This study aims to check how safe and well-tolerated a second dose of RSVpreF is when given during later pregnancies, and to see how long the immunity lasts from a single dose given during a previous pregnancy by examining the blood of nonpregnant participants who had the vaccine before.

This study will include pregnant participants and their unborn babies (Cohort 1 and Cohort 2) and non-pregnant participants (Cohort 3), as described below:

• Cohort 1- Pregnant participants who had previously received RSVpreF as part of a Pfizer clinical trial and have antibody data available from that trial will get a second dose of RSVpreF.
• Cohort 2- Pregnant participants who received the RSVpreF in a previous pregnancy, either through commercial means with Abrysvo® or as part of a Pfizer clinical trial, will be randomly chosen to get either one dose of the RSVpreF or one dose of a placebo. This applies to those who do not have antibody data available from the earlier trial.

For both groups of pregnant participants, the study will look at how safe and well-tolerated the vaccine is during pregnancy, as well as how the body responds to it.

Infants will be monitored for six months after birth to check safety and antibodies level.

• Cohort 3- Nonpregnant participants who received RSVpreF during a previous pregnancy as part of an earlier clinical trial will have blood samples taken to see how well their immune response has persisted over time. Participants will not receive any vaccine.

Pregnant Participants-Cohort 1 and Cohort 2 Key Inclusion Criteria

• Women aged 18 to 49 who are pregnant, between 24 and 36 weeks along, and expecting one baby without known risks for complications can participate.
• Had the RSVpreF or Abrysvo vaccine during a previous pregnancy.
• Had an ultrasound scan at 18 weeks or later during their current pregnancy, with no major fetal problems detected.
• Based on their medical history, physical check-up, and the doctor's judgment, they are found suitable to join the study.
• Agrees to let their baby take part in the study and gives their permission.
• Able to sign a consent form, agreeing to follow the rules and conditions of the study.

Key Exclusion Criteria

• Received any approved or experimental RSV vaccine since their previous pregnancy.
• Has a pre-pregnancy body mass index (BMI) over 40 kg/m2.
• History of a severe bad reaction to a vaccine or a serious allergic reaction (like anaphylaxis) to any ingredient in the study vaccine or a similar vaccine.
• Current pregnancy problems or issues at the time of giving consent.
• Previous pregnancy issues or problems at the time of giving consent.
• Women who are breastfeeding at the time of enrollment

Infant Participants

• Proof that the parent(s) or legal guardian(s) has signed and dated a consent form.
• Parent(s) or legal guardian(s) must agree to attend scheduled visits and follow the study plan, including laboratory tests and other procedures.

Nonpregnant Participants-Cohort 3 Key Inclusion Criteria

• Have already received one dose of the RSVpreF vaccine during their previous pregnancy as part of the Pfizer clinical trial, and the results from that time can be used for this study.
• Able to sign a consent form, agreeing to follow the rules and requirements of the study.

Key Exclusion Criteria

• Received any approved or experimental RSV vaccine after participating in the Pfizer clinical trial.
• Taking part in other studies with new drugs within 28 days before giving consent or during the study period.

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.

Blinding only applies to Cohort 2 of the study.