1.Age = 18 years if volunteer self-reports pregnant OR HIV-positive, OR age = 40 years with = 1 co-morbidity known to be associated with severe COVID-19, for example: • Hypertension • Type 2 diabetes mellitus • Obesity (body mass index [BMI] of 30 kg/m2 or higher but < 40 kg/m2) • Severe Obesity (BMI = 40 kg/m2) • Heart conditions, such as heart failure, coronary artery disease, or cardiomyopathies • Chronic kidney disease • COPD (chronic obstructive pulmonary disease) • Cancer • Non-HIV immunocompromised state (weakened immune system) or solid organ transplant • Pregnancy • Sickle cell disease • Smoking 2.Willingness to be followed and remain in the catchment area for the planned duration of the study. 3.Ability and willingness to provide informed consent. 4.Willingness to discuss HIV infection risks and willing to receive HIV testing, risk reduction counseling, and appropriate referrals to minimize HIV acquisition, if volunteer is pregnant 5.Assessment of understanding: volunteer demonstrates understanding of this study, completes a questionnaire prior to first vaccination with demonstration of understanding of all questionnaire items answered incorrectly. 6.Willing to defer SARS-CoV-2 vaccination until the crossover visit (anticipated at approximately Month 3.5). 7.Agrees not to enroll in another study of an investigational research agent until the end of the study. Enrollment in studies of investigational research agents for the treatment of COVID-19 is allowed for participants who develop COVID-19 disease.

1.Age = 18 years if volunteer self-reports pregnant OR HIV-positive, OR age = 40 years with = 1 co-morbidity known to be associated with severe COVID-19, for example: • Hypertension • Type 2 diabetes mellitus • Obesity (body mass index [BMI] of 30 kg/m2 or higher but < 40 kg/m2) • Severe Obesity (BMI = 40 kg/m2) • Heart conditions, such as heart failure, coronary artery disease, or cardiomyopathies • Chronic kidney disease • COPD (chronic obstructive pulmonary disease) • Cancer • Non-HIV immunocompromised state (weakened immune system) or solid organ transplant • Pregnancy • Sickle cell disease • Smoking 2.Willingness to be followed and remain in the catchment area for the planned duration of the study. 3.Ability and willingness to provide informed consent. 4.Willingness to discuss HIV infection risks and willing to receive HIV testing, risk reduction counseling, and appropriate referrals to minimize HIV acquisition, if volunteer is pregnant 5.Assessment of understanding: volunteer demonstrates understanding of this study, completes a questionnaire prior to first vaccination with demonstration of understanding of all questionnaire items answered incorrectly. 6.Willing to defer SARS-CoV-2 vaccination until the crossover visit (anticipated at approximately Month 3.5). 7.Agrees not to enroll in another study of an investigational research agent until the end of the study. Enrollment in studies of investigational research agents for the treatment of COVID-19 is allowed for participants who develop COVID-19 disease.