Impact and Effectiveness of ABRYSVO® Vaccination During Pregnancy

This hospital-based retrospective study will be conducted in a research network of independent hospital sites. We will collect data from hospital medical records, supplemented with information from vaccine registries. This study will use three retrospective design approaches: a test negative design (TND) to evaluate real-world vaccine effectiveness (VE) of maternal ABRYSVO against RSV-associated LRTD hospitalization and other outcomes in infants, a descriptive cohort design to evaluate the clinical features of infants hospitalized with RSV-positive LRTD, and an ecologic before-and-after design to evaluate the impact of ABRYSVO vaccination during pregnancy on infant RSV-associated and all-cause respiratory outcomes.

The TND study will include infants through 9 months of age who were admitted to one of the participating hospital sites with symptoms of respiratory infection, born at 32 weeks of gestational age or greater, met the definition of LRTD, had a respiratory specimen collected with an RSV test result through standard of care testing, born 14 days or more after start of the first seasonal ABRYSVO vaccination campaign, and born to individuals who were expected to reach the indicated ABRYSVO vaccination window during a local ABRYSVO vaccination season. To complement the VE estimates generated in the TND study, we will use the cases from the primary objective of the TND analysis as a cohort of participants to describe the endpoints in the descriptive cohort study. The ecologic before-and-after study will include information for infants and children ≤24 months of age meeting eligibility criteria in post-ABRYSVO program implementation years and in several historical RSV seasons pre-ABRYSVO program implementation.

For the TND study, a multivariable logistic regression model, adjusted for confounding, will be used to compute an adjusted odds ratio (aOR), comparing the odds of maternal ABRYSVO vaccination during pregnancy between test-positive cases and test-negative controls. VE will be estimated as (1-aOR) x 100%. Secondary and exploratory objectives will evaluate VE estimates stratified by several characteristics. For the descriptive cohort study of RSV-positive LRTD hospitalized infants ≤9 months of age (i.e., the cases from the primary objective of the TND study), infant characteristics, timing, severity/clinical features, and use of healthcare resources during the index hospitalization will be described. For the ecologic before-and-after study, the impact of maternal ABRYSVO introduction on rates of RSV-associated and all-cause outcomes among infants ≤6 months (compared with older age groups) over multiple RSV seasons will be described overall and by epidemiological week, calendar year, calendar month, age group, and hospital site. These analyses will begin in the 2024 RSV season in Argentina and continue in future RSV seasons in Argentina and Uruguay (2025 and 2026), with comparison to several pre-ABRYSVO implementation seasons. Quasi-experimental approaches will also be used to quantitatively compare incidence of study outcomes among infants aged ≤6 months with older age groups.