial virus, or RSV—had 83% lower odds of being hospitalized for RSV than infants who did not, in a pooled analysis of 27 real-world studies from five countries.¹ RSV is a common seasonal virus that infects nearly all children by age two, and it is a leading reason young infants end up in the hospital.

An antibody shot, not a vaccine

Nirsevimab works differently from a vaccine. A vaccine trains an infant’s immune system to build its own defenses, which takes time. Nirsevimab skips the training: it delivers a supply of ready-made antibodies—proteins that latch onto the virus and block it from infecting cells—in a single injection. It is closer to lending a newborn armor than to teaching them to forge their own. One dose is designed to last through an infant’s first RSV season, the months when severe illness is most likely, and several countries began offering it to all infants in 2023.

Literature Searched Through February 2025, 27 Studies Pooled

Researchers systematically searched the medical literature from January 2023 through February 2025 and found 32 real-world studies of infant nirsevimab programs in France, Italy, Luxembourg, Spain and the United States; 27 had data that could be pooled. The studies compared infants in their first year who received nirsevimab in routine care with infants who did not, counting who was admitted to the hospital for RSV, who needed intensive care, and who developed an RSV-related lower respiratory tract infection—an infection reaching the airways and lungs, such as bronchiolitis or pneumonia—over the RSV season.

81% Lower Intensive Care Odds, Same Hospital Stay

Immunized infants had 83% lower odds of RSV-related hospitalization, 81% lower odds of an intensive-care admission, and 75% lower odds of an RSV-related lung infection. Among infants who were hospitalized anyway, the length of the hospital stay was about the same whether or not they had been immunized. The real-world numbers closely match what nirsevimab achieved in the clinical trials that led to its approval—which is exactly what an analysis like this is designed to check.

Five High-Income Countries, and Some Immunized Infants Hospitalized

These were observational studies, not randomized trials, so they show association rather than proof of cause—families who chose immunization may differ in other ways from families who did not. All of the data came from five high-income countries with strong health systems, so the findings may not transfer everywhere. And the protection was substantial but not total: some immunized infants were still hospitalized.

HARMONIE Randomized More Than 8,000 European Infants

Randomized trials point the same way. In HARMONIE, which assigned more than 8,000 European infants to either nirsevimab or no preventive treatment, RSV hospitalization was 83% lower in the treated group—almost exactly the real-world figure.² An earlier trial comparing nirsevimab with a dummy injection found 75% fewer RSV lung infections needing medical care in healthy late-preterm and term infants.³

Key Takeaways

• Across 27 real-world studies in five countries, infants who received the RSV antibody nirsevimab had 83% lower odds of RSV hospitalization and 81% lower odds of intensive care than unimmunized infants.
• Nirsevimab is not a vaccine—it supplies ready-made antibodies in one injection designed to cover an infant’s first RSV season.
• The real-world results closely mirror nirsevimab’s clinical trials, but the studies were observational, so they show association rather than proof of cause.

References

1. Sumsuzzman DM, Wang Z, Langley JM, Moghadas SM. Real-world effectiveness of nirsevimab against respiratory syncytial virus disease in infants: a systematic review and meta-analysis. The Lancet Child & Adolescent Health. 2025;9(6):393–403. link
2. Drysdale SB, Cathie K, Flamein F, et al. Nirsevimab for prevention of hospitalizations due to RSV in infants. New England Journal of Medicine. 2023;389(26):2425–2435. link.
3. Hammitt LL, Dagan R, Yuan Y, et al. Nirsevimab for prevention of RSV in healthy late-preterm and term infants. New England Journal of Medicine. 2022;386(9):837–846. link.