inclusion criteria: 1. willing and able to provide written informed consent prior to performing study procedures. 2. males and non-pregnant females who are between 18 to 59 years of age. 3. asymptomatic, rt-pcr negative (at screening) and without a known prior history of covid-19 infection (requiring a negative sars-cov-2 nucleocapsid antibody test result at screening). 4. provides documentation showing completion of an fda authorized or approved covid-19 vaccination regimen, where the last administration was ≥ 6 months (180 days) from the study enrollment date. 5. if female participant: a female participant is eligible to participate if she is not pregnant or breastfeeding, and at least one of the following conditions applies: * is not a woman of child-bearing potential (wocbp); or * is a wocbp and using an acceptable contraceptive method during the intervention period (for a minimum of 90 days after ndv-hxp-s vaccination). the investigator should evaluate the effectiveness of the contraceptive method in relationship to the first dose of study intervention. only highly effective methods of contraception that have a low user dependency or a combination of highly effective methods that are user dependent may be used. 6. if male participant: agrees to the following requirements during the intervention period and for at least 90 days after ndv-hxp-s vaccination, which corresponds to the time needed to eliminate reproductive safety risk of the study intervention(s): * refrain from donating sperm and be abstinent from heterosexual intercourse with a female of childbearing potential as their preferred and usual lifestyle (abstinent on a long-term and persistent basis) and agree to remain abstinent; or * must agree to use a male condom when engaging in any activity that allows for passage of ejaculate to another person. in addition to male condom use, a highly effective method of contraception may be considered in wocbp partners of male participants. 7. participant understands and agrees to comply with planned study procedures. 8. participant agrees to not participate in another clinical trial for treatment of covid-19 or sars-cov-2 through day 365. 9. participant agrees to not receive any other vaccination (including covid-19 vaccines) through day 56 of the study. 10. provides consent for release of information for hospitalization records and other medically attended visits during the study.

inclusion criteria: 1. willing and able to provide written informed consent prior to performing study procedures. 2. males and non-pregnant females who are between 18 to 59 years of age. 3. asymptomatic, rt-pcr negative (at screening) and without a known prior history of covid-19 infection (requiring a negative sars-cov-2 nucleocapsid antibody test result at screening). 4. provides documentation showing completion of an fda authorized or approved covid-19 vaccination regimen, where the last administration was ≥ 6 months (180 days) from the study enrollment date. 5. if female participant: a female participant is eligible to participate if she is not pregnant or breastfeeding, and at least one of the following conditions applies: * is not a woman of child-bearing potential (wocbp); or * is a wocbp and using an acceptable contraceptive method during the intervention period (for a minimum of 90 days after ndv-hxp-s vaccination). the investigator should evaluate the effectiveness of the contraceptive method in relationship to the first dose of study intervention. only highly effective methods of contraception that have a low user dependency or a combination of highly effective methods that are user dependent may be used. 6. if male participant: agrees to the following requirements during the intervention period and for at least 90 days after ndv-hxp-s vaccination, which corresponds to the time needed to eliminate reproductive safety risk of the study intervention(s): * refrain from donating sperm and be abstinent from heterosexual intercourse with a female of childbearing potential as their preferred and usual lifestyle (abstinent on a long-term and persistent basis) and agree to remain abstinent; or * must agree to use a male condom when engaging in any activity that allows for passage of ejaculate to another person. in addition to male condom use, a highly effective method of contraception may be considered in wocbp partners of male participants. 7. participant understands and agrees to comply with planned study procedures. 8. participant agrees to not participate in another clinical trial for treatment of covid-19 or sars-cov-2 through day 365. 9. participant agrees to not receive any other vaccination (including covid-19 vaccines) through day 56 of the study. 10. provides consent for release of information for hospitalization records and other medically attended visits during the study.

inclusion criteria: willing and able to provide written informed consent prior to performing study procedures. males and non-pregnant females who are between 18 to 59 years of age. asymptomatic, rt-pcr negative (at screening) and without a known prior history of covid-19 infection (requiring a negative sars-cov-2 nucleocapsid antibody test result at screening). provides documentation showing completion of an fda authorized or approved covid-19 vaccination regimen, where the last administration was ≥ 6 months (180 days) from the study enrollment date. if female participant: a female participant is eligible to participate if she is not pregnant or breastfeeding, and at least one of the following conditions applies: is not a woman of child-bearing potential (wocbp); or is a wocbp and using an acceptable contraceptive method during the intervention period (for a minimum of 90 days after ndv-hxp-s vaccination). the investigator should evaluate the effectiveness of the contraceptive method in relationship to the first dose of study intervention. only highly effective methods of contraception that have a low user dependency or a combination of highly effective methods that are user dependent may be used. if male participant: agrees to the following requirements during the intervention period and for at least 90 days after ndv-hxp-s vaccination, which corresponds to the time needed to eliminate reproductive safety risk of the study intervention(s): refrain from donating sperm and be abstinent from heterosexual intercourse with a female of childbearing potential as their preferred and usual lifestyle (abstinent on a long-term and persistent basis) and agree to remain abstinent; or must agree to use a male condom when engaging in any activity that allows for passage of ejaculate to another person. in addition to male condom use, a highly effective method of contraception may be considered in wocbp partners of male participants. participant understands and agrees to comply with planned study procedures. participant agrees to not participate in another clinical trial for treatment of covid-19 or sars-cov-2 through day 365. participant agrees to not receive any other vaccination (including covid-19 vaccines) through day 56 of the study. provides consent for release of information for hospitalization records and other medically attended visits during the study.

