Respiratory syncytial virus, or RSV, is a common respiratory virus that usually causes mild, cold-like symptoms in most people. However, it can be particularly dangerous for infants, especially those under two months old, and is recognized as the leading cause of hospitalization for infants in the United States. This past winter marked a significant turning point in the fight against severe RSV illness in babies, thanks to the widespread availability of new preventative tools. A new study from the US Centers for Disease Control and Prevention (CDC) and provisional data on infant mortality suggest these tools are having a dramatic and positive public health impact.
The Threat of RSV to Infants
Infants are especially vulnerable to RSV for several reasons. Firstly, they often lack pre-existing immunity to the virus, making the disease potentially more severe. Secondly, their airways are very small. When an RSV infection causes mucus and inflammation in the lungs, it can make it extremely difficult for babies to get enough air, described as being “like they’re breathing through a tighter and tighter straw”.
The consequences for infants can be severe. While some may only experience mild symptoms, others can become critically ill and require hospitalization. RSV can lead to bronchiolitis, a lung infection that commonly fills pediatric hospital wards in the winter. Symptoms of respiratory complications in babies can include using stomach muscles to breathe and becoming less alert. Critically, even infants without other obvious symptoms can experience periods of apnea, where they suddenly stop breathing.
Heather Auerbach, an emergency physician, shared a personal account of her son’s severe RSV infection. Born full-term and without complications, her son was hospitalized at just three weeks old in January 2020. His breathing suddenly worsened, he became less alert, and started using his stomach muscles to breathe. After his hospital admission, when he returned home, Dr. Auerbach and her husband, also a physician, had to regularly suction away his mucus. This experience highlights the significant burden and fear a child’s RSV hospitalization can place on parents, in addition to being disruptive and expensive. Dr. Auerbach noted that kids can get “very, very sick” from RSV without preventative measures.
The impact of severe RSV can also potentially have long-term effects. Dr. Auerbach shared that her son started wheezing a few months after his illness and received an asthma diagnosis at a very early and unusual age. She, her husband, and their son’s pulmonologist believe this was related to his RSV infection, although there’s no guarantee he wouldn’t have developed asthma eventually. She doesn’t believe he would have developed it at six months old without the RSV infection. This raises the question of whether the new shots could potentially prevent such additional complications down the road.
New Tools for Prevention
The winter of 2023-24 was the first RSV season where new preventative options were widely available in the United States. These tools provide antibodies to help protect infants from severe RSV illness.
One approach is a vaccine given to pregnant people, specifically Pfizer’s Abrysvo. This vaccine is offered during the third trimester of pregnancy, typically between September and January, coinciding with the RSV season. When administered to the pregnant person, antibodies are transferred to the fetus, providing protection to the newborn for several months after birth.
The second tool is a monoclonal antibody treatment, known as nirsevimab. Unlike a vaccine that prompts the body to create antibodies, this treatment provides lab-made antibodies directly to the infant. Nirsevimab helps the baby’s immune system fight off the virus and provides protection for at least five months. It is recommended for babies under eight months old born during or entering their first RSV season (typically October through March). It is also given to babies aged eight to 19 months old if they are at high risk due to underlying medical conditions. Both the maternal vaccine and the monoclonal antibody treatment target a specific protein that the RSV virus needs to enter cells. The CDC recommends using one of these two preventative tools.
Dr. Auerbach reflected on her son’s hospitalization in 2020, stating she wished the shot had been available then, as she genuinely thinks it could have saved him a hospitalization.
Dramatic Decline in Infant Hospitalizations
Studies are now showing a clear link between the availability and use of these new preventative tools and a significant reduction in RSV hospitalizations among young infants.
Compared with the RSV seasons from 2018 to 2020, which were typical seasons before the COVID-19 pandemic altered transmission patterns and before the new preventatives were available, this past winter saw a substantial drop in infant hospitalizations. Data from two surveillance systems tracked by the CDC confirm this decline.
According to the RSV-Associated Hospitalization Surveillance Network (RSV-Net), which includes data from around 300 hospitals in 13 states, the RSV hospitalization rate for infants up to 7 months old in the 2024-25 season was 8.5 per 1,000 children. This represents a 43% drop compared to the average rate of 15 per 1,000 children in the 2018-2020 seasons. For babies under the age of seven months, hospitalizations dropped by as much as 56%, depending on the data used.
The New Vaccine Surveillance Network (NVSN), which monitors hospitalizations in seven pediatric medical centers across seven major cities, also showed a decrease. The RSV hospitalization rate for infants up to 7 months old fell by 28%, from around 15 per 1,000 children in 2018–2020 to around 11 per 1,000 children this past winter. When researchers excluded data from Houston, which saw an early RSV season before the preventatives were widely available, the drop in NVSN data was even more pronounced, reaching 56%.
The most significant decrease was observed in the youngest and most vulnerable group: newborns under the age of two months. In this age group, RSV hospitalizations dropped by 52% according to RSV-Net data. According to NVSN data, the rate dropped by 45%, but when Houston was excluded, the decrease was a remarkable 71%.
