inclusion criteria: 1. subjects must have recent exposure to sars-cov-2 (such as repeated or extensive exposure to an infected individual(s) or cohabiting with a sars-cov-2 positive individual). 2. subjects must be 50 years or older if the exposure is due to cohabitation. 3. subjects must be free of clinical signs or symptoms of a potential covid-19 diagnosis (ordinal scale of clinical improvement score of 0) with a sars-cov-2 infection symptom score (fever, cough, shortness of breath, and fatigue) of 0 in each category. 4. spirometry (forced expiratory volume in one second fev1 and forced vital capacity \[fvc\]) ≥70% of predicted value. 5. if female, the subject must be surgically sterile or ≥ 1 year postmenopausal. if of child-bearing potential (including being \< 1years postmenopausal) and, if participating in sexual activity that may lead to pregnancy, the subject agrees to use an effective dual method of birth control (acceptable methods include intrauterine device, spermicide, barrier, male partner surgical sterilization, and hormonal contraception) during the study and through 30 days after completion of the study. 6. if female, must not be pregnant, plan to become pregnant, or nurse a child during the study and through 30 days after completion of the study. a pregnancy test must be negative at the screening visit, prior to dosing on day 1. 7. if male, must be surgically sterile or, if not surgically sterile and if participating in sexual activities that may lead to pregnancy, be willing to practice two effective methods of birth control (acceptable methods include barrier, spermicide, or female partner surgical sterilization) during the study and through 30 days after completion of the study. 8. ability to understand and give informed consent.

inclusion criteria: 1. subjects must have recent exposure to sars-cov-2 (such as repeated or extensive exposure to an infected individual(s) or cohabiting with a sars-cov-2 positive individual). 2. subjects must be 50 years or older if the exposure is due to cohabitation. 3. subjects must be free of clinical signs or symptoms of a potential covid-19 diagnosis (ordinal scale of clinical improvement score of 0) with a sars-cov-2 infection symptom score (fever, cough, shortness of breath, and fatigue) of 0 in each category. 4. spirometry (forced expiratory volume in one second fev1 and forced vital capacity \[fvc\]) ≥70% of predicted value. 5. if female, the subject must be surgically sterile or ≥ 1 year postmenopausal. if of child-bearing potential (including being \< 1years postmenopausal) and, if participating in sexual activity that may lead to pregnancy, the subject agrees to use an effective dual method of birth control (acceptable methods include intrauterine device, spermicide, barrier, male partner surgical sterilization, and hormonal contraception) during the study and through 30 days after completion of the study. 6. if female, must not be pregnant, plan to become pregnant, or nurse a child during the study and through 30 days after completion of the study. a pregnancy test must be negative at the screening visit, prior to dosing on day 1. 7. if male, must be surgically sterile or, if not surgically sterile and if participating in sexual activities that may lead to pregnancy, be willing to practice two effective methods of birth control (acceptable methods include barrier, spermicide, or female partner surgical sterilization) during the study and through 30 days after completion of the study. 8. ability to understand and give informed consent.

inclusion criteria: subjects must have recent exposure to sars-cov-2 (such as repeated or extensive exposure to an infected individual(s) or cohabiting with a sars-cov-2 positive individual). subjects must be 50 years or older if the exposure is due to cohabitation. subjects must be free of clinical signs or symptoms of a potential covid-19 diagnosis (ordinal scale of clinical improvement score of 0) with a sars-cov-2 infection symptom score (fever, cough, shortness of breath, and fatigue) of 0 in each category. spirometry (forced expiratory volume in one second fev1 and forced vital capacity [fvc]) ≥70% of predicted value. if female, the subject must be surgically sterile or ≥ 1 year postmenopausal. if of child-bearing potential (including being < 1years postmenopausal) and, if participating in sexual activity that may lead to pregnancy, the subject agrees to use an effective dual method of birth control (acceptable methods include intrauterine device, spermicide, barrier, male partner surgical sterilization, and hormonal contraception) during the study and through 30 days after completion of the study. if female, must not be pregnant, plan to become pregnant, or nurse a child during the study and through 30 days after completion of the study. a pregnancy test must be negative at the screening visit, prior to dosing on day 1. if male, must be surgically sterile or, if not surgically sterile and if participating in sexual activities that may lead to pregnancy, be willing to practice two effective methods of birth control (acceptable methods include barrier, spermicide, or female partner surgical sterilization) during the study and through 30 days after completion of the study. ability to understand and give informed consent.

