A Study of mRNA-1345 Vaccine Targeting Respiratory Syncytial Virus in Pregnant Women and 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 theinformed consent.

• Will be 28 to 36 weeks pregnant at the time of vaccination (confirmed by anobstetric 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/orhave their infant undergo all study procedures, and to comply with studyrequirements, including a means of communication (for example, phone, text messageor email) with study site staff.

• Have engaged with local antenatal care and will continue to do so through theremainder of their pregnancy.

• Have had an antenatal obstetric ultrasound at ≥18 weeks of pregnancy or be willingto 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 parentsand 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 degreesFahrenheit [°F]) within 72 hours prior to or at the Screening Visit or Day 1.

• Reported history of anaphylaxis or severe hypersensitivity reaction after receipt ofany mRNA vaccine or therapeutic or any components of an mRNA-1345 vaccine.

• Any medical, psychiatric, or occupational condition, including reported history ofdrug or alcohol abuse, that, in the opinion of the Investigator, might poseadditional risk due to participation in the study or could interfere with theinterpretation of study results.

• Have conditions in the current pregnancy or in previous pregnancies that may affectparticipant 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 weeksbefore or after Day 1 and/or any other nonstudy vaccine within 2 weeks before orafter Day 1.

• Previous vaccination with any licensed or investigational RSV vaccine or plannedreceipt 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 governmentbody acting in accordance with powers conferred on them by laws and regulation (forexample, foster care). This does not include a child who is adopted or has anappointed LAR.

Other inclusion and exclusion criteria may apply.