The Sudden Golden Age of RSV Prevention in Infants

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Introduction

For fifty years, RSV prevention was a graveyard. The first vaccine, trialled in 1966, did not just fail — it primed vaccinated infants for worse disease, and two of them died.[1] For the next quarter-century the only tool we had was a monthly injection for a handful of high-risk babies. Then, almost overnight, we found ourselves with a maternal vaccine and two long-acting antibodies, all aimed at protecting every infant. This is the story of what unstuck the field — and how the new tools actually compare.

Why this matters

Respiratory syncytial virus is still the single most common reason a previously healthy infant ends up in hospital. Globally it caused an estimated 33 million lower-respiratory episodes, 3.6 million admissions and around 101,400 deaths in children under five in 2019, with over 97% of those deaths in low- and middle-income countries.[2] The uncomfortable part, and the one that reframes everything that follows, is that the majority of those hospitalised infants were not the classic high-risk babies — they were born at term with no underlying condition.[3] For the first time we have products that can protect all of them. Understanding why they exist, and where they differ, is now a bedside question, not a research one.

A field frozen by its first attempt

To understand the speed of the last three years, you have to understand the fifty years of stillness before them. Between 1965 and 1966, a formalin-inactivated RSV vaccine (FI-RSV) was given to infants before their first exposure to the virus. Of 31 vaccinated children, 20 were later infected naturally, 16 were hospitalised, and two died.[1] The vaccine had induced a Th2-skewed response and a flood of non-neutralising antibodies that formed immune complexes rather than protecting — a phenomenon we now call vaccine-associated enhanced respiratory disease.[1]

The consequence was caution measured in decades. Any non-live vaccine given to an RSV-naïve infant was treated as suspect, and infant vaccine development effectively stopped. When people describe RSV as the field that “everyone gave up on,” this is what they mean.

The one insight that changed everything

The logjam broke because of structural biology, not luck. RSV’s fusion (F) protein is a shape-shifter: it sits in a fragile, metastable prefusion form and then snaps irreversibly into a stable postfusion form. It turns out that the human antibodies that neutralise RSV most powerfully bind an epitope called site Ø, which exists only on the prefusion shape.[4] The trouble was that prefusion F falls apart the moment you try to use it as an immunogen.

In 2013, structure-based design solved this. By engineering disulphide bonds and filling internal cavities, researchers locked F into its prefusion conformation — the DS-Cav1 construct — and showed it elicited neutralising activity many times the protective threshold, driven by antibodies 10- to 100-fold more potent than the old drug palivizumab.[4] Every modern RSV product — the maternal vaccine, nirsevimab and clesrovimab — is a direct descendant of that single stabilised protein.

Two further advances turned a good target into a practical one. First, antibody engineering: modifying the Fc region of a monoclonal antibody (the “YTE” mutation) roughly tripled its half-life, from around three weeks to about 70 days for nirsevimab, so a single dose could cover an entire season rather than requiring monthly injections.[5] Second, durability: unlike SARS-CoV-2, where every therapeutic antibody was rendered obsolete within three years by viral escape, RSV’s key epitopes are structurally constrained and highly conserved — clesrovimab’s target, site IV, is 99.8% conserved across more than 11,000 sequenced RSV strains — so the virus cannot easily mutate away from these antibodies.[6]

So when the question comes up — is this progress about biology, antibody engineering or cost? — the honest answer is biology first. The 2013 prefusion structure was the necessary condition. Engineering made it a once-a-season product. Conservation made it durable. Cost is why it is now scaling, not why it became possible.

Why RSV wrecks the infant lung

RSV has what one major review calls a “propensity for causing bronchiolitis” that is intrinsic to how it infects.[7] It targets the ciliated epithelial cells lining the airways; those cells die and slough into the lumen, and together with mucus, fibrin, neutrophils and DNA-rich debris they form dense plugs that physically obstruct the smallest airways.[7,8] The classic post-mortem picture in fatal infant RSV is exactly this — bronchiolar lumens occluded by sloughed necrotic epithelium, with surrounding inflammation and oedema.[7] The virus also disables the cilia’s ability to clear mucus within a day of infection and drives excess mucus production, compounding the blockage.[7]

