This article is part of “Innovations In: RSV,” an editorially independent special report that was produced with financial support from MSD, Sanofi and AstraZeneca.
While pregnant with her third child last year, Alison Carroll pondered options that hadn’t been available during her first two pregnancies: not one but two ways to help prevent her newborn from ending up in the hospital, fighting for breath because of a severe infection with respiratory syncytial virus (RSV).
A pediatric hospitalist herself, she already had for years witnessed and treated the worst of RSV’s ravages in children, including a scary situation that landed her own daughter, Stella, in the same wing of Vanderbilt Children’s Hospital in Nashville, where Carroll works.
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So Carroll didn’t need to think long before saying yes to one of the new preventative options that have revolutionized the medical world’s ability to prevent the most severe symptoms of RSV, especially in infants, who are particularly vulnerable. Since they debuted in 2023, the preventatives—a vaccine for pregnant people and an antibody shot for newborns—have reduced hospitalizations of the youngest babies by up to half.
“That’s amazing,” says Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia. “It actually caused a slight decrease in the infant mortality rate in this country,” a figure that wasn’t more dramatic mainly because RSV is responsible for a small proportion of infant deaths relative to other causes.
Although the infant mortality rate is low, respiratory syncytial virus is the leading cause of hospitalization among infants in the U.S., with newborns up to the age of two months being at the highest risk, according to the U.S. Centers for Disease Control and Prevention. Roughly 60,000 to 80,000 children under age five are hospitalized with RSV every year in the U.S., and an estimated 100 to 300 children in that age group die annually.
In some babies, RSV progresses beyond coldlike symptoms, spreading from the upper respiratory tract (primarily the nose and throat) to the lower, where it affects their lungs and ultimately causes severe breathing difficulties, low oxygen levels and other serious complications.
That’s why pediatricians and infectious disease specialists have reacted with such enthusiasm to the development of the preventative measures, which, in both clinical trials and early real-world results, have produced dramatic reductions in RSV infections that require medical attention or hospitalization among babies. A CDC analysis of data from two different groups of children in the U.S. 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.
Looking at the data more narrowly, the researchers saw an even larger effect: for infants aged zero to two months, the decreases were 45 percent and 52 percent for the two groups, respectively. That 45 percent reduction jumped to 71 percent when the Houston, Tex., region was excluded because of its early-onset RSV season, which began before the new drugs were available.
Offit says the CDC data were not limited to babies who received antibodies through the maternal vaccine or monoclonal antibody injections but rather included all RSV-related infant hospitalizations. “This wasn’t an efficacy study,” he says. “They were looking from 30,000 feet, saying, ‘Has there been a decrease [in hospitalizations]?’”
“And there has,” he adds, “which is remarkable.”
Offit isn’t the only expert offering superlatives.
“I think the results have been stunning, actually,” says Yvonne Maldonado, chief of pediatric infectious diseases at Stanford University School of Medicine and a former member of the federal Advisory Committee on Immunization Practices. “Those two treatments, or preventive measures, have resulted in massive reductions in infections and, among infected kids, massive reductions in hospitalizations.”
The protective measures work in different ways. With an injection of the Pfizer vaccine Abrysvo at 32 to 36 weeks of gestation, a pregnant person develops antibodies against RSV that are conferred to the fetus through the placenta, giving babies crucial protection that lasts for the first six to nine months of life.
The other option is for the infant to receive a monoclonal antibody directly via injection.
There are now two monoclonal antibody options: Sanofi and AstraZeneca’s nirsevimab, which was approved in 2023 and is the subject of the recent efficacy studies. A second, Merck’s clesrovimab, won approval in June of this year and is expected to be available ahead of the 2025–2026 RSV season, which typically starts in fall and continues through winter.
The antibodies that were either passed on by the pregnant person (who received the vaccine) or directly via the monoclonal antibody injections gradually wane in effectiveness over time, and even initially, they do not prevent infection. Rather, they give the body the tools to fight the virus before it becomes serious.
“You’re not immunizing the child; you’re just giving them antibody, and when it’s gone, it’s gone,” says Ruth Karron, director of the Johns Hopkins Vaccine Initiative and a recognized global expert in pediatric RSV. “But if we use our products effectively, I think we’re going to see RSV hospitalizations virtually disappear, which would be remarkable.”
It isn’t yet clear whether the maternal vaccine or an infant antibody injection is more effective. “That question is something we’re still trying to figure out,” Carroll says. “But I would encourage all of my mom friends to get the vaccine when they’re pregnant and, if that’s not available, to make sure their baby can get nirsevimab as soon as possible.”
Each of Carroll’s children has landed somewhere on the timeline of progress toward prevention of severe RSV. Her first child, Stella, was born before the medicines were available. In 2022, when she was not quite four years old, Stella came home from day care with respiratory symptoms that quickly worsened. Her cough got so bad that she began to throw up. She could not eat. “I tried my best at home for her and eventually decided that she was too dehydrated and needed to go to the hospital,” Carroll says. There, a blood test confirmed RSV.
“She was admitted for about three and a half days, and she needed a lot of hydration and then antibiotics for the secondary bacterial pneumonia, which is one of the concerns with RSV,” Carroll says. “I was on the other side of that equation as the parent of a child hospitalized with RSV.” (Stella ultimately made a strong recovery.)
