A Dose Escalation Study to Evaluate Safety, Reactogenicity, and Immunogenicity of mRNA-1345 in Healthy Adults and in Children Who Are Respiratory Syncytial Virus (RSV)-Seropositive

Exclusion Criteria

• •Has Screening laboratory values Grade ≥1 (younger adult, women of child-bearing potential, and pediatric participants) or \>Grade 1 (older adult participants, including Japanese older adult participants).
• •Is acutely ill or febrile on the day of the first injection.
• •Has a significant medical history, including but not limited to:
• •* Congenital or acquired immunodeficiency, including human immunodeficiency virus (HIV) infection.
• •* Chronic hepatitis or suspected active hepatitis.
• •* Bleeding disorder that is considered a contraindication to intramuscular (IM) injection or phlebotomy.
• •* Dermatologic conditions that could affect local solicited AR assessments.
• •* Any history of allergic or anaphylactic reactions following a vaccination that required medical intervention.
• •* Autoimmune disease except for Hashimoto's disease.
• •* Systemic immunosuppressants or immune-modifying drugs for \>14 days in total within 6 months prior to the day of enrollment. Topical tacrolimus is allowed if not used within 14 days prior to the day of enrollment. Inhaled, nasal, and topical steroids are allowed.
• •* Intravenous blood products (red cells, platelets, and immunoglobulins \[Ig\]) within 3 months prior to enrollment.
• •Has received or plans to receive any licensed or authorized vaccine, to include COVID-19 vaccines, ≤ 28 days prior to the first injection (Day 1) or plans to receive a licensed vaccine within 28 days before or after any study vaccine injection, with the exception of licensed influenza vaccines, which may be received more than 14 days before or after any study vaccine injection. Nonstudy vaccinations should not be delayed.
• •Has a history of myocarditis, pericarditis, or myopericarditis.
• •Known history of poorly controlled hypertension (per determination of the Investigator) or systolic blood pressure \>160 millimeters of mercury (mmHg) at the Screening visit.
• •Known history of hypotension or systolic blood pressure \<85 mmHg at the Screening visit.
• •Poorly controlled diabetes mellitus (per determination of the Investigator).
• •Diagnosis of significant chronic pulmonary disease (per determination of the Investigator) (such as, chronic obstructive pulmonary disease, asthma).
• •Significant chronic cardiovascular disease (per determination of the Investigator).
• •Resides in a nursing home.
• •Anticipates the need for immunosuppressive treatment at any time during participation in the study.
• •Diagnosis of malignancy within previous 10 years (excluding non-melanoma skin cancer and cervical carcinoma in-situ).
• •Has received any monoclonal antibody at any time prior to Screening.
• •Prior hospitalization for RSV disease in the last 2 years.
• •Receipt of any prior systemic immunosuppressants or immune-modifying drugs.
• •Any history of febrile seizures (inclusive of single simple febrile seizure).
• •History of epilepsy.
• •History of meningitis.