Why the very youngest, specifically

Three things converge in the first few months of life. The bronchioles are at their narrowest, so a plug or a millimetre of oedema causes disproportionate obstruction, gas-trapping and rising respiratory rate.[7] The infant’s own neutralising-antibody response is poor, and maternal antibody — transferred late in the third trimester — is already waning.[9] The result is that the peak of serious RSV disease falls at around two months of age, precisely when maternal antibody reliably protects against most other pathogens but not RSV.[10] The epidemiology tracks the biology: US surveillance found the highest admission rate, 25.1 per 1,000, in one-month-old infants, and Japanese data placed the peak of hospitalisations at two months.[3,11]

Why immunity never lasts

RSV reinfects throughout life, sometimes within the same season, despite having only a single serotype.[10] Natural infection induces only partial, short-lived neutralising antibody that often does not survive to the next season.[12] It takes three to six seasons of repeated exposure for a child’s neutralising titres to reach adult levels.[13] RSV-specific T cells neither protect against reinfection nor are boosted by it.[12] And the virus actively evades antibody — its secreted G glycoprotein acts as a decoy.[10] This is why “vaccinate once and forget” was never going to work for RSV, and why passive antibody, delivered directly or through the mother, fits the narrow newborn window so cleanly.

Where the burden actually sits

The headline figures from the definitive global estimate deserve to be stated plainly: in 2019, RSV caused 33.0 million lower-respiratory episodes, 3.6 million hospital admissions, and around 101,400 attributable deaths in children under five — roughly one in fifty of all under-five deaths, with the proportion highest (3.6%) in infants aged 28 days to six months.[2]

The framing point for clinicians is the term-healthy majority. In US population-based surveillance, 67% of RSV-hospitalised infants had no prematurity and no underlying condition; in Japan, 90% of hospitalised under-twos had no known risk factor.[3,11] Prematurity, chronic lung disease and congenital heart disease raise individual risk, but because those groups are numerically small, they account for a minority of total admissions. This is the single most important reason the old palivizumab model could never move the population needle, and why the new products target all infants.

Are we flying blind in low-income settings? Partly, and honestly so. Only 26% of RSV deaths occur in hospital overall, falling to just 18% of young-infant deaths in low-income countries — roughly four community deaths for every one in hospital, and in one remote Indian setting as many as thirteen.[2] The authors of the global estimate call their mortality figure a likely underestimate, drawn from sparse data in under-resourced settings.[2] We should quote the numbers, but with that caveat attached rather than dramatised.

Seasonality: why the Gulf and South Asia break the rule

In temperate regions RSV is a sharp winter epidemic lasting around five to six months. In subtropical and tropical climates the pattern differs: peaks often coincide with the warmest months or the monsoon, seasons run longer — up to ten months — and in some settings RSV circulates essentially year-round.[14,15] Saudi hospital data still show a discernible winter and early-spring rise but with detection year-round, and older work explicitly recommended extended dosing because the warm-climate season outruns the temperate five-month assumption.[16,17] A dosing schedule copied from North America or the UK will misfire where the season is longer, shifted or absent — which strengthens the case for year-round or birth-dose delivery in these regions.

How is a “season” actually declared? Surveillance uses statistical epidemic thresholds, most commonly the Moving Epidemic Method: prior seasons set a threshold, and onset is declared when weekly activity crosses it. In Guatemala this captured 70–99% of annual detections, but onsets varied by up to five months between years and between two sites in the same country.[14] We do not declare the season by the calendar — we declare it when the virus crosses a moving statistical threshold.

The four routes, from palivizumab to clesrovimab

Palivizumab (1998): how to remember it

The IMpact-RSV trial gave preterm and chronic-lung-disease infants five monthly intramuscular doses and achieved a 55% relative reduction in RSV hospitalisation — but the absolute reduction was 5.8%, a number-needed-to-treat of 17.[18] It binds site II, present on both conformations of F and therefore less potent than the prefusion-specific antibodies; it has a three-to-four-week half-life demanding monthly dosing; and it was restricted to a narrow high-risk group and was famously expensive.[18] It is now being withdrawn — the manufacturer has announced it will be unavailable after 31 December 2025, superseded by the newer products.[19] The fair way to remember it: the honourable first draft that proved passive antibody works, protected high-risk babies for 25 years, and taught us the target — retired because the field it opened outgrew it.