inclusion criteria: willing and able to provide written informed consent prior to performing study procedures. males and non-pregnant females who are between 18 to 59 years of age. asymptomatic, rt-pcr negative (at screening) and without a known prior history of covid-19 infection (requiring a negative sars-cov-2 nucleocapsid antibody test result at screening). provides documentation showing completion of an fda authorized or approved covid-19 vaccination regimen, where the last administration was ≥ 6 months (180 days) from the study enrollment date. if female participant: a female participant is eligible to participate if she is not pregnant or breastfeeding, and at least one of the following conditions applies: is not a woman of child-bearing potential (wocbp); or is a wocbp and using an acceptable contraceptive method during the intervention period (for a minimum of 90 days after ndv-hxp-s vaccination). the investigator should evaluate the effectiveness of the contraceptive method in relationship to the first dose of study intervention. only highly effective methods of contraception that have a low user dependency or a combination of highly effective methods that are user dependent may be used. if male participant: agrees to the following requirements during the intervention period and for at least 90 days after ndv-hxp-s vaccination, which corresponds to the time needed to eliminate reproductive safety risk of the study intervention(s): refrain from donating sperm and be abstinent from heterosexual intercourse with a female of childbearing potential as their preferred and usual lifestyle (abstinent on a long-term and persistent basis) and agree to remain abstinent; or must agree to use a male condom when engaging in any activity that allows for passage of ejaculate to another person. in addition to male condom use, a highly effective method of contraception may be considered in wocbp partners of male participants. participant understands and agrees to comply with planned study procedures. participant agrees to not participate in another clinical trial for treatment of covid-19 or sars-cov-2 through day 365. participant agrees to not receive any other vaccination (including covid-19 vaccines) through day 56 of the study. provides consent for release of information for hospitalization records and other medically attended visits during the study.

inclusion criteria: willing and able to provide written informed consent prior to performing study procedures. males and non-pregnant females who are between 18 to 59 years of age. asymptomatic, rt-pcr negative (at screening) and without a known prior history of covid-19 infection (requiring a negative sars-cov-2 nucleocapsid antibody test result at screening). provides documentation showing completion of a fda authorized or approved covid-19 vaccination regimen, where the last administration was ≥ 6 months (180 days) from the study enrollment date. if female participant: a female participant is eligible to participate if she is not pregnant or breastfeeding, and at least 1 of the following conditions applies: is not a woman of child-bearing potential (wocbp); or is a wocbp and using an acceptable contraceptive method during the intervention period (for a minimum of 90 days after ndv-hxp-s vaccination). the investigator should evaluate the effectiveness of the contraceptive method in relationship to the first dose of study intervention. only highly effective methods of contraception that have a low user dependency or a combination of highly effective methods that are user dependent may be used. if male participant: agrees to the following requirements during the intervention period and for at least 90 days after ndv-hxp-s vaccination, which corresponds to the time needed to eliminate reproductive safety risk of the study intervention(s): refrain from donating sperm and be abstinent from heterosexual intercourse with a female of childbearing potential as their preferred and usual lifestyle (abstinent on a long-term and persistent basis) and agree to remain abstinent; or must agree to use a male condom when engaging in any activity that allows for passage of ejaculate to another person. in addition to male condom use, a highly effective method of contraception may be considered in wocbp partners of male participants. participant understands and agrees to comply with planned study procedures. participant agrees to not participate in another clinical trial for treatment of covid-19 or sars-cov-2 through day 365. participant agrees to not receive any other vaccination (including covid-19 vaccine) through day 56. provides consent for release of information for hospitalization records and other medically attended visits during the study.

