Safety and Immunogenicity Study of a SARS-CoV-2 (COVID-19) Variant Vaccine (mRNA-1273.351) in Naïve and Previously Vaccinated Adults

Inclusion Criteria

• Provides written informed consent prior to initiation of any study procedures.
• Be able to understand and agrees to comply with planned study procedures and be available for all study visits.
• Agrees to the collection of venous blood per protocol.
• Cohort 1: previously received 2 doses of mRNA-1273 intramuscular (IM) as part of DMID 20-0003.
• Cohort 1: Male or non-pregnant female, >/= 18 years of age at time of enrollment. Cohort 2: Male or non-pregnant female, 18 through 55 years of age at time of enrollment.
• Women of childbearing potential* must agree to practice abstinence or use at least one acceptable primary form of contraception.**, *** Note: These criteria are applicable to females in a heterosexual relationship and child-bearing potential (i.e., the criteria do not apply to subjects in a same sex relationship).
• Not of childbearing potential - post-menopausal females (defined as having a history of amenorrhea for at least one year) or a documented status as being surgically sterile (hysterectomy, bilateral oophorectomy, tubal ligation/salpingectomy, or Essure(R) placement).

** Acceptable forms of primary contraception include monogamous relationship with a vasectomized partner who has been vasectomized for 180 days or more prior to the subject's first vaccination, intrauterine devices, birth control pills, and injectable/implantable/insertable hormonal birth control products.

*** Must use at least one acceptable primary form of contraception for at least 30 days prior to the first vaccination and at least one acceptable primary form of contraception for 60 days after the last vaccination.

• In good health.*
• As determined by medical history and physical examination to evaluate acute or ongoing chronic medical diagnoses/conditions that have been present for at least 90 days, which would affect the assessment of safety of subjects. Chronic medical diagnoses/conditions should be stable for the last 60 days (no hospitalizations, emergency room, or urgent care for condition or need for supplemental oxygen). This includes no change in chronic prescription medication, dose, or frequency as a result of deterioration of the chronic medical diagnosis/condition in the 60 days before enrollment. Any prescription change that is due to change of health care provider, insurance company, etc., or done for financial reasons, and in the same class of medication, will not be considered a deviation of this inclusion criterion. Any change in prescription medication due to improvement of a disease outcome or for dose optimization, as determined by the participating site PI or appropriate sub-investigator, will not be considered a deviation of this inclusion criterion. Subjects may be on chronic or as needed (prn) medications if, in the opinion of the participating site PI or appropriate sub-investigator, they pose no additional risk to subject safety or assessment of reactogenicity and immunogenicity, and do not indicate a worsening of medical diagnosis/condition. Similarly, medication changes subsequent to enrollment and study vaccination are acceptable provided the change was not precipitated by deterioration in the chronic medical condition, and there is no anticipated additional risk to the subject or interference with the evaluation of responses to study vaccination.
• Oral temperature is less than 100.0 degrees Fahrenheit (37.8 degrees Celsius).
• Must agree to have samples stored for secondary research.
• Agrees to adhere to Lifestyle Considerations throughout study duration.
• Must agree to refrain from donating blood or plasma during the study (outside of this study).

Exclusion Criteria

• Positive pregnancy test prior to each vaccine administration.
• BMI > 40.0 kg / m^2.
• Female subject who is breastfeeding.
• Has any medical disease or condition that, in the opinion of the participating site PI or appropriate sub-investigator, precludes study participation.*
• Including ac