inclusion criteria: subjects must have recent exposure to sars-cov-2 (such as repeated or extensive exposure to an infected individual(s) or cohabiting with a sars-cov-2 positive individual). subjects must be 50 years or older if the exposure is due to cohabitation. subjects must be free of clinical signs or symptoms of a potential covid-19 diagnosis (ordinal scale of clinical improvement score of 0) with a sars-cov-2 infection symptom score (fever, cough, shortness of breath, and fatigue) of 0 in each category. spirometry (forced expiratory volume in one second fev1 and forced vital capacity [fvc]) ≥70% of predicted value. if female, the subject must be surgically sterile or ≥ 1 year postmenopausal. if of child-bearing potential (including being < 1years postmenopausal) and, if participating in sexual activity that may lead to pregnancy, the subject agrees to use an effective dual method of birth control (acceptable methods include intrauterine device, spermicide, barrier, male partner surgical sterilization, and hormonal contraception) during the study and through 30 days after completion of the study. if female, must not be pregnant, plan to become pregnant, or nurse a child during the study and through 30 days after completion of the study. a pregnancy test must be negative at the screening visit, prior to dosing on day 1. if male, must be surgically sterile or, if not surgically sterile and if participating in sexual activities that may lead to pregnancy, be willing to practice two effective methods of birth control (acceptable methods include barrier, spermicide, or female partner surgical sterilization) during the study and through 30 days after completion of the study. ability to understand and give informed consent.

inclusion criteria: 1. subjects must have recent exposure to sars-cov-2 (such as repeated or extensive exposure to an infected individual(s) or cohabiting with a sars-cov-2 positive individual). 2. subjects must be 50 years or older if the exposure is due to cohabitation. 3. subjects must be free of clinical signs or symptoms of a potential covid-19 diagnosis (ordinal scale of clinical improvement score of 0) with a sars-cov-2 infection symptom score (fever, cough, shortness of breath, and fatigue) of 0 in each category. 4. spirometry (forced expiratory volume in one second fev1 and forced vital capacity [fvc]) ≥70% of predicted value. 5. if female, the subject must be surgically sterile or ≥ 1 year postmenopausal. if of child-bearing potential (including being < 1years postmenopausal) and, if participating in sexual activity that may lead to pregnancy, the subject agrees to use an effective dual method of birth control (acceptable methods include intrauterine device, spermicide, barrier, male partner surgical sterilization, and hormonal contraception) during the study and through 30 days after completion of the study. 6. if female, must not be pregnant, plan to become pregnant, or nurse a child during the study and through 30 days after completion of the study. a pregnancy test must be negative at the screening visit, prior to dosing on day 1. 7. if male, must be surgically sterile or, if not surgically sterile and if participating in sexual activities that may lead to pregnancy, be willing to practice two effective methods of birth control (acceptable methods include barrier, spermicide, or female partner surgical sterilization) during the study and through 30 days after completion of the study. 8. ability to understand and give informed consent.

