Nirsevimab for Infant RSV: What Real-World Data Show

Nirsevimab has delivered the result parents and health systems want most from RSV prevention: fewer infants in the hospital. Across recent real-world meta-analyses, a single dose was associated with roughly 84% to 89% fewer RSV-related lower respiratory tract infection hospitalizations during one RSV season. That is a large effect on a patient-important outcome, not a laboratory marker.

The tempting twist is that these observational estimates are numerically higher than the randomized-trial benchmarks. Resist the victory lap. Randomized trials and real-world comparisons answer related but different questions, and healthy-vaccinee bias can make an intervention look better outside a trial than it truly is. The evidence supports confidence that nirsevimab works well; it does not establish that it works better in routine care than in trials.

A single dose has held up across hospital systems

Nirsevimab is a long-acting monoclonal antibody directed at RSV's prefusion F protein. It gives infants passive antibodies rather than asking their immune systems to make them. That distinction matters mainly for timing: protection can begin without waiting for an active vaccine response, which is useful when RSV season and a newborn's first few months collide.

The largest synthesis described here, Soudani et al. (2025), pooled 16 observational studies—seven cohort studies and nine case-control studies—covering 141,550 infants. It assessed eight prespecified outcomes, including RSV-related lower respiratory tract infection hospitalization, ICU admission, bronchiolitis hospitalization, bronchiolitis ICU admission, and ventilatory support. Across this and other meta-analyses, pooled real-world estimates for RSV-related hospitalization reduction fell in the 84% to 89% range.

Independent reviews point in the same direction. A 2025 Lancet Child & Adolescent Health meta-analysis synthesized real-world evidence from all-infant programs in high-income countries. A later JAMA Pediatrics meta-analysis examined both lower respiratory tract infection hospitalizations and emergency-department visits, adding a major source of healthcare use below the admission threshold. The concept document does not provide confidence intervals for these pooled estimates, so the exact precision cannot be assessed here. Still, convergence across separate analyses makes a chance finding less plausible.

The most concrete program-level estimate came from Galicia, Spain. In the prospective NIRSE-GAL cohort study, infants in the 2023–2024 universal nirsevimab program had an 89% reduction in RSV-related lower respiratory tract infection hospitalizations. All eligible infants were offered nirsevimab through the public program. That design does not turn the study into a randomized trial, but it reduces some of the self-selection that can distort opt-in programs. The investigators also characterized breakthrough infections, which appeared less severe.

The benefit reaches severe RSV, not only mild illness

Hospital admission is already a meaningful endpoint, but RSV's hardest cases require intensive care or ventilatory support. In Soudani and colleagues' analysis, nirsevimab's association with protection extended to RSV-related bronchiolitis requiring PICU admission and to bronchiolitis-related ventilatory support. A separate 2025 Spanish universal-program analysis also reported significant reductions in general-ward hospitalizations and PICU admissions for RSV-associated lower respiratory tract infection.

These findings matter because ICU-level disease carries the greatest immediate clinical burden. They do not prove a reduction in mortality: the evidence summarized here reports hospitalization and intensive-care outcomes, not a mortality estimate. It is better to state the result precisely than to inflate it into an endpoint the studies did not measure.

Preterm infants also appear to benefit. A 2025 preterm-specific meta-analysis by Wang et al. found that real-world effectiveness was preserved across gestational-age strata. This group has disproportionate risk of RSV hospitalization and historically received the bulk of palivizumab prophylaxis. Nirsevimab's single-dose schedule is a practical advance over palivizumab's monthly injections during RSV season.

Why the real-world numbers exceed trial benchmarks

The randomized-trial comparison creates the article's central puzzle. A 2026 meta-analysis and time-to-event analysis by Sayed et al. reported trial-derived efficacy of 74.5% against medically attended lower respiratory tract infection in term and late-preterm infants born at 35 weeks' gestation or later. In preterm infants born at 29 to 34 weeks, efficacy was 70.1% against medically attended lower respiratory tract infection and 78.4% against RSV-related hospitalization.

Those are not interchangeable endpoints. The 74.5% and 70.1% figures concern medically attended illness, while the 78.4% figure concerns hospitalization in a preterm cohort. The 84% to 89% real-world range largely concerns hospitalization. A higher apparent effect against hospitalization may reflect a difference in endpoint, population, comparator, circulating RSV, healthcare use, or study design. It cannot establish that the antibody gained biological power after leaving the trial.

Healthy-vaccinee bias is the most obvious concern. Families whose infants receive nirsevimab can differ from unvaccinated families in access to care, health literacy, socioeconomic circumstances, and baseline infant health. If those differences also affect hospitalization risk, an observational comparison can overestimate the antibody's causal effect. Case-control studies are particularly exposed to this problem; nine of the 16 studies in Soudani et al.'s meta-analysis used that design. Galicia's universal-offer model helps, but it does not erase residual confounding across the full evidence base.

Protection appears to cover one season; later seasons remain unknown

Soudani et al. included subgroups at zero to three months and three to six months after administration, and the broader evidence base did not report a dramatic within-season collapse in effectiveness. Sayed and colleagues added time-to-event analysis that examined onset and the trajectory of protection across the season. This fits the intended use of one dose before or at the start of an RSV season.

The durability conclusion has boundaries. Small subgroup cell sizes can miss moderate waning, so a lack of statistically significant heterogeneity between early and later intervals does not prove identical protection throughout the season. More importantly, the available real-world evidence covers a single season, mainly 2022–2023 or 2023–2024. It does not answer whether repeat dosing provides equivalent protection in a second season, whether repeated passive immunization affects later immune priming, or whether multi-year programs produce herd effects.

Coverage and cost determine population impact

An effective dose does not automatically create an effective program. The 2025 Austrian implementation study by Höck et al. found incomplete uptake, implementation friction, and cost concerns after Austria introduced nirsevimab and maternal RSVpreF vaccination. Nirsevimab requires a separate neonatal injection during a narrow seasonal window: programs must identify eligible infants, obtain the product, and administer it before exposure.

That logistics problem changes the public-health arithmetic. If a product reduces risk by 89% among recipients but reaches only 60% of eligible infants, its population effect will be much smaller than its recipient-level effectiveness. In European markets, the reported per-dose cost of roughly €400 to €500 also makes reimbursement central to access. Evidence from early-adopter European programs cannot automatically predict uptake or impact in lower-resource settings with different seasonality, comorbidity, and healthcare access.

Maternal RSVpreF vaccination offers a different delivery route through prenatal care. In a 2025 living review by Lee et al., maternal RSVpreF vaccination had approximately 68% real-world effectiveness against RSV-associated acute respiratory illness, lower than the pooled nirsevimab hospitalization estimates but against a different endpoint. The two approaches may be complementary, especially when timing of transplacental antibody transfer is uncertain.

The verdict

Nirsevimab has a strong clinical case for preventing serious RSV disease in infants during one season. Multiple real-world analyses associate a single dose with large reductions in hospitalization, and the evidence extends to preterm infants and severe outcomes such as PICU admission. The trial evidence provides an appropriate anchor: efficacy was substantial before the product entered universal programs.

What remains unsettled is why observational estimates look even higher. Bias, endpoint differences, and program context can explain some or all of the gap. Antigenic variants at the nirsevimab binding site also remain a forward-looking risk; current effectiveness estimates largely reflect seasons in which circulating strains were susceptible. For policy, the useful question is less whether real life beat the trial and more whether a health system can deliver the antibody to enough infants, early enough, at a price it can sustain.