Second-Season Nirsevimab Immunization Protects High-Risk Toddlers Against RSV Hospitalization

A second-season dose of the monoclonal antibody nirsevimab effectively keeps high-risk toddlers out of the hospital when they contract respiratory syncytial virus (RSV). Real-world surveillance data shows the immunization reduces RSV-associated hospitalizations by 75 percent among eligible children aged 8 to 23 months.[1]

The findings, presented at the IDWeek 2026 conference in October, provide the first large-scale confirmation that the antibody works during a child's second respiratory season. Until now, public health recommendations for the second-year dose relied on indirect evidence, such as drug blood levels, rather than direct hospitalization outcomes.[1][3]

"These data provide valuable real-world evidence of nirsevimab effectiveness against RSV hospitalizations for nirsevimab-eligible children in their second RSV season," said Angela Campbell, a researcher at the Centers for Disease Control and Prevention (CDC). She presented the findings to clinicians seeking clarity on the intervention.[1]

Real-world effectiveness

The estimates support the initial strategy of using the antibody to protect vulnerable toddlers. The CDC-led study analyzed data from the Overcoming RSV Network, which tracked patients across 26 hospitals in 24 states between November 2024 and March 2026.[1]

Researchers matched 231 nirsevimab-eligible hospitalized toddlers who had respiratory symptoms and a positive RSV test against a control group with negative tests. Among the hospitalized children, RSV positivity was significantly less common in those who had received nirsevimab.[1]

Only 20 percent of the immunized toddlers tested positive for the virus, compared to 42 percent of those who did not receive the antibody. The timing of the injection heavily influenced its protective power.[1]

The adjusted effectiveness against RSV-associated hospitalization reached 86 percent when the dose was given within 60 days of symptom onset. That protection dropped to 66 percent when the antibody was administered earlier in the season.[1]

Estimating this effectiveness has historically been difficult because the high-risk groups eligible for a second dose make up only 2 to 4 percent of the pediatric population. The study participants primarily included toddlers with chronic lung disease of prematurity and severe immunocompromise.[1]

Differing eligibility guidelines

The new efficacy data arrives just as the American Academy of Pediatrics (AAP) and the CDC diverge on exactly which toddlers should receive the second-season dose. Both organizations agree that nirsevimab, marketed as Beyfortus, is the only approved option for children entering their second RSV season.[2][3][5]

The CDC currently recommends the 200-milligram second-season dose for four specific groups. These include children with chronic lung disease of prematurity who required medical support in the previous six months, those with severe immunocompromise, children with severe cystic fibrosis, and American Indian or Alaska Native children.[3]

In September 2026, the AAP published a policy statement expanding its own list to include four additional groups. The pediatric academy now recommends the second-season dose for all children born before 32 weeks gestation, regardless of their need for prior medical support.[3]

The AAP also added children with hemodynamically significant congenital heart disease, those with anatomic pulmonary abnormalities or neuromuscular disorders, and children with Down syndrome. These chromosomal differences and structural anomalies significantly raise the risk of severe respiratory complications during an infection.[3]

Navigating the discrepancy

The AAP expanded its criteria after reviewing hospitalization burdens, noting that 92 percent of children requiring intensive care for second-season RSV were previously ineligible for the immunization. The pediatric group aims to protect toddlers who face longer hospital stays when infected.[3][5]

The CDC's narrower list stems from a 2023 vote by its vaccine advisory committee, which based its initial recommendations on the eligibility criteria for an older, discontinued antibody called palivizumab. The CDC rated the certainty of its second-season evidence as very low at the time, citing a lack of direct efficacy data.[3]

This discrepancy leaves pediatricians and families to navigate the gap. While the federal Vaccines for Children program covers the cost for eligible uninsured or underinsured children, private insurance coverage for the newly added AAP groups may vary until the CDC officially adopts the expanded criteria.[3][4][5]

"We are so lucky to have these products now to prevent what has historically been the leading cause of hospitalization among young children," said Sean T. O'Leary, who leads the AAP's RSV immunization subgroup. He noted that the expanded groups represent a relatively small population that faces severe risks.[3]

Administration and safety

Nirsevimab is a passive immunization, meaning it delivers lab-made antibodies directly to the child rather than prompting the immune system to generate its own. The protection is designed to last at least five months, covering the length of a typical winter respiratory season.[4]

The AAP and CDC both advise administering the dose shortly before the RSV season begins, typically in October or November. The injection can be given simultaneously with other routine childhood vaccines without interfering with the immune response.[3][4]

Safety data from clinical trials and real-world use show the antibody is well-tolerated. The most common side effects are mild, including temporary pain, redness, or swelling at the injection site, and occasional rashes.[4]

With the Overcoming RSV Network losing its funding after the 2025-2026 season, future surveillance will rely on other platforms. For now, the IDWeek findings provide the firmest evidence yet that the second-season dose successfully shields the highest-risk toddlers from severe respiratory failure.[1][5]