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Scientists have made monumental strides in their quest to protect vulnerable babies from respiratory syncytial virus (RSV): in just the past few years, monoclonal antibody injections for infants and a maternal vaccine that delivers protection through the placenta have substantially reduced infant hospitalizations and deaths.

But researchers still are grasping at a broader goal: to make a vaccine available for toddlers and preschoolers, who are also vulnerable to severe RSV disease.

That breakthrough, medical researchers say, could be right around the corner.

The most promising candidate is a two-dose vaccine administered as a nasal spray. Should it prove safe and effective in clinical trials, which are currently underway, it could help prevent severe RSV disease in children over the first several years of their lives—not just the first months.

“Where we’re heading next is the possibility of having true vaccines, not monoclonal antibodies, for kids after their first birthday,” says James Campbell, an infectious disease pediatrician at the University of Maryland.

Before the arrival in 2023 of Pfizer’s maternal vaccine and a preventive monoclonal antibody drug called nirsevimab, developed by Sanofi and AstraZeneca, RSV brought an annual scourge upon children’s hospitals. Historically, it has been the number one cause of infant hospitalization; about 2 to 3 percent of infants in their first year of life are hospitalized with RSV each year in the U.S.

During the fall and early winter, RSV “fills up our hospitals with sick children and may potentially have long-term ramifications in terms of a higher likelihood of wheezing,” says Jennifer Nayak, an infectious disease pediatrician at the University of Rochester Medical Center. “The fact that there are prevention strategies out there … has actually moved the landscape quite far in this area.”

A second monoclonal antibody, Merck’s clesrovimab, was approved for use in infants this year.

The monoclonal antibodies and maternal vaccination both protect babies through passive immunization, which means the infants are given antibodies. For RSV, the antibodies are either directly injected as nirsevimab or clesrovimab, or passed through the placenta after the vaccine is given during pregnancy. Those antibodies are ready to fight the disease, but they eventually wear off. Because infants do not receive a vaccine that prompts their immune systems to make their own antibodies—known as active immunity—they aren’t primed to fight RSV after the passively received antibodies have waned.

“Unlike active immunization, where you establish immune memory, passive immunization doesn’t do that,” Nayak says.

And while RSV is the most dangerous to babies, it’s far from harmless in older kids.

A Centers for Disease Control and Prevention analysis of data from two different groups of children in the U.S. revealed this stark reality. In it, scientists found that RSV-related hospitalization rates among infants seven months and younger decreased by about 28 and 43 percent, respectively, during the peak of the 2024–2025 RSV season, when both preventatives were available, compared with pre-COVID-pandemic RSV seasons from 2018 to 2020.

Those same statistics also underscored the ongoing vulnerability of older children: While RSV-related hospitalizations declined in infants, they increased in older kids. For children aged eight months to 19 months, hospitalization rates from RSV were 26 and 34 percent higher in the two groups, respectively, in the 2024–2025 season than they were in 2018–2020. In one of the cohorts, the hospitalization rate rose from five per 1,000 children to nearly seven. Hospitalization rates for 20- to 59-month-olds in the two groups were 1.7 and 2.5 per 1,000 children, 55 and 67 percent higher than they were in those prepandemic seasons.

The existing passive immunization products are reducing the impact on infants, but “what we want to do for them after that is going to be the question,” Campbell says. “We all know that in the second and third season, there is still a burden in two-year-olds and three-year-olds.”

The disease is even more deadly in low- and middle-income countries that don’t have the resources to provide supportive care to children with severe infections. Worldwide, RSV is responsible for more than 3.6 million hospitalizations and about 100,000 deaths in children under age five each year, according to the World Health Organization.

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Chiara Vercesi

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