The maternal vaccine (RSVpreF / Abrysvo)

A single dose of bivalent prefusion-F vaccine in pregnancy; the mother makes antibody and transfers it across the placenta. In the phase 3 MATISSE trial, efficacy against medically-attended severe RSV lower-respiratory illness in infants was 81.8% by 90 days and 69.4% by 180 days; efficacy against any medically-attended RSV illness was 57.1% at 90 days and did not formally meet the statistical bar.[20] No safety signals were seen in mothers or infants.[20]

The window question that Shiva’s audience will ask is why it varies by country. Antibody needs about 14 days after vaccination to develop and cross the placenta, so an infant born within 14 days of the dose is not protected by it.[21] The US window is 32–36 weeks, deliberately narrower than the trial’s 24–36 weeks, set to minimise any theoretical preterm-birth risk: a different maternal RSV vaccine had shown a preterm signal, and MATISSE itself showed a small numerical imbalance (preterm birth 5.7% vaccine versus 4.7% placebo, not significant overall, but with a regional signal in South Africa).[22,21] The UK chose from 28 weeks, year-round, on a different reading of the same risk-benefit balance.[23] So yes — the window genuinely differs: 32–36 weeks seasonally in the US, from 28 weeks year-round in the UK.

Nirsevimab (2023): why it was more than a better palivizumab

A prefusion-F-specific (site Ø) monoclonal with the YTE half-life extension of around 70 days, given as a single weight-banded dose. What set it apart from palivizumab was not incremental: one seasonal dose replaces five monthly ones, it is aimed at all infants rather than the high-risk few, and its efficacy is higher — around 70% in preterm infants, 74–76% against medically-attended RSV illness in term and late-preterm infants, and 83% against hospitalisation with 76% against very severe RSV in the large pragmatic HARMONIE trial.[24,25,6] Neutralising-antibody levels stay well above baseline for at least five months — a full season — while still allowing the infant to mount their own response to natural RSV.[9] Real-world effectiveness has matched the trials, pooling to around 81% against hospitalisation across dozens of observational studies.[26] One honest caveat: moderately preterm infants retain some residual risk despite prophylaxis.[27]

Clesrovimab (2025): what it adds

A second long-acting monoclonal, approved in June 2025, targeting site IV — an even more conserved epitope — with a half-life of around 44 days and, crucially, given as a single fixed dose regardless of weight.[6,28] In the phase 3 CLEVER trial it reduced medically-attended RSV illness through five months by 60.4% and RSV hospitalisation by 84.2%, with a placebo-like safety profile.[26] What it brings that nirsevimab does not is a weight-independent single dose — simpler logistics, no weight-band decision, attractive for programmatic roll-out and for the newborn who has not yet been weighed — and a differently conserved target.[26,28]

So which route is the default?

The evidence does not crown a winner. Pooled real-world effectiveness against hospitalisation is near-identical — around 81% for nirsevimab and 79% for the maternal vaccine.[26] A reasonable framing: the infant monoclonal offers a controlled, guaranteed dose independent of the mother’s immune response and covers babies of unvaccinated mothers; the maternal route protects from birth with no gap in the first days and needs no infant visit. The answer is context and health system, not intrinsic superiority. And for the two monoclonals head-to-head, there is no direct trial — the differences between them are mechanistic and logistic, not proven clinical superiority.

Where the guidelines have landed

In the United States, ACIP recommends either the maternal vaccine (at 32–36 weeks, seasonally from September to January in most of the country) or an infant monoclonal — but not both for most infants, since both simply deliver antibody to the same baby.[21] Nirsevimab is specifically recommended for all infants born before 34 weeks, because a 32-week maternal dose only protects from around 34 weeks, and clesrovimab has been added as an alternative to nirsevimab.[21,6] In the UK, a universal year-round maternal programme from 28 weeks runs alongside a selective nirsevimab programme for high-risk and very preterm infants, which from the 2025 season replaces palivizumab.[23] Because nirsevimab is an antibody rather than a live vaccine, it can be co-administered with the routine childhood immunisations.[21]

The elephant in the room: cost and access

The trials were run largely in high-income systems, and the products are expensive. Palivizumab was never cost-effective outside narrow high-risk groups, and nirsevimab’s value depends heavily on price and implementation.[18,29] The uncomfortable arithmetic is that over 97% of RSV deaths occur in low- and middle-income countries, which is precisely where these products are least available and least studied in real-world use.[2] Modelling in Saudi Arabia projected that universal nirsevimab could avert around 58% of hospitalisations, but such analyses assume supply and financing that most low-income settings do not yet have.[30] For now, the golden age is a high-income and Gulf golden age. Extending it depends on tiered pricing and delivery systems — not on any further scientific breakthrough.

