This study will use a retrospective cohort design and will be conducted within routinely collected national healthcare and statutory demographic datasets held by PHS and National Records of Scotland (NRS). As such, there will be no active enrollment of study participants, no direct contact with study participants, no collection of any primary data outside of the standard of care (SOC), and no requirement for informed consent.

This study design was chosen due to several advantages, over other possible designs, including the ability to evaluate incidence of study outcomes in exposed and unexposed infants, ability to follow infants longitudinally to evaluate study outcomes through 12 months of age, and ability to evaluate all-cause outcomes.

Study endpoints, including RSV-associated LRTD hospitalization and RSV-associated hospitalization, among infants born to ABRYSVO-vaccinated mothers (exposed group) will be compared with those among infants born to ABRYSVO-unvaccinated mothers (comparison group) initially from birth through 6 months of age, with later analysis from birth through 12 months as the infants reach this age threshold and their data become available.

Infants must meet all inclusion criteria to be eligible for inclusion in the study:

1. Live birth in Scotland from 1st September 2024 to 28th February 2026
2. Gestational age at birth ≥28(0/7) weeks of gestation (earliest gestational age eligible for ABRYSVO vaccination)

Infants meeting any of the following criteria will not be included in the study:

1. Born to a mother who received any licensed or investigational RSV vaccine other than ABRYSVO at any time during pregnancy
2. Meets UK national guidelines for monoclonal antibody receipt

This will be a whole population birth cohort study of all live-born infants born in Scotland during the study period. In Scotland, all infants are assigned a unique identifier, the Community Health Index (CHI) at birth. CHI is a common identifier across all National Health Service (NHS) healthcare encounters and allows linkage of all healthcare data to statutory datasets such as death records. The patient population will include all live-born infants born in Scotland over an 18-month accrual period, from 1st September 2024 - 28th February 2026 (or end of RSV season) and their mothers (approximately 69,000 mother-infant pairs). Selection criteria for the population were based on the time period of the RSV season in Scotland and the earliest gestational age eligible for ABRYSVO vaccination, per the MHRA. All infants will be followed for 12 months after birth; thus, the last outcome assessment will be approximately 28th February 2027.

Pfizer will provide access to individual de-identified participant data and related study documents (e.g. protocol, Statistical Analysis Plan (SAP), Clinical Study Report (CSR)) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions. Further details on Pfizer's data sharing criteria and process for requesting access can be found at: https://www.pfizer.com/science/clinical\_trials/trial\_data\_and\_results/data\_requests.