This is a Phase 2/3 open-label study to evaluate the safety and immunogenicity of a booster dose of the XBB.1.5 severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) recombinant (r) spike (S) protein nanoparticle vaccine (SARS-CoV-2 rS) adjuvanted with Matrix-M™ in previously mRNA COVID-19 vaccinated adult participants ≥18 years of age and baseline SARS CoV-2 seropositive COVID-19 vaccine naïve participants ≥18 years of age.

Novavax, Inc. developed a recombinant prototype COVID-19 vaccine constructed from the full-length ancestral (Wuhan) SARS CoV-2 S glycoprotein (GP) adjuvanted with the saponin-based Matrix-M adjuvant (NVX-CoV2373). Subsequently, the SARS CoV 2 Omicron variant and subvariants emerged with enhanced transmissibility and the most significant number of mutations in any strain to date. Current evidence demonstrates that variant strain mutations such as those in the Omicron XBB.1.5 sublineage confer the ability to evade both natural and vaccine-induced neutralizing antibodies.

Part 1 of the study aims to investigate the safety and immunogenicity of the Novavax XBB.1.5 SARS-CoV-2 rS vaccine (NVX-CoV2601) adjuvanted with Matrix-M in previously COVID-19 mRNA vaccinated participants to determine if it induces superior antibody responses compared to a historical control of the prototype vaccine (original Wuhan strain), NVX-CoV2373.

Part 2 of the study aims to investigate the safety and immunogenicity of 1 dose of NVX CoV2601 in baseline SARS-CoV-2 seropositive COVID-19 vaccine naïve participants to determine if it induces non-inferior antibody responses compared to 1 booster dose of NVX-CoV2601 in previously COVID-19 mRNA vaccinated individuals participating in Part 1.

Part 1:

Approximately 330 previously mRNA COVID-19 vaccinated participants will receive a booster dose of XBB.1.5 Omicron subvariant vaccine (NVX-CoV2601) on Day 0. Immunogenicity and 28-day safety data will be used for an interim analysis, while participants remain on the study for immunogenicity and safety data collection up to Day 180 post-vaccination.

Part 2:

After completion of Part 1, approximately 330 unvaccinated participants with a clinical history of COVID-19-like disease during the previous year will receive a booster dose of NVX-CoV2601 on Day 0. Immunogenicity and 28-day safety data will be used for an interim analysis, while participants remain on the study for immunogenicity and safety data collection up to Day 180 post vaccination.

Inclusion Criteria:

1. Adults ≥ 18 years of age at time of study vaccination.

2. Part 1: Previously vaccinated with ≥ 3 doses of the Moderna and/or Pfizer /BioNTech prototype monovalent and/or BA.4/5 containing bivalent COVID-19 vaccines with the last dose administered ≥ 90 days prior to study vaccination.

   Part 2: Clinical history of COVID-19-like disease during the previous year.

3. Willing and able to give informed consent prior to study enrollment and to comply with study procedures.

4. 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 in-active 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.

   1. Condoms (male or female) with spermicide (if acceptable in country)
   2. Diaphragm with spermicide
   3. Cervical cap with spermicide
   4. Intrauterine device
   5. Oral or patch contraceptives
   6. Norplant®, Depo-Provera®, or other in country regulatory approved contraceptive method that is designed to protect against pregnancy.
   7. Abstinence, as a form of contraception, is acceptable if in line with the participant's lifestyle NOTE: Periodic abstinence (eg, calendar, ovulation, sympto-thermal, post-ovulation methods) and withdrawal are not acceptable methods of contraception.

5. Is medically stable, as determined by the investigator (based on review of health status, vital signs [to include body temperature], medical history, and physical examination [to include body weight]). Vital signs must be within medically acceptable ranges prior to study vaccination.

6. Agrees to not participate in any research involving receipt of investigational products (drug/biologic/device) including other SARS-CoV-2 prevention or treatment trials for the duration of the study.

NOTE: For participants who become hospitalized with COVID-19, participation in investigational treatment studies is permitted.

Exclusion Criteria:

1. Received COVID-19 vaccines other than Moderna and/or Pfizer-BioNTech 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 (Day 0).

3. Received influenza vaccination within 14 days prior to study vaccination, or any other vaccine within 30 days prior to 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.

   NOTE: Stable endocrine disorders (eg, thyroiditis, pancreatitis), including stable diabetes mellitus with no history of diabetic ketoacidosis are NOT excluded.

7. Chronic administration (defined as > 14 continuous days) of immunosuppressant, systemic glucocorti-coids, or other immune-modifying drugs within 90 days prior to study vaccination (Day 0).

   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 any prohibited medication (see Section 7.4.1), immunoglobulin, blood-derived products, or immunosuppressant drugs within 90 days prior to study vaccination (Day 0).

9. Active cancer (malignancy) on chemotherapy within 3 years prior to first study vaccination (with the exception of adequately treated non-melanomatous skin carcinoma or lentigo malign 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 study vaccination 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 (includ-ing 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, clinical research organization [CRO], and study site personnel involved in the conduct or planning of the study).

14. Known history of myocarditis or pericarditis.

15. Respiratory symptoms in the past 3 days (ie, cough, sore throat, difficulty breathing).

16. Temperature of > 38°C within 24 hours of planned study vaccination (site measured or participant meas-ured).

17. Blood pressure of ≥ 160/100 mmHg.