Earlier that year Carroll had enrolled her then six-month old son, Vincent, in one of the clinical trials testing the safety of the monoclonal antibody nirsevimab, whose efficacy had already been established. The trial was conducted at Vanderbilt, and several of Carroll’s colleagues with young children did the same, “just because we believe so strongly in supporting these kinds of [clinical] trials, especially for RSV, something that we take care of all the time and see how sick kids can get from it,” she says.
Carroll chose to receive the vaccine herself while pregnant last year with Edward, her third child. She says her decision was driven in part by timing: Edward was born in December, the height of respiratory virus season, so protecting him immediately after birth was essential. For infants of pregnant people who do not receive the vaccine, the antibody injection should be administered within the first week of life if they’re born in October through March, when RSV has traditionally been most active, the CDC says. For babies born outside that time period, their shot should come shortly before the start of their first RSV season.
Nearly all children will get RSV, whose initial symptoms are similar to the common cold, before their second birthday, according to the CDC.
In some cases, RSV’s complications can pile up quickly. One of its first manifestations is a narrowing of the airways; once it reaches the lungs as bronchiolitis, children begin having difficulty breathing, may start breathing very rapidly, can suffer from low levels of oxygen and often struggle to eat. The resulting dehydration is often what lands them in the hospital, but some need to be intubated and placed on a ventilator, which Maldonado says can lead to permanent scarring of the lungs and other long-term effects. Some studies have linked early RSV infection with developing childhood asthma.
The development of the pregnancy vaccine and infant antibody shots was a long time coming—it took more than half a century, with the first attempts to prevent RSV emerging in the 1960s, about a decade after the virus was discovered.
Those early efforts focused on producing a vaccine, but the resulting candidate actually exacerbated RSV’s effects in children when they naturally contracted the virus. “That really hampered research for 30 years or more,” Maldonado explains.
It was not until the 2000s that a group of researchers at the National Institutes of Health realized that the structure of the protein on which the initial vaccine had been based was the wrong version.
“There is a form of the protein on the surface of the virus that looks one way before infection, and then after it infects the human cell, it folds in on itself and looks very different,” says Maldonado, who practices at Lucile Packard Children’s Hospital Stanford. Researchers “were making the vaccine to that [postinfection] version, which wasn’t helping very much.”
The NIH researchers discovered that when attacking the prefusion version of the protein instead, the result was “about 70 to 80 percent prevention against serious infection among those who got sick,” Maldonado says.
“Contrary to what most people think, we’re not trying to prevent infection in every child,” Maldonado says. “We’re trying to keep them out of the hospital—to keep them from dying, number one, and keep them from having complications…. These two products have really delivered on that, and they’ve been very safe.”
For the medical field, the next battle is for more widespread uptake of both types of preventative measures. A June report by the CDC found that 57 percent of infants born between April 2024 and March 2025 received protection, either through maternal vaccination or the monoclonal antibody.
“That’s really good for a first-year, or first couple of years’, uptake,” Karron says. “And the first year that nirsevimab was available, there were product shortages, and that’s not going to be the case this year, especially with a second product on the market.”
The chance of these preventatives nearly stamping out RSV as a cause of childhood hospitalization, though, will ultimately hinge on an increase in usage. “Think about how much less suffering and hospitalization and death can occur if we just fully complied with this vaccine program,” Offit says. “But we don’t.”
A July study by researchers at the Children’s Hospital of Philadelphia found that most infants who received the antibody shot were white and privately insured. “Notably, patients who were publicly insured, Black, or living in areas with lower childhood opportunity were significantly less likely to receive nirsevimab,” wrote the report’s authors, who urged a close examination of the drivers of those disparities in order to ensure more equitable uptake.
Most private insurance companies, as well as Medicaid and the Children’s Health Insurance Program, now cover the preventatives. And the majority of pregnant people and infants who receive these drugs do so at their doctor’s offices or clinics, though Carroll says there will likely be an effort to make the monoclonal antibodies available at all hospitals during RSV season.
Immunization experts are also keeping a wary eye on growing antivaccine sentiment, which could slow a more widespread acceptance of either option. “I think that we’re just living in an age … where people are skeptical about vaccines or at least unrealistically skeptical,” Offit says. “Maybe a better word is cynical. They just don’t buy it. And certainly [Robert F. Kennedy], Jr., as the head of [the Department of Health and Human Services], hasn’t helped with that.”
Offit cites the resurgence of measles in the U.S. as an example of the harms of antivax messaging. There have been more cases of measles this year than any year since 1992, according to the CDC. “Sadly, there’s so much misinformation and disinformation out there, so much distrust,” Offit says.
Others suggest that the uptake may be gradual simply because health care infrastructure is often too cumbersome to adapt quickly to new products. “Some of the questions now are ‘Can you get it in a hospital?’ ‘Will it be paid for?’ ‘Who can administer it?’ ‘Is the hospital pharmacy going to stock it?’” Vanderbilt pediatric hospitalist James Antoon says. “There’s still a lot of implementation yet to be done.”
Those who’ve seen the dramatic effect of the new RSV treatments on infants and young children hope that the new products’ remarkable efficacy will convince skeptics. The reduction in hospital visits alone make the treatments “game changers,” Carroll says. One of the next steps is to get that message out to ob-gyns and other health providers to whom expectant parents turn for advice and direction.
“Now we’ve proven it,” Offit says. “You can dramatically lower hospitalizations and intensive care unit admissions and deaths. You can do that. So let’s do it.”
