key inclusion criteria: * working or residing in an environment with high risk of exposure to sars-cov-2 for whom exposure may be relatively prolonged or for whom personal protective equipment (ppe) may be inconsistently used, especially in confined settings. * phase 2 only: screening laboratory results within normal limits for testing laboratory or are deemed not clinically significant by the investigator. * be post-menopausal or be surgically sterile or have a partner who is sterile or use medically effective contraception with a failure rate of \< 1% per year when used consistently and correctly from screening until 3 months following last dose (phase 2) or until last dose (phase 3). key

key inclusion criteria: * working or residing in an environment with high risk of exposure to sars-cov-2 for whom exposure may be relatively prolonged or for whom personal protective equipment (ppe) may be inconsistently used, especially in confined settings. * phase 2 only: screening laboratory results within normal limits for testing laboratory or are deemed not clinically significant by the investigator. * be post-menopausal or be surgically sterile or have a partner who is sterile or use medically effective contraception with a failure rate of \< 1% per year when used consistently and correctly from screening until 3 months following last dose (phase 2) or until last dose (phase 3). key

key inclusion criteria: - working or residing in an environment with high risk of exposure to sars-cov-2 for whom exposure may be relatively prolonged or for whom personal protective equipment (ppe) may be inconsistently used, especially in confined settings. - phase 2 only: screening laboratory results within normal limits for testing laboratory or are deemed not clinically significant by the investigator. - be post-menopausal or be surgically sterile or have a partner who is sterile or use medically effective contraception with a failure rate of < 1% per year when used consistently and correctly from screening until 3 months following last dose (phase 2) or until last dose (phase 3). key

key inclusion criteria: - working or residing in an environment with high risk of exposure to sars-cov-2 for whom exposure may be relatively prolonged or for whom personal protective equipment (ppe) may be inconsistently used, especially in confined settings. - phase 2 only: screening laboratory results within normal limits for testing laboratory or are deemed not clinically significant by the investigator. - be post-menopausal or be surgically sterile or have a partner who is sterile or use medically effective contraception with a failure rate of < 1% per year when used consistently and correctly from screening until 3 months following last dose (phase 2) or until last dose (phase 3). key

key inclusion criteria: - working or residing in an environment with high risk of exposure to sars-cov-2 for whom exposure may be relatively prolonged or for whom personal protective equipment (ppe) may be inconsistently used, especially in confined settings. - screening laboratory results within normal limits for testing laboratory or are deemed not clinically significant by the investigator. - be post-menopausal or be surgically sterile or have a partner who is sterile or use medically effective contraception with a failure rate of < 1% per year when used consistently and correctly from screening until 3 months following last dose. key

key inclusion criteria: - working or residing in an environment with high risk of exposure to sars-cov-2 for whom exposure may be relatively prolonged or for whom personal protective equipment (ppe) may be inconsistently used, especially in confined settings. - screening laboratory results within normal limits for testing laboratory or are deemed not clinically significant by the investigator. - be post-menopausal or be surgically sterile or have a partner who is sterile or use medically effective contraception with a failure rate of < 1% per year when used consistently and correctly from screening until 3 months following last dose. key