Practical implications

For clinicians, three shifts follow from the evidence. First, prevention is no longer a high-risk-only conversation — the term, healthy infant is now the target, and eligibility should be checked routinely rather than reserved for the neonatal unit. Second, timing is everything: the protective window is the first few months, and both maternal and infant strategies are built around delivering antibody before the season and before those first vulnerable weeks. Third, in warm-climate practice, the temperate “winter season” framing does not transfer cleanly, and local seasonality should drive local scheduling. Where the source evidence stops — long-term outcomes, exact pricing, direct comparison of the two monoclonals — we should stop too, and say so.

Key Takeaways

  • RSV prevention was frozen for 50 years by a 1966 vaccine that worsened disease; the 2013 prefusion-F structure is the single insight that unlocked every modern product.[1,4]
  • Severe RSV peaks at around two months of age because infant airways are narrowest and immune defences weakest, while immunity from natural infection is short-lived and reinfection is lifelong.[7,10,12]
  • The majority of hospitalised infants are born at term with no risk factors — 67% in US data, 90% in Japanese data — which is why prevention now targets all infants, not just high-risk groups.[3,11]
  • Maternal RSVpreF vaccine: 81.8% efficacy against severe RSV at 90 days; window is 32–36 weeks in the US but from 28 weeks in the UK.[20,21,23]
  • Nirsevimab (single weight-banded dose, site Ø) and clesrovimab (single fixed dose, site IV) both prevent 80–84% of RSV hospitalisations; there is no head-to-head trial to separate them.[26]
  • The unresolved problem is access: over 97% of RSV deaths are in low- and middle-income countries, where these products remain largely unavailable.[2]

