Globally, respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract disease (LRTD) in infants and children. Pfizer has developed ABRYSVO-a bivalent RSV prefusion F protein-based vaccine (RSVpreF) composed of two prefusion F proteins to protect against both RSV-A and RSV-B. In the United States, ABRYSVO has been approved and recommended for active immunization of pregnant individuals from 320/7 to 366/7 weeks' gestational age for the prevention of LRTD and severe LRTD caused by RSV in infants from birth through 6 months of age. Post-licensure data from the United States and Argentina have also demonstrated real world effectiveness. Data from the US VISION and NVSN networks reported 79% (95% CI 55-90) and 70% (95% CI 28-88%) VE respectively against RSV-associated hospitalization among infants in the second RSV season and a study from Argentina reported 78.6% (95% CI 62.1-87.9) and 71.3% (95% CI 53.3-82.3) VE against RSV-associated LRTD leading to hospitalization among infants aged 0 to ≤3 and 0 to ≤6 months respectively.

To generate critical evidence to support vaccine policy and implementation, Pfizer will collaborate with University of Pittsburgh to study vaccine effectiveness (VE) of ABRYSVO vaccination during pregnancy against RSV-associated outcomes in infants. The study will take place in a real-world population in Western Pennsylvania over multiple seasons, beginning in the 2023-2024 season, and will use a test negative design (TND) approach.

There will be no active enrollment of study participants, no direct contact with study participants, and no collection of any primary data outside of the Standard of Care (SOC).

This study will use a TND to evaluate real-world VE of maternal ABRYSVO against RSV-associated outcomes in infants. Additionally, we will describe RSV-associated medically-attended visits for infants exposed to ABRYSVO and RSV monoclonal antibodies (e.g., Beyfortus and Enflonsia).

Population included in ABYRSVO VE analyses:

1. Infants ≤ 6 months (≤ 180 days) of age on the index date.
2. Index date within the time period for data collection (approximately 01 October to 15 April in the 2023-2024, 2024-2025, 2025-2026, and 2026-2027 RSV seasons).
3. Infants born at ≥32 weeks of gestation.
4. Infants born to individuals who were expected (based on estimated date of delivery) to reach the indicated ABRYSVO vaccination window (320/7 to 366/7 weeks' gestation) during the local vaccination season.
5. Infant's EHR links to the maternal EHR.
6. Hospitalized at a UPMC facility during the RSV season (according to local hospital epidemiology) with acute respiratory tract illness symptoms meeting the protocol-defined clinical case, and for whom RSV testing results from a specimen collected 10 days prior to hospital admission through 3 days after hospital admission are known.
7. Ability to document maternal ABRYSVO receipt or not.

Population excluded from ABYRSVO VE analyses:

1. Infants who received any licensed or investigational RSV preventive product (e.g., RSV monoclonal antibodies, RSV vaccine (including investigational products or due to medication error)) since birth.
2. Born to birth parent for whom ABRYSVO vaccination status cannot be verified in the maternal EHR or Pennsylvania Immunization Electronic Registry System (PIERS) system.
3. Infants who were born to an individual vaccinated with ABRYSVO prior to the current pregnancy or vaccinated with more than one dose of ABRYSVO or vaccinated with any other licensed or investigational RSV vaccine during pregnancy.

The study population will include infants who were hospitalized with ARI, have documented RSV test results and born to an individual eligible for ABRYSVO vaccination in pregnancy based on the timing of birth relative to the local seasonal ABRYSVO vaccination program.

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.