This is a Phase 2/3, randomized, double-blind study to evaluate the safety and immunogenicity of different booster dose levels of the monovalent severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) recombinant (r) spike (S) protein nanoparticle (SARS-CoV-2 rS) vaccines with Matrix-M™ adjuvant (NVX-CoV2373 [prototype Wuhan vaccine with Matrix-M adjuvant] or NVX-CoV2601 [Omicron XBB.1.5 subvariant vaccine with Matrix-M adjuvant]).

The ongoing COVID-19 pandemic has reached a stage where it is necessary to stablish the framework for periodic national vaccination campaigns.The present study aims to investigate the safety and immunogenicity of different booster dose levels of monovalent and bivalent vaccines in adults ≥ 50 years of age who have already been immunized with ≥ 3 doses of a COVID-19 prototype or bivalent licensed mRNA vaccine. The Boosters of investigational products will be administered ≥ 90 days after the participants received their third dose of a COVID-19 prototype or bivalent licensed mRNA vaccine.

Approximately 1,980 participants ≥ 50 years of age who have received a regimen of ≥ 3 doses of a coronavirus disease 2019(COVID-19) vaccine (the last vaccine could have been a bivalent licensed mRNA vaccine) will be included in this study. The last COVID-19 vaccine dose should have been administered ≥ 90 days prior to Day 0.

Approximately 1,800 participants will be randomly assigned in a 1:2:2:2:2:1 ratio to receive NVX-CoV2373 or NVC-CoV2601 in a double-blinded fashion into 1 of 6 monovalent vaccine groups (vaccine groups A to G). Following completion of enrollment into the 6 monovalent vaccine groups, 180 participants will be enrolled in vaccine group G to receive a bivalent licensed mRNA vaccine in an open-label fashion.

Inclusion Criteria:

1. Adults ≥ 50 years of age at screening.
2. Willing and able to give informed consent prior to study enrollment and to comply with study procedures.
3. Female participants of childbearing potential (defined as any participant who has experienced menarche and who is NOT surgically sterile [ie, hysterectomy, bilateral tubal ligation, or bilateral oophorectomy] or postmenopausal [defined as amenorrhea ≥ 12 consecutive months]) must agree to be heterosexually inactive from at least 28 days prior to enrollment and through the end of the study OR agree to consistently use a medically acceptable method of contraception listed below from ≥ 28 days prior to enrollment and through the end of the study.
4. Is medically stable, as determined by the investigator (based on review of health status, vital signs [to include body temperature], medical history, and targeted physical examination [to include body weight]). Vital signs must be within medically acceptable ranges prior to the initial study vaccination.
5. Agrees to not participate in any other SARS-CoV-2 prevention or treatment trials for the duration of the study.
6. Have previously received ≥ 3 doses of a COVID-19 prototype or bivalent licensed mRNAvaccine with the last dose having been given ≥ 90 days previously prior to first study booster.

Exclusion Criteria:

1. Received COVID-19 vaccines other than a COVID-19 prototype or bivalent licensed mRNA vaccine 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, or any other vaccine within 30 days prior to first study vaccination.
4. Any known allergies to products contained in the investigational product. 5. Any history of anaphylaxis to any prior vaccine.

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

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

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

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

10. Participants who are breastfeeding, pregnant, or who plan to become pregnant prior to the end of study.

11. Suspected or known history of alcohol abuse or drug addiction within 2 years prior to the study vaccine dose that, in the opinion of the investigator, might interfere with protocol compliance.

12. Any other condition that, in the opinion of the investigator, would pose a health risk to the participant if enrolled or could interfere with evaluation of the study vaccine or interpretation of study results (including neurologic or psychiatric conditions likely to impair the quality of safety reporting).

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

14. Participants with a history of myocarditis or pericarditis.