Preventing Respiratory Syncytial Virus Illness in Infants
May 1, 2025
By Philip R. Fischer, MD, DTM&H
Synopsis: Prevention of respiratory syncytial virus (RSV) illness by using a monoclonal antibody, nirsevimab, in young infants entering the winter RSV season was recommended in 2023. Subsequently, effectiveness was evaluated in children in the northeastern United States. Nirsevimab was 68% effective in preventing RSV-associated need for medical care and 81% effective in preventing hospitalization for RSV-related illness.
Source: Xu H, Aparicio C, Wats A, et al. Estimated effectiveness of nirsevimab against respiratory syncytial virus. JAMA Netw Open. 2025;8(3):e250380.
Worldwide, respiratory syncytial virus (RSV) accounts for 1,400,000 hospitalizations of children during the first six months of life each year; 1% of these children do not survive the illness. Nirsevimab, a monoclonal antibody, was licensed in the United States in July 2023 for RSV prevention in infants. Pre-licensure clinical trials had shown efficacy against RSV illness prompting medical care (79%), RSV requiring hospitalization (81%), and RSV requiring critical care services (90%). Nirsevimab is recommended for children in the United States younger than 8 months of age who are heading into their first RSV (winter) season; it also is recommended for older high-risk children.
Xu and colleagues in Connecticut undertook a controlled study to see how effective nirsevimab was in a community setting. Ill children who tested negative for RSV (controls) were compared to ill children who tested positive for RSV (cases). Study subjects received care in a large integrated health system affiliated with Yale University and were eligible for recommended RSV prophylactic treatment with nirsevimab during the October 2023 to May 2024 RSV season.
During the 2023-2024 RSV season, 3,090 RSV tests were done in this setting in infants with a median age of 6.7 months; 22% of tests were positive. Overall, 10.7% of the study subjects had received the recommended nirsevimab (most with the 50 mg dose, 29% with the 100 mg dose). (Less than 1% of subjects’ mothers had received RSV vaccination during pregnancy.) Of RSV-positive patients, 24% were hospitalized, half of whom received more than 2 L/minute of oxygen support and 14% of whom required intensive care.
Comparing children who tested negative for RSV with those who tested positive, receipt of nirsevimab was 68% effective in preventing medical care (62% effective in preventing outpatient visits, 81% effective in preventing hospitalization) and 85% effective in preventing severe (requiring high-flow oxygen support or intensive care) RSV illness. However, effectiveness waned following receipt of nirsevimab, from 79% two weeks post-administration to 55% at 14 weeks post-administration.
The effectiveness of nirsevimab seen in this “real world” study was similar to the effectiveness noted in pre-licensure trials. Uniquely, the study population tested was more than 50% non-Caucasian, while most pre-licensure studies had involved mostly Caucasian children. This study also was somewhat unique in including not just hospitalized children, but also outpatients with RSV infection and illness. The follow-up through the full RSV season also was different from some previous studies and, unfortunately, showed some reduction in protective effect of nirsevimab during the months following administration.
In summary, the authors rightly assessed that their new data support current American guidelines recommending nirsevimab for all infants younger than 8 months of age entering their first RSV season. Future studies in settings with higher rates of nirsevimab use will be helpful in determining the effects of this RSV prevention when overall community rates of RSV infection are decreased.
Commentary
These new data from Xu and colleagues affirm findings from Spain and the United States that nirsevimab effectively prevents RSV-related hospitalization in otherwise healthy infants.1-3 In Spain, nirsevimab use reduced approximately 70% of RSV-related hospitalizations as compared with the two preceding years.1 In the United States, nirsevimab reduced RSV-related hospitalizations by 90% to 93%.2-3
The findings of Xu and colleagues also extend some of the previous Spanish and American findings in showing that nirsevimab use also reduced the risk (by 68%) of less severe RSV illness prompting outpatient medical care but not requiring hospitalization.1,2 By using RSV-negative ill children as the comparison group, Xu and colleagues overcame confounding possibilities that the likelihood that the favorable effectiveness of nirsevimab in reducing the apparent need for outpatient care was merely due to access-to-care and family thresholds for seeking medical care.
In the United States, RSV protection is recommended for older adults and for infants.4 The Centers for Disease Control and Prevention recommends that adults older than 75 years of age (and those 60-74 years of age deemed to be at high-risk of severe consequences of RSV infection) receive RSV vaccination.4 Infants can either be protected by vaccination of pregnant women of 32-36 weeks’ gestation from September through January or by use of nirsevimab for infants during the October through March respiratory virus season.4
There is some synergy between respiratory viruses and bacteria infecting infants. Reducing the risk of illness due to one organism also can reduce rates of illness by other organisms. A recent study from Israel demonstrated that the rates of RSV infection (during the years prior to use of nirsevimab) dropped significantly with the implementation of widespread infant immunization with a pneumococcal conjugate vaccine.5 Specifically, hospitalization rates for RSV-related pneumonia dropped by 29% during the initial years following generalized use of pneumococcal vaccines.5
Ongoing studies of the effectiveness of RSV prevention efforts in infants will be helpful as the use of nirsevimab becomes more generalized, but it is anticipated that the favorable effects will persist. At the same time and especially during the current era of public and political doubt about the value of vaccination in America, Xu and colleagues remind us that these new data “provide evidence that may help boost public confidence in the immunization program.”
Philip R. Fischer, MD, DTM&H, is Professor of Pediatrics, Department of Pediatric and Adolescent Medicine, Mayo Clinic, Rochester, MN.
References
1. Rius-Peris JM, Palomo-Atance E, Muro-Díaz E, et al. Nirsevimab immunisation significantly reduces respiratory syncytial virus-associated bronchiolitis hospitalisations and alters seasonal patterns. Acta Paediatr. 2025; Mar 26. doi: 10.1111/apa.70066. [Online ahead of print].
2. Moline HL, Tannis A, Toepfer AP, et al. Early estimate of nirsevimab effectiveness for prevention of respiratory syncytial virus-associated hospitalization among infants entering their first respiratory syncytial virus season — New Vaccine Surveillance Network, October 2023-February 2024. MMWR Morb Mortal Wkly Rep. 2024;73(9):209-214.
3. Moline HL, Toepfer AP, Tannis A, et al. Respiratory syncytial virus disease burden and nirsevimab effectiveness in young children from 2023-2024. JAMA Pediatr. 2025;179(2):179-187.
4. Centers for Disease Control and Prevention. RSV vaccines. Aug. 30, 2024. https://www.cdc.gov/rsv/vaccines/index.html
5. Dagan R, van der Beek BA, Grupel T, et al. Decline of community-acquired alveolar pneumonia positive for respiratory syncytial virus in hospitalized children following implementation of PCV in Israel. Clin Infect Dis. 2025; Mar 7:ciaf102. doi: 10.1093/cid/ciaf102. [Online ahead of print].
Prevention of respiratory syncytial virus (RSV) illness by using a monoclonal antibody, nirsevimab, in young infants entering the winter RSV season was recommended in 2023. Subsequently, effectiveness was evaluated in children in the northeastern United States. Nirsevimab was 68% effective in preventing RSV-associated need for medical care and 81% effective in preventing hospitalization for RSV-related illness.
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