Maternal Immunization With RSVpreF Vaccine: Effectiveness in Preventing Respiratory Syncytial Virus-associated Hospitalizations in Infants Under 6 Months in Argentina: Multicenter Case-control Study.

Background: Respiratory syncytial virus (RSV) is a leading cause of morbidity and mortality in infants, particularly in those under 6 months. Starting March 2024, Argentina implemented the bivalent RSVpreF vaccine for pregnant women between 32.0 and 36.6 weeks of gestation during the RSV season. This report study aimed to assess the effectiveness of maternal immunization in preventing RSV-associated hospitalizations in infants under 6 months born to vaccinated and not vaccinated women, including their clinical and epidemiological characteristics.

Methods: A multicenter, prospective, observational, nested case-control study using a test-negative design was conducted. Data were collected through active epidemiological surveillance of respiratory infections in children under 18 years of age hospitalized at 4 pediatric referral hospitals in Argentina.

Results: Between January and October 2024, 1340 children were hospitalized for respiratory infections, including 187 infants born after March 15: 91 RSV-positive cases and 96 RSV-negative controls. RSV cases peaked in epidemiological week 26 (June). Compared to controls, RSV cases were younger (P < 0.001) and had lower frequencies of comorbidities (P < 0.001), prematurity (P = 0.016) and chronic respiratory diseases (P < 0.001). RSV maternal immunization was less frequent among RSV cases than controls (17.6% vs. 44.8%; P < 0.001). Vaccinated RSV cases had shorter duration of oxygen therapy (4 vs. 7 days; P < 0.001) and hospital stay (5 vs. 8 days; P < 0.001). The crude effectiveness of RSV immunization in infants under 6 months was 68.2% (95% confidence interval: 33.1%-84.9%) and 78.7% (95% confidence interval: 51.4%-90.7%) after adjustment for age under 3 months, prematurity and chronic respiratory disease.

Conclusions: In its first season, Argentina's maternal RSVpreF immunization demonstrated substantial effectiveness in reducing RSV-associated hospitalizations in infants and decreasing RSV disease severity.