inclusion criteria: 1. subjects must have recent exposure to sars-cov-2 (such as repeated or extensive exposure to an infected individual(s) or cohabiting with a sars-cov-2 positive individual). 2. subjects must be 50 years or older if the exposure is due to cohabitation. 3. subjects must be free of clinical signs or symptoms of a potential covid-19 diagnosis (ordinal scale of clinical improvement score of 0) with a sars-cov-2 infection symptom score (fever, cough, shortness of breath, and fatigue) of 0 in each category. 4. spirometry (forced expiratory volume in one second fev1 and forced vital capacity [fvc]) ≥70% of predicted value. 5. if female, the subject must be surgically sterile or ≥ 1 year postmenopausal. if of child-bearing potential (including being < 1years postmenopausal) and, if participating in sexual activity that may lead to pregnancy, the subject agrees to use an effective dual method of birth control (acceptable methods include intrauterine device, spermicide, barrier, male partner surgical sterilization, and hormonal contraception) during the study and through 30 days after completion of the study. 6. if female, must not be pregnant, plan to become pregnant, or nurse a child during the study and through 30 days after completion of the study. a pregnancy test must be negative at the screening visit, prior to dosing on day 1. 7. if male, must be surgically sterile or, if not surgically sterile and if participating in sexual activities that may lead to pregnancy, be willing to practice two effective methods of birth control (acceptable methods include barrier, spermicide, or female partner surgical sterilization) during the study and through 30 days after completion of the study. 8. ability to understand and give informed consent.

inclusion criteria: 1. subjects must have documented exposure to sars-cov-2 (defined as repeated daily exposure to an infected individual(s) [such as cohabiting with a sars-cov-2 positive individual] or involved in the assessment or care of covid-19 patients without adequate ppe (such as non availability of a n95 mask). 2. subjects must be 50 years or older if the exposure is due to cohabitation. 3. subjects must be free of clinical signs or symptoms of a potential covid-19 diagnosis (ordinal scale of clinical improvement score of 0) with a sars-cov-2 infection symptom score (fever, cough, shortness of breath, and fatigue) of 0 in each category. 4. spirometry (forced expiratory volume in one second fev1 and forced vital capacity [fvc]) ≥70% of predicted value. 5. if female, the subject must be surgically sterile or ≥ 1 year postmenopausal. if of child-bearing potential (including being < 1years postmenopausal) and, if participating in sexual activity that may lead to pregnancy, the subject agrees to use an effective dual method of birth control (acceptable methods include intrauterine device, spermicide, barrier, male partner surgical sterilization, and hormonal contraception) during the study and through 30 days after completion of the study. 6. if female, must not be pregnant, plan to become pregnant, or nurse a child during the study and through 30 days after completion of the study. a pregnancy test must be negative at the screening visit, prior to dosing on day 1. 7. if male, must be surgically sterile or, if not surgically sterile and if participating in sexual activities that may lead to pregnancy, be willing to practice two effective methods of birth control (acceptable methods include barrier, spermicide, or female partner surgical sterilization) during the study and through 30 days after completion of the study. 8. ability to understand and give informed consent.

inclusion criteria: 1. subjects must have documented exposure to sars-cov-2 (defined as repeated daily exposure to an infected individual(s) [such as cohabiting with a sars-cov-2 positive individual] or involved in the assessment or care of covid-19 patients without adequate ppe (such as non availability of a n95 mask). 2. subjects must be 50 years or older if the exposure is due to cohabitation. 3. subjects must be free of clinical signs or symptoms of a potential covid-19 diagnosis (ordinal scale of clinical improvement score of 0) with a sars-cov-2 infection symptom score (fever, cough, shortness of breath, and fatigue) of 0 in each category. 4. spirometry (forced expiratory volume in one second fev1 and forced vital capacity [fvc]) ≥70% of predicted value. 5. if female, the subject must be surgically sterile or ≥ 1 year postmenopausal. if of child-bearing potential (including being < 1years postmenopausal) and, if participating in sexual activity that may lead to pregnancy, the subject agrees to use an effective dual method of birth control (acceptable methods include intrauterine device, spermicide, barrier, male partner surgical sterilization, and hormonal contraception) during the study and through 30 days after completion of the study. 6. if female, must not be pregnant, plan to become pregnant, or nurse a child during the study and through 30 days after completion of the study. a pregnancy test must be negative at the screening visit, prior to dosing on day 1. 7. if male, must be surgically sterile or, if not surgically sterile and if participating in sexual activities that may lead to pregnancy, be willing to practice two effective methods of birth control (acceptable methods include barrier, spermicide, or female partner surgical sterilization) during the study and through 30 days after completion of the study. 8. ability to understand and give informed consent.