This clinical study will assess the safety, reactogenicity and immunogenicity of mRNA-1653, a combined human metapneumovirus and human parainfluenza virus type 3 vaccine in healthy adults.

Inclusion Criteria:

• Agrees to comply with the study procedures and provides written informed consent
• 18 to 49 years of age
• Body mass index between 18 and 35 kg/m2
• In good health based on medical history, physical examination, vital sign measurements and laboratory safety tests performed prior to initial study vaccination
• Negative urine pregnancy test at the screening visit and the day of each vaccination for females of childbearing potential
• Female subjects must either be of non-childbearing potential or use acceptable methods of contraception from at least 30 days prior to enrollment and through 3 months following last vaccination
• Willing to comply with the requirements of the protocol (eg, complete Diary Cards, return for follow-up visits, be available for safety phone calls)

Exclusion Criteria:

• Any ongoing, symptomatic acute or chronic illness requiring medical or surgical care
• A history of malignancy in the last 10 years
• If female and of childbearing potential, is pregnant or lactating, has not adhered to an adequate contraception method from at least 30 days before study entry, or does not plan to do so for at least 3 months after the last vaccination
• Abnormal screening safety laboratory test results including liver enzyme tests
• Administration of any investigational or non-registered product (drug or vaccine) other than the study vaccine within 30 days preceding the first dose of study vaccine or has plans for administration during the study period
• Prior administration of investigational agent using lipid nanoparticle formulations
• A positive test result for hepatitis B surface antigen, hepatitis C virus antibody, or human immunodeficiency virus types 1 or 2 antibodies
• A positive test result for drugs of abuse
• Chronic administration of potentially hepatotoxic drugs or have other medical conditions that affect the liver (eg, alcohol abuse)
• A history of idiopathic urticaria
• Plans for administration or has been administered a vaccine within the period from 30 days before through 30 days after each study vaccination, with the exception of any licensed influenza vaccine administered ≥15 days before or after any study vaccination
• Any chronic administration of an immunosuppressant or other immune modifying drug
• Prior administration of immunoglobulins and/or any blood products within the 3 months before the first study vaccine or has plans for administration during the study period
• Any known or suspected immune-mediated disease or immunosuppressive condition as determined by medical history and/or physical examination
• A history of hypersensitivity or serious reactions to previous vaccinations
• Any bleeding disorder considered a contraindication to IM injection or blood draw
• Any acute illness or fever at screening
• Any condition that, in the opinion of the investigator, would pose a health risk to the subject if enrolled or could interfere with evaluation of the study drug or interpretation of study results
• Donation of blood or blood products > 450 mL within 30 days of dosing.
• Is an immediate family member or household member of study personnel