The purpose of the study is to evaluate the reactogenicity, safety, and immunogenicity of an investigational respiratory syncytial virus (RSV) vaccine, mRNA-1345, in pregnant women, and safety and immunogenicity in infants born to vaccinated mothers.

Key Inclusion Criteria:

Maternal Participants

• Are adults ≥18 years to <40 years of age inclusive, at the time of signing the informed consent.
• Will be 28 to 36 weeks pregnant at the time of vaccination (confirmed by an obstetric ultrasound report).
• Intend to deliver at a maternity unit where study procedures can be performed.
• Are willing and able to attend all study visits, to undergo all study procedures/or have their infant undergo all study procedures, and to comply with study requirements, including a means of communication (for example, phone, text message or email) with study site staff.
• Have engaged with local antenatal care and will continue to do so through the remainder of their pregnancy.
• Have had an antenatal obstetric ultrasound at ≥18 weeks of pregnancy or be willing to have an antenatal obstetric ultrasound at Screening if not already conducted.

Specific inclusion criteria for Japanese pregnant women:

• Japanese participants are defined as individuals born in Japan, with both parents and 4 grandparents who were born in Japan.

Infant Participants

• Have consent from infant participant's parent(s)/legally authorized representative (LAR) if required by local regulations.

Key Exclusion Criteria:

Maternal Participants

• Acutely ill or febrile (temperature ≥38.0 degrees Celsius [℃]/100.4 degrees Fahrenheit [°F]) within 72 hours prior to or at the Screening Visit or Day 1.
• Reported history of anaphylaxis or severe hypersensitivity reaction after receipt of any mRNA vaccine or therapeutic or any components of an mRNA-1345 vaccine.
• Any medical, psychiatric, or occupational condition, including reported history of drug or alcohol abuse, that, in the opinion of the Investigator, might pose additional risk due to participation in the study or could interfere with the interpretation of study results.
• Have conditions in the current pregnancy or in previous pregnancies that may affect participant safety, assessment of safety endpoints, assessment of immune response, or adherence to study procedures.
• Received or plans to receive any mRNA vaccine or COVID-19 vaccine within 4 weeks before or after Day 1 and/or any other nonstudy vaccine within 2 weeks before or after Day 1.
• Previous vaccination with any licensed or investigational RSV vaccine or planned receipt during study participation.
• Intend for their infant to receive RSV monoclonal antibodies after delivery.

Infant Participants

• Is a child who has been placed under the control or protection of an agency, organization, institution, or entity by the courts, the government, or a government body acting in accordance with powers conferred on them by laws and regulation (for example, foster care). This does not include a child who is adopted or has an appointed LAR.

Other inclusion and exclusion criteria may apply.

Observer-blind study