A Single Shot That Is Quieting RSV Season for Babies

What a large Italian study of 13,624 newborns (Cocchi et al., JAMA Network Open 2025) tells families about nirsevimab, RSV, and who still needs extra care

A new antibody shot called nirsevimab, given to newborns before they leave the hospital, dramatically lowered the number of babies hospitalized with the winter virus RSV in a large real-world study from Italy. Across nearly 14,000 babies, hospital admissions for RSV lung infections dropped by about two-thirds after the shot was offered to everyone, and babies who caught RSV anyway tended to be less sick. But the study also found that premature babies and babies with older brothers or sisters at home stayed at higher risk even after the shot — a reminder that one dose does a great deal, yet not everything.

Why This Research Matters

Almost every parent has heard of RSV, respiratory syncytial virus, even if they did not know its name until a baby in the family caught it. The Italian study at the center of this article, led by Dr. Enrico Cocchi and colleagues [1], set out to answer a simple but important question: when a new preventive shot is offered to every newborn, what actually happens to the number of babies hospitalized with this virus? RSV is the single most common reason infants around the world end up in the hospital with breathing trouble, causing an estimated 3.6 million hospital admissions each year in children under five [2]. For most babies RSV is an ordinary cold, but in the very young it can settle into the lungs as bronchiolitis, making it hard to breathe and feed, and sometimes requiring oxygen, a hospital stay, or intensive care.

To understand why this is such a milestone, it helps to know what families and doctors faced before. For 25 years, the only tool to prevent RSV was a medicine called palivizumab. It worked — a landmark 1998 trial showed it lowered RSV hospitalizations in high-risk babies [3] — but it had to be given as an injection every month through the winter, it was very expensive, and it was only offered to a small group of the most fragile premature or heart-and-lung patients. That left the great majority of babies, healthy full-term newborns, with no protection at all. Parents could only rely on hand-washing, avoiding crowds, and hoping. Anyone who has sat up through a long night with a wheezing infant knows how helpless that felt.

Nirsevimab changed the picture because it is different in one crucial way: a single dose protects a baby for an entire RSV season. In carefully run medical trials it lowered RSV illness and hospitalizations by roughly 70% to 75% [4],[5], was shown to be as safe as the older medicine even in the most delicate babies [6], and cut RSV hospital admissions by 83% under real-world trial conditions [7]. A separate approach — a vaccine given to mothers during pregnancy that passes protection to the baby — offers families another option [8]. On this strength, health authorities in the United States recommended nirsevimab for all infants entering their first RSV season [9], and Italy chose to offer it to every baby born during the winter virus period. What nobody yet knew was whether the impressive trial results would hold up when the shot was rolled out to a whole population at once.

What the Study Did

The researchers looked at every baby born across five hospitals in the Romagna region of northern Italy over two back-to-back winters [1]. The first winter (2023–2024) was before nirsevimab was routine; the second (2024–2025) was after it began being offered to all newborns before they went home from the hospital. In all, 13,624 babies were included — no baby was left out because of prematurity or other health problems. About 5 in 100 were born premature, and about half had an older sibling at home. Nearly 80% of the newborns in the second winter received the shot.

The team then counted how many babies were hospitalized with RSV lung infections, confirmed by a nasal swab test, and compared the two winters. They were careful to account for the fact that RSV comes and goes with the seasons, and they double-checked their work by also counting hospital stays for other, non-RSV chest infections — if those stayed the same while RSV admissions dropped, it would show the change was really about RSV and not just a quieter winter overall.

What They Found

The main result was striking. In the winter before the shot, 220 babies were hospitalized with RSV; in the winter after, only 72 were [1]. And of those 72, three out of four were babies who had not received the shot. When the researchers zoomed in on individual babies, those who got nirsevimab had roughly 89% lower odds of being hospitalized with RSV. Looking at the whole population — including the one in five babies who did not get the shot — RSV hospitalizations fell by about 68%. Both numbers tell the same story from different angles: the shot protects an individual baby very strongly, and even a not-quite-complete rollout protects a community a great deal. Meanwhile, hospital stays for other chest infections did not change, confirming that this was RSV protection at work, not a fluke of an easy season.

The study also delivered an honest and important caution. Even among babies who received nirsevimab, two groups remained at higher risk: babies born prematurely, and babies living with older siblings [1]. Premature babies were nearly three times as likely to be hospitalized with RSV, and babies with older brothers or sisters were more than four times as likely. This does not mean the shot works less well for them — the researchers checked, and it does not. Rather, premature babies start out more vulnerable because their lungs and immune systems are less mature, and children who bring viruses home from school or daycare simply expose a baby to RSV more often. Protection is powerful, but a determined virus and a fragile patient can still meet.

There was reassurance, too, for the babies who did catch RSV despite the shot. They were less likely to need high-flow oxygen support, a sign that their illness ran a milder course [1]. Their hospital stays were not clearly shorter and intensive-care numbers were too small to draw firm conclusions, but the overall pattern suggested that even a breakthrough infection tended to be gentler in a baby who had been protected.

