∙ Participants are eligible to be included in the study only if all of the following criteria apply:

• Male and non-pregnant female participant must be ≥50 years of age inclusive, at the time of signing the informed consent.

‣ Arms A, B, C, J, K, L, and M will enroll participants ≥50 years of age

⁃ Arms D, E, F, G, H, and I will enroll participants ≥50 to \<70 years of age

• Participants who are healthy as determined by medical evaluation including comprehensive medical history, comprehensive physical examination, vital signs\*, and screening laboratory tests conducted no more than 30 days prior to first study injection administration (Day 0).

• • Vital signs within grade 1 on the severity grading scale, excluding temperature, are allowed. If vital sign parameter meets grade 2 criteria, then subject must be excluded. Vital signs may be repeated 3 times after a period of rest as needed if transient abnormal fluctuation is suspected.

• (Applicable only for arms A through I) Completed an Emergency Use Authorization (EUA) or Conditional Marketing Authorization or licensed initial COVID-19 vaccine series (as applicable) ≥30 days prior to enrollment (i.e., at least 30 days prior to the Day 0 visit).

• Screening laboratory values \[sodium, potassium, blood urea nitrogen (BUN), Alanine Aminotransferase (ALT), Aspartate Aminotransferase (AST), total bilirubin, alkaline phosphatase, creatinine, random glucose, white blood cell count with differential, hemoglobin, and platelet count\] must be within normal ranges or considered not clinically significant by the PI.\*\*

• Negative HIV 1/2 antibody, hepatitis B surface antigen (HBsAg), and hepatitis C virus (HCV) antibody at screening. If HIV 1/2 antibody is positive, and confirmation testing is negative the participant may be enrolled.

• Normal urinalysis or, if abnormal, urinalysis determined to be not clinically significant by the PI at screening.\*\*

‣ Screening laboratory values that are abnormal but are considered abnormal due to an acute illness or process may be repeated once. Careful consideration regarding enrolling subjects with screening lab values meeting grade 2 severity that are considered not clinically significant must be made as there is less room for fluctuations to increase to gr ade 3 (severe) lab events.

• A female participant is eligible to participate if she is not pregnant or breastfeeding, and one of the following conditions applies:

‣ Is a woman of nonchildbearing potential (WONCBP). OR

⁃ Is a woman of childbearing potential (WOCBP) and using an acceptable contraceptive method as defined below during the 30 days prior to Day 0 (i.e., the first study vaccine) and is willing to continue to do so during the study until at least 90 days after the last dose of study vaccine, and agrees not to donate eggs (ova, oocytes) for the purpose of reproduction during this period. The investigator should evaluate the potential for contraceptive method failure (e.g., noncompliance, recently initiated) in relation to the timing of the first dose of study vaccine.

⁃ A WOCBP must have a negative serum pregnancy test at screening and a negative urine pregnancy test on the day of each study vaccine (prior to vaccination). If a urine test cannot be confirmed as negative (e.g., an ambiguous result), a serum pregnancy test is required. In such cases, the participant must be excluded from participation if the serum pregnancy result is positive.

• WONCBP is as defined as

⁃ Postmenopausal: A postmenopausal state is defined as least 12 months of spontaneous amenorrhea.

⁃ Permanently sterile: Permanent sterilization methods include hysterectomy, bilateral salpingectomy, bilateral tubal occlusion/ligation procedures, bilateral oophorectomy, and successful Essure® placement (permanent, non-surgical, non-hormonal sterilization) with documented radiological confirmation test at least 90 days after the procedure, and still menstruating or \<1 year since of the last menses if menopausal.

• WOCBP is defined as

• • Any woman or adolescent who has begun menstruation. Acceptable contraception methods include, but are not limited to, sexual abstinence, monogamous relationship with vasectomized partner who has been vasectomized for 6 months or more prior to the participant receiving study product, barrier methods such as condoms or diaphragms with spermicide or foam, effective intrauterine devices, NuvaRing®, and licensed hormonal methods such as implants, injectables, or oral contraceptives (the pill).

• Capable of understanding and giving signed informed consent, which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in this protocol prior to any screening procedures.

• Willing to abstain from donating whole blood or blood derivatives until after Day 84 visit, and within 60 days prior to each study visit after Day 84.

⁃ Be able and willing to participate in all study visits and be reachable by telephone or personal contact by the study site personnel.

⁃ Participants who the investigator believes will comply with the requirements of the protocol (e.g., completion of the diary cards, return for follow-up visits, have regular contact to allow evaluation during the study).

⁃ Capable of understanding and completing diary card. Arms A, B, and C use electronic diary and require access to web browser. Arms D through M use paper diary