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 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 …