Argentina's maternal RSV vaccine cut infant hospitalisations by a third in year one

Argentina introduced the bivalent RSV prefusion F maternal vaccine into its programme recently enough that 2024 was effectively its first full season [s1]. A study published in The Lancet Regional Health – Americas on 19 November reports what happened to infant hospitalisations in Buenos Aires afterwards [s1].

Respiratory syncytial virus is a leading cause of hospitalisation and death in young infants worldwide [s1]. Maternal immunisation works by transferring antibody across the placenta, protecting the baby during the first months of life — the window in which RSV does most of its damage and in which no infant vaccine can act fast enough.

What was measured

The study used a hospital-based, multicentre, retrospective surveillance cohort design across three Argentine tertiary hospitals, drawing on data collected from hospitalised infants under 18 months of age over seven years [s1]. Expected RSV-associated acute lower respiratory tract infection hospitalisations were compared with observed cases using a Poisson model [s1].

Nested inside that was a test-negative case-control analysis to estimate vaccine effectiveness [s1]. It included infants hospitalised with acute lower respiratory tract infection who were born between 1 March and 9 November 2024, were under six months old when tested for RSV, and whose mothers had been eligible for prenatal RSV immunisation [s1]. Estimates were adjusted for age, sex, comorbidities and epidemiological week [s1].

The impact analysis covered 3,373 participants; 323 of them were born during the vaccination period and were eligible for the effectiveness analysis [s1].

The findings

Vaccine effectiveness against RSV-associated lower respiratory tract infection hospitalisation was 80.8% (95% CI 62.8 to 90.5) in infants under three months and 66.1% (95% CI 30.1 to 83.8) in infants under six months [s1].

Against the more severe outcomes it was higher still. In infants under six months, effectiveness was 87.2% (95% CI 52.6 to 97.0) against paediatric intensive care unit admission and 88.6% (95% CI 62.3 to 97.1) against extended hospital stays [s1].

At population level, the vaccine reduced RSV-associated hospitalisations in infants under six months by 33.6% (95% CI 29.5 to 37.2) in 2024 compared with expected cases based on previous years [s1]. The number needed to immunise to prevent one RSV-related hospitalisation was 83.9 (95% CI 65.9 to 185.4) [s1].

The declining effectiveness from under-three-months to under-six-months is the expected shape for passive antibody: transferred maternal antibody wanes, so protection is strongest earliest.

How it compares with the licensing trial

MATISSE, the phase 3 trial that established the vaccine, was conducted in 18 countries and randomised pregnant women at 24 to 36 weeks' gestation 1:1 to a single 120 μg intramuscular dose of the bivalent RSVpreF vaccine or placebo [s2]. At the prespecified interim analysis, 3,682 maternal participants received vaccine and 3,676 placebo, with 3,570 and 3,558 infants evaluated respectively [s2].

Medically attended severe RSV-associated lower respiratory tract illness occurred within 90 days of birth in six infants in the vaccine group and 33 in the placebo group — vaccine efficacy 81.8% (99.5% CI 40.6 to 96.3) [s2]. Within 180 days it was 19 versus 62 cases, efficacy 69.4% (97.58% CI 44.3 to 84.1) [s2]. Against medically attended RSV-associated lower respiratory tract illness of any severity within 90 days, efficacy was 57.1% (99.5% CI 14.7 to 79.8), which did not meet the trial's statistical success criterion [s2]. No safety signals were detected in mothers or in infants and toddlers followed to 24 months [s2].

The Buenos Aires numbers land close to the trial's severe-disease figures, in a different population, against a hospitalisation endpoint, under programme rather than trial conditions. That correspondence is the study's main contribution: it is one of the first real-world confirmations from a middle-income country that has actually deployed the vaccine at scale.

What to hold lightly

This is a retrospective observational study, not a randomised one. Test-negative designs control for healthcare-seeking behaviour reasonably well but cannot rule out residual confounding — mothers who accept vaccination in pregnancy differ from those who do not in ways that are hard to measure fully.

The effectiveness estimate rests on 323 infants, which is why the confidence intervals are wide: the under-six-month estimate spans 30.1% to 83.8% [s1]. The number needed to immunise carries a similarly wide interval, 65.9 to 185.4 [s1]. And the impact analysis compares 2024 against expected counts derived from earlier years, which assumes RSV seasons are otherwise comparable — an assumption that post-pandemic RSV epidemiology has repeatedly strained.

The study was funded by the Gates Foundation and the Thrasher Research Fund [s1]. The authors frame their findings as evidence for policymakers and health authorities [s1], and that is the right register: for countries in the region weighing introduction, this is the first sizeable programme data from a neighbour.

Sources

• [s1] Impact and effectiveness of RSV maternal immunization on infant hospitalizations in Buenos Aires: a hospital-based, multicentre, retrospective surveillance cohort study, The Lancet Regional Health – Americas, 19 November 2025. https://doi.org/10.1016/j.lana.2025.101296
• [s2] Bivalent Prefusion F Vaccine in Pregnancy to Prevent RSV Illness in Infants, The New England Journal of Medicine, 5 April 2023. https://doi.org/10.1056/NEJMoa2216480