Phase 2/3 Heterologous Boosting Study With Different Dose Levels of Monovalent SARS-CoV-2 rS Vaccines

Inclusion Criteria:

1. Adults ≥ 50 years of age at screening.

2. Willing and able to give informed consent prior to study enrollment and to complywith study procedures.

3. Female participants of childbearing potential (defined as any participant who hasexperienced menarche and who is NOT surgically sterile [ie, hysterectomy, bilateraltubal ligation, or bilateral oophorectomy] or postmenopausal [defined as amenorrhea ≥ 12 consecutive months]) must agree to be heterosexually inactive from at least 28days prior to enrollment and through the end of the study OR agree to consistentlyuse a medically acceptable method of contraception listed below from ≥ 28 days priorto enrollment and through the end of the study.

4. Is medically stable, as determined by the investigator (based on review of healthstatus, vital signs [to include body temperature], medical history, and targetedphysical examination [to include body weight]). Vital signs must be within medicallyacceptable ranges prior to the initial study vaccination.

5. Agrees to not participate in any other SARS-CoV-2 prevention or treatment trials forthe duration of the study.

6. Have previously received ≥ 3 doses of a COVID-19 prototype or bivalent licensedmRNAvaccine with the last dose having been given ≥ 90 days previously prior to firststudy booster.

Exclusion Criteria:

1. Received COVID-19 vaccines other than a COVID-19 prototype or bivalent licensed mRNAvaccine in the past, inclusive of clinical trial COVID-19 vaccines.

2. Participation in research involving receipt of investigational products (drug/biologic/device) within 90 days prior to study vaccination.

3. Received influenza vaccination within 14 days prior to first study vaccination, orany other vaccine within 30 days prior to first study vaccination.

4. Any known allergies to products contained in the investigational product. 5. Anyhistory of anaphylaxis to any prior vaccine.

5. Autoimmune or immunodeficiency disease/condition (iatrogenic or congenital)requiring ongoing immunomodulatory therapy.

6. Chronic administration (defined as > 14 continuous days) of immunosuppressant,systemic glucocorticoids, or other immune-modifying drugs within 90 days prior tostudy vaccination. NOTE: An immunosuppressant dose of glucocorticoid is defined as asystemic dose ≥ 10 mg of prednisone per day or equivalent. The use of topical orintranasal glucocorticoids is permitted. Topical tacrolimus and ocular cyclosporinare permitted. Use of inhaled glucocorticoids is prohibited.

7. Received immunoglobulin, blood-derived products, or immunosuppressant drugs within 90 days prior to study vaccination, except for rabies immunoglobulin which may begiven if medically indicated.

8. Active cancer (malignancy) on therapy within 3 years prior to study vaccination (with the exception of adequately treated non-melanomatous skin carcinoma or lentigomaligna and uterine cervical carcinoma in situ without evidence of disease, at thediscretion of the investigator).

9. Participants who are breastfeeding, pregnant, or who plan to become pregnant priorto the end of study.

10. Suspected or known history of alcohol abuse or drug addiction within 2 years priorto the study vaccine dose that, in the opinion of the investigator, might interferewith protocol compliance.

11. Any other condition that, in the opinion of the investigator, would pose a healthrisk to the participant if enrolled or could interfere with evaluation of the studyvaccine or interpretation of study results (including neurologic or psychiatricconditions likely to impair the quality of safety reporting).

12. Study team member or immediate family member of any study team member (inclusive ofSponsor, contract research organization (CRO), and study site personnel involved inthe conduct or planning of the study).

13. Participants with a history of myocarditis or pericarditis.