Sources and further reading

  1. Babawale PI, Martínez-Espinoza I, Mitchell A, et al. Preventing RSV infection in children: current passive immunizations and vaccine development. Pathogens. 2025;14(2):104. DOI: 10.3390/pathogens14020104
  2. Li Y, Wang X, Blau DM, et al. Global, regional, and national disease burden estimates of acute lower respiratory infections due to RSV in children younger than 5 years in 2019. The Lancet. 2022;399(10340):2047–2064. DOI: 10.1016/S0140-6736(22)00478-0
  3. Rha B, Curns AT, Lively JY, et al. Respiratory syncytial virus–associated hospitalizations among young children: 2015–2016. Pediatrics. 2020;146(1). DOI: 10.1542/peds.2019-3611
  4. McLellan JS, Chen M, Joyce M, et al. Structure-based design of a fusion glycoprotein vaccine for respiratory syncytial virus. Science. 2013;342(6158):592–598. DOI: 10.1126/science.1243283
  5. Mankad VS, Leach A, Chang Y, et al. Comprehensive summary of safety data on nirsevimab from all pivotal randomized clinical trials. Pathogens. 2024;13(6):503. DOI: 10.3390/pathogens13060503
  6. Guo H, Wang H, Wu S, et al. Determinants of success and failure of antibody-based strategies against respiratory viruses: insights from RSV and SARS-CoV-2. Frontiers in Immunology. 2026;17. DOI: 10.3389/fimmu.2026.1818721
  7. Pickles RJ, DeVincenzo JP. Respiratory syncytial virus (RSV) and its propensity for causing bronchiolitis. The Journal of Pathology. 2014;235(2):266–276. DOI: 10.1002/path.4462
  8. Cortjens B, de Boer OJ, de Jong R, et al. Neutrophil extracellular traps cause airway obstruction during respiratory syncytial virus disease. The Journal of Pathology. 2015;238(3):401–411. DOI: 10.1002/path.4660
  9. Wilkins D, Yuan Y, Chang Y, et al. Durability of neutralizing RSV antibodies following nirsevimab administration. Nature Medicine. 2023;29(5):1172–1179. DOI: 10.1038/s41591-023-02316-5
  10. Bukreyev A, Yang L, Fricke J, et al. The secreted form of RSV G glycoprotein helps the virus evade antibody-mediated restriction of replication. Journal of Virology. 2008;82(24):12191–12204. DOI: 10.1128/JVI.01604-08
  11. Kobayashi Y, Togo K, Agosti Y, et al. Epidemiology of respiratory syncytial virus in Japan: a nationwide claims database analysis. Pediatrics International. 2021;64(1). DOI: 10.1111/ped.14957
  12. Bont L, Versteegh JFM, Swelsen W, et al. Natural reinfection with respiratory syncytial virus does not boost virus-specific T-cell immunity. Pediatric Research. 2002;52(3):363–367. DOI: 10.1203/00006450-200209000-00009
  13. Kosanovich JL, Eichinger KM, Lipp MA, et al. Formulation of the prefusion RSV F protein with a Th1/Th2-balanced adjuvant. Vaccine. 2020;38(41):6357–6362. DOI: 10.1016/j.vaccine.2020.08.023
  14. Hamid S, Grajeda LM, de León O, et al. Variability in the timing of respiratory syncytial virus epidemics in Guatemala, 2008–2018. Influenza and Other Respiratory Viruses. 2024;18(7). DOI: 10.1111/irv.13334
  15. Yang L, Chan KH, Suen LKP, et al. Age-specific epidemic waves of influenza and respiratory syncytial virus in a subtropical city. Scientific Reports. 2015;5(1). DOI: 10.1038/srep10390
  16. Almuqati NM, Al-Hindi MY, Moussa HA, et al. Healthcare utilization and economic burden of pediatric lower respiratory tract infections across five tertiary hospitals in Saudi Arabia. Pediatric Reports. 2026;18(3):71. DOI: 10.3390/pediatric18030071
  17. Al-Alaiyan S, Pollack P, Notario G, et al. Safety and pharmacokinetics of extended use of palivizumab in Saudi Arabian infants and children. Drugs in Context. 2015;4:1–10. DOI: 10.7573/dic.212270
  18. Embleton ND. Palivizumab for preterm infants. Is it worth it? Archives of Disease in Childhood — Fetal & Neonatal Edition. 2005;90(4):F286–F289. DOI: 10.1136/adc.2004.058081
  19. American Academy of Pediatrics, AAP News. Sobi discontinues RSV injection Synagis. 2025. publications.aap.org/aapnews/news/32817
  20. Kampmann B, Madhi SA, Munjal I, et al. Bivalent prefusion F vaccine in pregnancy to prevent RSV illness in infants. New England Journal of Medicine. 2023;388(16):1451–1464. DOI: 10.1056/NEJMoa2216480
  21. Fleming-Dutra KE, Jones JM, Roper LE, et al. Use of the Pfizer RSV vaccine during pregnancy for the prevention of RSV-associated lower respiratory tract disease in infants: recommendations of the ACIP — United States, 2023. MMWR. 2023;72(41):1115–1122. DOI: 10.15585/mmwr.mm7241e1
  22. Madhi SA, Kampmann B, Simões EAF, et al. Preterm birth frequency and associated outcomes from the MATISSE trial of the bivalent RSV prefusion F protein vaccine. Obstetrics & Gynecology. 2025;145(2):147–156. DOI: 10.1097/AOG.0000000000005817
  23. NHS England. NHS to roll out long-lasting ‘suit of armour’ jab to protect thousands of premature babies from RSV. 2025. england.nhs.uk
  24. Arbetter D, Gopalakrishnan V, Aksyuk AA, et al. Lower respiratory tract infections following RSV monoclonal antibody nirsevimab versus placebo: analysis from a phase 3 randomized clinical trial (MELODY). Clinical Infectious Diseases. 2024;81(3):634–644. DOI: 10.1093/cid/ciae596
  25. Loe MWC, Soenong H, Lee E, et al. Nirsevimab: alleviating the burden of RSV morbidity in young children. Journal of Paediatrics and Child Health. 2024;60(10):489–498. DOI: 10.1111/jpc.16643
  26. Oliva I, Oliveira CR. RSV prophylactics for pediatric populations: a product review and meta-analysis of real-world effectiveness. Human Vaccines & Immunotherapeutics. 2026;22(1). DOI: 10.1080/21645515.2026.2690750
  27. Cocchi E, Bloise S, Lorefice A, et al. Multicentre study on nirsevimab: Bayesian analysis reveals persisting risk for preterm infants. BMJ Paediatrics Open. 2025;9(1):e003665. DOI: 10.1136/bmjpo-2025-003665
  28. Hu Z, Hellmann F, Zang X, et al. Population pharmacokinetics of clesrovimab in preterm and full-term infants. Clinical Pharmacology & Therapeutics. 2026;119(4):1036–1046. DOI: 10.1002/cpt.70199
  29. Alharbi A, Yousef AA, Zubani A, et al. RSV burden in infants in the Kingdom of Saudi Arabia and the impact of all-infant RSV protection: a modeling study. Advances in Therapy. 2024;41(4):1419–1435. DOI: 10.1007/s12325-024-02798-w
  30. Villenave R, Thavagnanam S, Sarlang S, et al. In vitro modeling of respiratory syncytial virus infection of pediatric bronchial epithelium. PNAS. 2012;109(13):5040–5045. DOI: 10.1073/pnas.1110203109

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