Another way to look at the impact is by examining the peak in hospitalizations. According to RSV-Net data, the peak in hospitalizations for children under the age of one this year was roughly half of the peak seen at the height of the 2023-24 season and about one-third of the peak in the 2022-23 season.
Dr. Heather Auerbach noted this trend in her own practice, observing “definitely less RSV this year compared to the last two to three years”. She also saw fewer critically ill children from RSV this year, needing lower levels of care. She explained the difference between needing basic support like suction and oxygen versus needing more intensive care like high flow oxygen, BiPap, or admission to the pediatric intensive care unit (PICU).
Evidence Linking the Decline to Prevention, Not Other Factors
The studies and experts point strongly to the new preventative tools as the primary reason for this dramatic drop in infant hospitalizations. A key piece of evidence supporting this link is the hospitalization rates observed in older children.
Children aged 1-4 and 5-17, who were largely not eligible for the new RSV prevention products, had higher rates of hospitalization from RSV this winter compared with the previous winter. According to Jasmine Reed, a CDC spokesperson, these higher rates in older children suggest that this past season may have been more severe overall than previous seasons. Yet, despite the potential for a more severe season, hospitalization rates dropped markedly among babies. This indicates that the shots are likely working well, potentially even better than the numbers initially suggest, as the drop occurred despite a potentially worse season for those without access to prevention.
The contrast between the outcomes for eligible young infants and largely ineligible older children suggests that the decline is due to the preventative interventions, rather than changes in behavior, testing patterns, or virus transmission rates across the board. Children older than eight months, who were largely not eligible, were hospitalized at higher rates this winter compared with the same times in 2018–20. This comparison reinforces the conclusion that the significant drop in young infants’ hospitalizations is directly linked to the widespread availability and use of the new preventatives. Researchers concluded that the observed drop in young infant hospitalizations could even be an underestimate of the tools’ true impact because the season may have been more severe.
Potential Impact on Infant Mortality
Beyond reducing hospitalizations, experts are also exploring whether the success in preventing severe RSV in infants has contributed to another positive public health trend: a drop in the US infant mortality rate.
Provisional data from the CDC posted in 2024 indicated that the nation’s infant mortality rate dropped last year. This follows two years (2022 and 2023) where the rate had hovered at a late-pandemic plateau, even seeing the first statistically significant jump in about two decades in 2022 and 2023, which experts attributed partly to a rebound in RSV and flu infections after pandemic precautions eased.
The provisional infant mortality rate dropped to about 5.5 infant deaths per 1,000 live births in 2024, down from about 5.6 per 1,000 live births in the previous two years. While this might seem like a small change, it represents a significant number of lives saved; U.S. infant deaths fell to about 19,900 last year, compared with about 20,150 in 2023. CDC officials believe the final numbers released later will largely reflect these provisional findings.
Some experts, including Dr. Amanda Williams, interim chief medical officer for the March of Dimes, think that one reason for the drop could be the vaccination campaign against RSV. The effort to implement the new antibody shot for infants and the RSV vaccine for pregnant women is considered a probable explanation for the improvement.
A separate CDC report released alongside the infant mortality data noted that infant hospitalizations in the 2024-25 respiratory virus season were more than 40% lower than past averages. This finding aligns with the other studies showing the dramatic reduction in RSV hospitalizations directly linked to the new prevention tools.
However, Dr. Williams also noted that more work is needed to fully understand all the reasons for the drop in infant mortality. She pointed out that much of the improvement in 2024 was seen in infants who were at least one month old when they died, suggesting that factors beyond RSV, such as fewer deaths from accidents, homicides, or SIDS, might also play a role. Nevertheless, the reduction in deaths and severe illness from RSV among the most vulnerable infants is a likely contributor to the overall decline.
Looking Ahead
The success of the new RSV prevention products represents a major breakthrough in protecting young babies from severe illness. Jasmine Reed of the CDC stated that the study findings demonstrate how a high uptake of these preventative tools “can lead to remarkable public health impact”. She added that increased use in future seasons could potentially lead to even larger reductions in RSV hospitalizations among infants.
Experts like Kawsar Talaat, a vaccine researcher and infectious disease physician, express excitement about the future. Based on the current data, she believes that “soon see a day when RSV hospitalizations in young infants will be much rarer than it is today” in the United States. While the focus of these studies is on the US, she also highlighted the global importance of these advancements, noting that most RSV hospitalizations and deaths occur in countries with less access to healthcare. Finding ways to make these products available to the most vulnerable children worldwide is crucial.
In conclusion, the 2023-24 RSV season in the US offered compelling evidence of the power of new public health interventions. The widespread availability of a maternal RSV vaccine and a preventative monoclonal antibody treatment is strongly linked to a significant and dramatic decline in severe RSV illness and hospitalizations among young infants. This reduction in severe outcomes for the most vulnerable population is a major public health achievement and may have played a role in the recent, welcome decrease in the national infant mortality rate. With continued and potentially increased use of these tools, the future looks brighter for protecting infants from the threat of RSV.