What This Means for Families and Their Baby's Care

The practical takeaway is encouraging and clear. Nirsevimab is a genuine advance, and for most families the decision to accept it for a newborn is well supported by evidence — both from the careful trials and now from this and other real-world studies. Because the shot is usually given in the hospital before a baby goes home, most families will be offered it at a natural moment, and saying yes offers strong, season-long protection from the most common cause of infant hospitalization.

At the same time, the study gently reframes what protection means. If your baby was born early, or if there are older children in the house, the shot still helps enormously — but your baby remains more vulnerable than a full-term single child, so ordinary precautions still matter through that first winter. Washing hands, keeping a sick sibling's coughs and kisses away from the newborn, and watching for fast or labored breathing are all still worthwhile. For families who cannot or choose not to have the shot, the maternal vaccine during pregnancy is another route to protecting a baby [8], and it is worth discussing options with your care team.

A Few Honest Limits

This was a real-world study rather than a controlled experiment, comparing one winter to the next, so it cannot rule out every other explanation with the certainty of a randomized trial. The follow-up was short, so it cannot yet say whether preventing RSV hospitalizations also reduces later wheezing or asthma. But its findings line up closely with independent data from the United States, where health surveillance has estimated nirsevimab to be roughly 80% to 90% effective at preventing RSV hospitalizations [10],[11] and effective at keeping babies out of intensive care [12]. When several separate studies on different continents reach the same conclusion, families can take that agreement as a reason for confidence.

What Researchers Are Working On Next

The next questions follow directly from what this study revealed. Scientists want to know how to better protect the babies who remain at higher risk — whether premature infants might benefit from a different or additional dose, whether reducing the spread of viruses within busy households would help, and whether preventing RSV in infancy leads to healthier lungs years later. For now, the message from nearly 14,000 Italian babies is a hopeful one: a single shot has made RSV season measurably quieter and gentler for infants, while pointing clearly to the families who still deserve a little extra care [1].

References

  1. Cocchi E, Bloise S, Lorefice A, et al. Nirsevimab Prophylaxis and Respiratory Syncytial Virus Hospitalizations Among Infants. JAMA Netw Open. 2025;8(11):e2544679. doi:10.1001/jamanetworkopen.2025.44679 ↩
  2. Li Y, Wang X, Blau DM, et al. Global, regional, and national disease burden estimates of acute lower respiratory infections due to respiratory syncytial virus in children younger than 5 years in 2019: a systematic analysis. Lancet. 2022;399(10340):2047–2064. doi:10.1016/S0140-6736(22)00478-000478-0) ↩
  3. The IMpact-RSV Study Group. Palivizumab, a humanized respiratory syncytial virus monoclonal antibody, reduces hospitalization from respiratory syncytial virus infection in high-risk infants. Pediatrics. 1998;102(3):531–537. doi:10.1542/peds.102.3.531 ↩
  4. Griffin MP, Yuan Y, Takas T, et al. Single-dose nirsevimab for prevention of RSV in preterm infants. N Engl J Med. 2020;383(5):415–425. doi:10.1056/NEJMoa1913556 ↩
  5. Hammitt LL, Dagan R, Yuan Y, et al. Nirsevimab for prevention of RSV in healthy late-preterm and term infants (MELODY). N Engl J Med. 2022;386(9):837–846. doi:10.1056/NEJMoa2110275 ↩
  6. Domachowske J, Madhi SA, Simões EAF, et al. Safety of nirsevimab for RSV in infants with heart or lung disease or prematurity (MEDLEY). N Engl J Med. 2022;386(9):892–894. doi:10.1056/NEJMc2112186 ↩
  7. Drysdale SB, Cathie K, Flamein F, et al. Nirsevimab for prevention of hospitalizations due to RSV in infants (HARMONIE). N Engl J Med. 2023;389(26):2425–2435. doi:10.1056/NEJMoa2309189 ↩
  8. Kampmann B, Madhi SA, Munjal I, et al. Bivalent prefusion F vaccine in pregnancy to prevent RSV illness in infants (MATISSE). N Engl J Med. 2023;388(16):1451–1464. doi:10.1056/NEJMoa2216480 ↩
  9. Jones JM, Fleming-Dutra KE, Prill MM, et al. Use of nirsevimab for the prevention of RSV disease among infants and young children: recommendations of the ACIP — United States, 2023. MMWR Morb Mortal Wkly Rep. 2023;72(34):920–925. doi:10.15585/mmwr.mm7234a4 ↩
  10. Moline HL, Toepfer AP, Tannis A, et al. Early estimate of nirsevimab effectiveness for prevention of RSV–associated hospitalization among infants entering their first RSV season — New Vaccine Surveillance Network, October 2023–February 2024. MMWR Morb Mortal Wkly Rep. 2024;73(9):209–214. doi:10.15585/mmwr.mm7309a4 ↩
  11. Moline HL, Tannis A, Goldstein L, et al. Effectiveness and impact of maternal RSV immunization and nirsevimab on medically attended RSV in US children. JAMA Pediatr. 2026;180(3):314–324. doi:10.1001/jamapediatrics.2025.5778 ↩
  12. Halasa N, et al. Nirsevimab effectiveness against intensive care unit admission for respiratory syncytial virus in infants — 24 states, December 2024–April 2025. MMWR Morb Mortal Wkly Rep. 2025;74(37). doi:10.15585/mmwr.mm7437a1 ↩