To generate critical evidence to support vaccine policy and implementation, Pfizer will collaborate with Kaiser Permanente Northern California (KPNC) to study the vaccine effectiveness (VE) of ABRYSVO vaccination during pregnancy in a real-world population. The overall research question of this study is: what is the effectiveness and impact of ABRYSVO vaccination during pregnancy against medically-attended (MA) respiratory syncytial virus (RSV)-associated and all-cause infant outcomes in a large, diverse, real-world population? This study will use a retrospective cohort design and will be conducted within an integrated delivery health care organization using electronic medical record (EMR) data collected during routine standard of care clinical encounters. Study outcomes 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 to ≤180 days of age, with later assessments from birth to ≤360 days of age and to ≤720 days of age as the infants reach these age thresholds and their data become available. There are two categories of outcomes of interest in this study: RSV-specific infant outcomes and non-specific all-cause infant outcomes, assessed within several follow-up windows (birth to ≤180 days of age, birth to ≤360 days of age, and/or birth to ≤720 days of age, depending on the outcome).

Identification of RSV-specific outcomes will be based on the first positive laboratory-confirmed PCR test from a respiratory specimen during a healthcare encounter in a KPNC healthcare setting, occurring during the relevant follow-up window. RSV-positive test results will be combined with International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM) diagnostic codes to define the RSV-specific outcomes. RSV-specific outcomes will include: PCR-confirmed RSV, PCR-confirmed RSV hospitalization, PCR-confirmed RSV LRTD, and PCR-confirmed RSV LRTD hospitalization.

Non-specific all-cause infant outcomes will include: all-cause LRTD, all-cause LRTD hospitalization, acute otitis media, and first antibiotic prescription (for any diagnosis). Identification of these outcomes will be based on ICD-10-CM diagnostic codes documented in infant EMRs during a healthcare encounter in a KPNC healthcare setting, occurring during the relevant follow-up window.

Inclusion Criteria

Pregnant Individuals - Inclusion Criteria:

Participants must meet all of the following inclusion criteria to be eligible for inclusion in the study:

1. Gestational age at delivery ≥32 0/7 weeks of gestation (earliest gestational age eligible for ABRYSVO vaccination) during the study period
2. Maternal age >/= 18 to 50 years at pregnancy onset
3. At least 1 prenatal visit between pregnancy onset and 20 weeks of gestation
4. Continuous KPNC enrollment for a minimum of 30 days prior to pregnancy onset through to the date of delivery, with no more than a 2-month administrative gap in coverage

Infants - Inclusion Criteria:

Participants must meet all of the following inclusion criteria to be eligible for inclusion in the study:

1. Born to a pregnant individual who has met all inclusion and exclusion criteria outlined above
2. Live birth
3. Enrolled as member in KPNC by 4 months of age (membership indicated in any of months 2-4 of life)

Exclusion Criteria

Pregnant Individuals - Exclusion Criteria:

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

1. Received any licensed or investigational RSV vaccine other than ABRYSVO at any time during pregnancy
2. Received ABRYSVO at ≥37 weeks' gestation
3. Received ABRYSVO at <32 0/7 weeks' gestation

Infants - Exclusion Criteria:

There are no exclusion criteria for infants. Those who receive any licensed or investigational RSV preventative product (e.g., monoclonal antibodies) at/after birth or any licensed or investigational RSV therapeutic (e.g., antiviral medications) will not be excluded, but their follow-up time will be censored on the date of receipt of the RSV preventative or therapeutic product.

The study population will comprise eligible maternal-infant pairs over a 2-year study period, identified from EMR records in the existing databases, which accrue in real-time as pregnancies/births occur. All pregnancies that reach 32 0/7 weeks' gestation during the 2-year study period, from September 22, 2023 (start of ABRYSVO vaccination season 1) to January 31, 2025 (estimated end of ABRYSVO vaccination season 2) will be eligible for inclusion, along with all live born infants from the eligible pregnancies. The KPNC pregnancy database will be used to identify the maternal-infant study population. This database links records of newborn infants with their mother, enabling integration of maternal data from across pregnancy and delivery encounters, along with infant birth information and longitudinal follow-up for infants who remain enrolled with KPNC. Infant healthcare encounters after the birth and throughout the follow-up period will be identified in the EMR.

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.