BEATRIX (group B strEptococcus mATeRnal and Infant VaX study) The purpose of this study is to learn about the safety and how the group B streptococcus (GBS) vaccine works in pregnant women and their babies.

This study is seeking healthy pregnant participants:

• aged 49 or younger who can join.
• between 24 and 36 weeks of gestation ("Gestational age" is a medical term used to describe how far along your pregnancy is)
• had a fetal ultrasound examination performed with no major fetal abnormalities observed
• documented negative for HIV, syphilis and Hepatitis B All participants in this study will receive only 1 shot in an arm. This could either be a group B streptococcus 6-valent polysaccharide conjugate vaccine (GBS6) or placebo. Placebo is an inactive substance used in the study for comparison purposes; in this study, the placebo injection will be saline (saltwater). The pregnant participants may take part in this study for a maximum of 14 months (6 months after delivery) , and their babies for about 12 months after they are born. The pregnant participants will need to visit the research site at least 3 to 4 times with some visits permitted to occur over the telephone.

A subset of infants will be asked to take part in the study for up to 19 months. The subset will receive diphtheria toxoid-containing vaccine and/or pneumococcal vaccine following each country's standard immunization plan and have blood drawn 1 month after completion of the primary and/or toddler (booster) doses.

Key Inclusion criteria- Maternal:

• Healthy pregnant women ≤49 years of age who are between 24 0/7 and 36 0/7 weeks of gestation on the day of planned vaccination, with an uncomplicated, singleton pregnancy, and who have no known increased risk of complications.
• Had a fetal anomaly ultrasound examination with no significant fetal abnormalities observed.
• Documented negative human immunodeficiency virus (HIV) antibody test, syphilis test, and hepatitis B virus (HBV) surface antigen test during this pregnancy and prior to randomization.
• Capable of giving personal signed informed consent.
• Willing to give informed consent for her infant to participate in the study.

Key Exclusion criteria- Maternal:

• Prepregnancy body mass index (BMI) of >40 kg/m2.
• Current pregnancy complications or abnormalities that may increase the risk associated with the participation in and completion of the study.
• Prior pregnancy complications or abnormalities that, based on the investigator's judgment, may increase the risk associated with the participation in and completion of the study.
• History of microbiologically proven invasive disease caused by GBS in the current pregnancy.
• A known or suspected infection during the current pregnancy that may increase the risk of complications in pregnancy (eg, active tuberculosis, syphilis, primary genital herpes simplex, malaria).

Key Inclusion criteria- Infant Participants

- Evidence of a signed and dated ICD signed by the parent(s)/legally authorized representative or legal guardian

Key Exclusion Criteria - Infant Participants:

- Children or grandchildren who are direct descendants of investigator site staff or sponsor and sponsor delegate employees directly involved in the conduct of the study.

Key Exclusion Criteria - Infant immunogenicity subset Participants:

- Children with a known or suspected contraindication to any vaccine administered in the infant vaccine immunogenicity subset.

Refer to the study contact for further eligibility details

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.