Nirsevimab proves up to 89% effective at preventing severe RSV in infants

Recent studies and public health initiatives have underscored the significant role of nirsevimab in preventing severe respiratory syncytial virus (RSV) outcomes in infants, with efficacy rates ranging from 80% to 89%. The findings, published in multiple journals and highlighted by health authorities, mark a pivotal shift in addressing a leading cause of infant hospitalization and mortality. RSV, which affects nearly all children by age 2, has long posed challenges due to its potential to cause bronchiolitis, pneumonia, and critical illness, particularly in vulnerable populations such as preterm infants or those with underlying conditions.

A multi-center study led by the Centers for Disease Control and Prevention (CDC) evaluated nirsevimab’s effectiveness against RSV-related ICU admissions and acute respiratory failure (ARF) in 457 infected infants and 302 controls across 24 U.S. States. The research, published in the *Morbidity and Mortality Weekly Report*, found that nirsevimab reduced ICU admissions by 80% and ARF by 83% when administered at least 7 days before symptom onset. The protection waned slightly over time, with 66% efficacy against ICU admission 60 to 183 days post-dose, but remained robust during the first RSV season. Researchers noted that most ICU patients were previously healthy, emphasizing the antibody’s ability to mitigate severe outcomes even in otherwise low-risk infants.

Another study in Italy and New York City analyzed the impact of universal nirsevimab prophylaxis in 13,624 infants. Coverage of 79.2% in five northern Italian provinces correlated with a 68% reduction in population-level RSV hospitalization risk. Among 292 hospitalized infants, nirsevimab was linked to an 89% lower risk of hospitalization when comparing infants born in the same month. However, the study highlighted persistent risks for preterm infants and those living with older siblings, suggesting the need for supplemental strategies to address high-risk groups. The antibody also reduced high-flow oxygen use but did not shorten hospital stays, underscoring its role in mitigating severity rather than accelerating recovery.

The CDC’s efforts to expand nirsevimab access through the Vaccines for Children (VFC) program further amplified its impact. A 2024–2025 initiative to boost birthing-hospital enrollment in VFC saw a 36% participation rate by the end of the RSV season, up from 27% in 2023–2024. This surge doubled nirsevimab doses ordered, with 102,057 doses distributed during the 2024–2025 season. The CDC’s policy requiring at least 30% of birthing hospitals to enroll in VFC by July 2025 contributed to this growth, particularly in the Northeast region, where enrollment rose to 74.5%. The agency also noted that Medicaid-insured infants, who face higher RSV risks, were prioritized in these efforts.

Clinical trials and real-world data have reinforced nirsevimab’s efficacy, with the CDC reporting a 43% reduction in RSV-related hospitalizations among infants seven months and younger during the 2024–2025 season. This aligns with broader advancements in RSV prevention, including maternal vaccines targeting the prefusion F glycoprotein. These vaccines, approved in 2023, provide passive immunity to newborns through placental transfer, complementing nirsevimab’s direct administration. Together, these interventions have driven a “stunning” decline in hospitalizations, according to pediatric infectious disease experts, with some regions reporting up to 71% reductions when accounting for seasonal variations.

Despite these strides, challenges remain. Disparities in nirsevimab uptake persist, with publicly insured, Black, and low-income infants less likely to receive the antibody, as noted in a June 2025 CDC report. Additionally, antivaccine sentiment and logistical hurdles, such as vaccine availability in hospitals, threaten broader adoption. The emergence of a second monoclonal antibody, clesrovimab, approved in June 2025, offers hope for increased accessibility but also raises questions about long-term efficacy and cost.

Historically, RSV prevention efforts faced setbacks, including the 1966 inactivated vaccine that worsened disease in vaccinated infants. The development of nirsevimab and maternal vaccines, grounded in understanding the prefusion F glycoprotein, represents a breakthrough after decades of research. As public health agencies continue to refine distribution strategies and address inequities, the prospect of significantly reducing RSV’s burden on infants and healthcare systems grows more tangible. For now, nirsevimab’s high efficacy and expanding access stand as a critical milestone in the fight against a virus that has long shaped pediatric care.