A Phase 3 Randomized Double Blind Efficacy and Safety Study of Oral Polio Vaccine and NA-831 for Covid-19

Inclusion Criteria: * Participants who are at high risk of SARS-CoV-2 infection, defined as adults whose locations or circumstances put them at appreciable risk of exposure to SARS-CoV-2 and COVID-19. * Understands and agrees to comply with the study procedures and provides written informed consent. * Able to comply with study procedures based on the assessment of the Investigator. * Female participants of non-childbearing potential may be enrolled in the study. Non-childbearing potential is defined as surgically sterile (history of bilateral tubal ligation, bilateral oophorectomy, hysterectomy) or postmenopausal (defined as amenorrhea for ≥12 consecutive months prior to Screening without an alternative medical cause). A follicle-stimulating hormone (FSH) level may be measured at the discretion of the Investigator to confirm postmenopausal status. * Female participants of childbearing potential may be enrolled in the study if the participant fulfills all the following criteria: * Has a negative pregnancy test at Screening and on the day of the first dose (Day 1). * Has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first dose (Day 1). * Has agreed to continue adequate contraception through 3 months following the second dose on Day 29. * Is not currently breastfeeding. * Male participants engaging in activity that could result in pregnancy of sexual partners must agree to practice adequate contraception and refrain from sperm donation from the time of the first dose and through 3 months after the second dose. * Healthy adults or adults with pre-existing medical conditions who are in stable condition. A stable medical condition is defined as disease not requiring significant change in therapy or hospitalization for worsening disease during the 3 months before enrollment. Exclusion Criteria: * Is acutely ill or febrile 72 hours prior to or at Screening. Fever is defined as a body temperature ≥38.0°C/100.4°F. Participants meeting this criterion may be rescheduled within the relevant window periods. Afebrile participants with minor illnesses can be enrolled at the discretion of the Investigator. * Is pregnant or breastfeeding. * Known history of SARS-CoV-2 infection. * Prior administration of an investigational coronavirus (SARS-CoV, Middle East Respiratory Syndrome \[MERS\]-CoV) vaccine or current/planned simultaneous participation in another interventional study to prevent or treat COVID-19. * Demonstrated inability to comply with the study procedures. * An immediate family member or household member of this study's personnel. * History of anaphylaxis, urticaria, or other significant adverse reaction requiring medical intervention after receipt of a vaccine. * Bleeding disorder considered a contraindication to intramuscular injection or phlebotomy. * Has received or plans to receive a vaccine within 28 days prior to the first dose (Day 1) or plans to receive a non-study vaccine within 28 days prior to or after any dose of investigational product (except for seasonal influenza vaccine). * Has participated in an interventional clinical study within 28 days prior to the day of enrollment. * Immunosuppressive or immunodeficient state, including human immunodeficiency virus (HIV) infection, asplenia, and recurrent severe infections. * Has received systemic immunosuppressants or immune-modifying drugs for \>14 days in total within 6 months prior to Screening (for corticosteroids ≥20 milligram (mg)/day of prednisone equivalent). Topical tacrolimus is allowed if not used within 14 days prior to Screening. * Has received systemic immunoglobulins or blood products within 3 months prior to the day of Screening. * Has donated ≥450 milliliters (mL) of blood products within 28 days prior to Screening.