inclusion criteria: willing and able to provide written informed consent prior to performing study procedures. males and non-pregnant females who are between 18 to 59 years of age. asymptomatic, rt-pcr negative (at screening) and without a known prior history of covid-19 infection (requiring a negative sars-cov-2 nucleocapsid antibody test result at screening). provides documentation showing completion of a fda authorized or approved covid-19 vaccination regimen, where the last administration was ≥ 6 months (180 days) from the study enrollment date. if female participant: a female participant is eligible to participate if she is not pregnant or breastfeeding, and at least 1 of the following conditions applies: is not a woman of child-bearing potential (wocbp); or is a wocbp and using an acceptable contraceptive method during the intervention period (for a minimum of 90 days after ndv-hxp-s vaccination). the investigator should evaluate the effectiveness of the contraceptive method in relationship to the first dose of study intervention. only highly effective methods of contraception that have a low user dependency or a combination of highly effective methods that are user dependent may be used. if male participant: agrees to the following requirements during the intervention period and for at least 90 days after ndv-hxp-s vaccination, which corresponds to the time needed to eliminate reproductive safety risk of the study intervention(s): refrain from donating sperm and be abstinent from heterosexual intercourse with a female of childbearing potential as their preferred and usual lifestyle (abstinent on a long-term and persistent basis) and agree to remain abstinent; or must agree to use a male condom when engaging in any activity that allows for passage of ejaculate to another person. in addition to male condom use, a highly effective method of contraception may be considered in wocbp partners of male participants. participant understands and agrees to comply with planned study procedures. participant agrees to not participate in another clinical trial for treatment of covid-19 or sars-cov-2 through day 365. participant agrees to not receive any other vaccination (including covid-19 vaccine) through day 56. provides consent for release of information for hospitalization records and other medically attended visits during the study.

inclusion criteria: willing and able to provide written informed consent prior to performing study procedures. males and non-pregnant females who are between 18 to 59 years of age. asymptomatic, rt-pcr negative (at screening) and without a known prior history of covid-19 infection. provides documentation showing completion of a fda authorized or approved covid-19 vaccination regimen, where the last administration was ≥ 6 months (180 days) from the study enrollment date. if female participant: a female participant is eligible to participate if she is not pregnant or breastfeeding, and at least 1 of the following conditions applies: is not a woman of child-bearing potential (wocbp); or is a wocbp and using an acceptable contraceptive method during the intervention period (for a minimum of 90 days after ndv-hxp-s vaccination). the investigator should evaluate the effectiveness of the contraceptive method in relationship to the first dose of study intervention. only highly effective methods of contraception that have a low user dependency or a combination of highly effective methods that are user dependent may be used. if male participant: agrees to the following requirements during the intervention period and for at least 90 days after ndv-hxp-s vaccination, which corresponds to the time needed to eliminate reproductive safety risk of the study intervention(s): refrain from donating sperm and be abstinent from heterosexual intercourse with a female of childbearing potential as their preferred and usual lifestyle (abstinent on a long-term and persistent basis) and agree to remain abstinent; or must agree to use a male condom when engaging in any activity that allows for passage of ejaculate to another person. in addition to male condom use, a highly effective method of contraception may be considered in wocbp partners of male participants. participant understands and agrees to comply with planned study procedures. participant agrees to not participate in another clinical trial for treatment of covid-19 or sars-cov-2 through day 365. participant agrees to not receive any other vaccination (including covid-19 vaccine) through day 56. provides consent for release of information for hospitalization records and other medically attended visits during the study.

inclusion criteria: willing and able to provide written informed consent prior to performing study procedures. males and non-pregnant females who are between 18 to 59 years of age. asymptomatic, rt-pcr negative (at screening) and without a known prior history of covid-19 infection. provides documentation showing completion of a fda authorized or approved covid-19 vaccination regimen, where the last administration was ≥ 6 months (180 days) from the study enrollment date. if female participant: a female participant is eligible to participate if she is not pregnant or breastfeeding, and at least 1 of the following conditions applies: is not a woman of child-bearing potential (wocbp); or is a wocbp and using an acceptable contraceptive method during the intervention period (for a minimum of 90 days after ndv-hxp-s vaccination). the investigator should evaluate the effectiveness of the contraceptive method in relationship to the first dose of study intervention. only highly effective methods of contraception that have a low user dependency or a combination of highly effective methods that are user dependent may be used. if male participant: agrees to the following requirements during the intervention period and for at least 90 days after ndv-hxp-s vaccination, which corresponds to the time needed to eliminate reproductive safety risk of the study intervention(s): refrain from donating sperm and be abstinent from heterosexual intercourse with a female of childbearing potential as their preferred and usual lifestyle (abstinent on a long-term and persistent basis) and agree to remain abstinent; or must agree to use a male condom when engaging in any activity that allows for passage of ejaculate to another person. in addition to male condom use, a highly effective method of contraception may be considered in wocbp partners of male participants. participant understands and agrees to comply with planned study procedures. participant agrees to not participate in another clinical trial for treatment of covid-19 or sars-cov-2 through day 365. participant agrees to not receive any other vaccination (including covid-19 vaccine) through day 56. provides consent for release of information for hospitalization records and other medically attended visits during the study.