* subjects at increased risk of exposure to sars-cov-2, including healthcare workers, emergency response personnel, and those with known contact with covid-19 patients * pregnant or lactating women or planning to conceive a child during the next 3 months * body mass index (bmi) \> 30.0 kg/m2 * acute covid-19, a positive test result for sars-cov2 infection, a positive sars-cov-2 serology for prior sars-cov-2 infection at screening, or exposure within 14 days to an individual with acute covid-19 * an acute respiratory illness * positive result for hiv, hepatitis b virus, or hepatitis c virus at screening * chronic or current cigarette smoking * any medical, psychiatric, or social condition or occupational or other responsibility that in the judgment of the investigator would interfere with or serve as a contraindication to protocol adherence, assessment of safety (including reactogenicity), or a subject's ability to give informed consent

* subjects at increased risk of exposure to sars-cov-2, including healthcare workers, emergency response personnel, and those with known contact with covid-19 patients * pregnant or lactating women or planning to conceive a child during the next 3 months * body mass index (bmi) \> 30.0 kg/m2 * acute covid-19, a positive test result for sars-cov2 infection, a positive sars-cov-2 serology for prior sars-cov-2 infection at screening, or exposure within 14 days to an individual with acute covid-19 * an acute respiratory illness * positive result for hiv, hepatitis b virus, or hepatitis c virus at screening * chronic or current cigarette smoking * any medical, psychiatric, or social condition or occupational or other responsibility that in the judgment of the investigator would interfere with or serve as a contraindication to protocol adherence, assessment of safety (including reactogenicity), or a subject's ability to give informed consent

- subjects at increased risk of exposure to sars-cov-2, including healthcare workers, emergency response personnel, and those with known contact with covid-19 patients - pregnant or lactating women or planning to conceive a child during the next 3 months - body mass index (bmi) > 30.0 kg/m2 - acute covid-19, a positive test result for sars-cov2 infection, a positive sars-cov-2 serology for prior sars-cov-2 infection at screening, or exposure within 14 days to an individual with acute covid-19 - an acute respiratory illness - positive result for hiv, hepatitis b virus, or hepatitis c virus at screening - chronic or current cigarette smoking - any medical, psychiatric, or social condition or occupational or other responsibility that in the judgment of the investigator would interfere with or serve as a contraindication to protocol adherence, assessment of safety (including reactogenicity), or a subject's ability to give informed consent

- subjects at increased risk of exposure to sars-cov-2, including healthcare workers, emergency response personnel, and those with known contact with covid-19 patients - pregnant or lactating women or planning to conceive a child during the next 3 months - body mass index (bmi) > 30.0 kg/m2 - acute covid-19, a positive test result for sars-cov2 infection, a positive sars-cov-2 serology for prior sars-cov-2 infection at screening, or exposure within 14 days to an individual with acute covid-19 - an acute respiratory illness - positive result for hiv, hepatitis b virus, or hepatitis c virus at screening - chronic or current cigarette smoking - any medical, psychiatric, or social condition or occupational or other responsibility that in the judgment of the investigator would interfere with or serve as a contraindication to protocol adherence, assessment of safety (including reactogenicity), or a subject's ability to give informed consent