participants meeting any of the following criteria will be excluded from the study. 1. received any additional covid-19 vaccine booster after the day 1 dose of nvx-cov2373 administered during participation in study 307. 2. history of laboratory-confirmed (by polymerase chain reaction \[pcr\] or rapid antigen test) covid-19 infection ≤ 4 months prior to day 1. 3. current participation in research involving receipt of an investigational product (drug/biologic/device). 4. any known allergies or history of anaphylaxis to the active substance or any of the other ingredients contained in the investigational product. 5. any autoimmune or immunodeficiency disease/condition (iatrogenic or congenital) or therapy that causes clinically significant immunosuppression. 6. received any vaccine ≤ 90 days prior to study vaccination, except for influenza vaccine which may be received \> 4 days prior to study vaccine, or rabies vaccine, which may be received at any time if medically indicated. 7. received immunoglobulin, blood-derived products, or immunosuppressant drugs within 90 days prior to study vaccination, except for rabies immunoglobulin which may be given if medically indicated. 8. active cancer (malignancy) on chemotherapy that is judged to cause significant immunocompromise within 1 year prior to first study vaccination (with the exception of malignancy cured via excision, at the discretion of the investigator). 9. participants who are breastfeeding, pregnant, or who plan to become pregnant prior to the eos visit. 10. suspected or known history of alcohol abuse or drug addiction within 3 months prior to the study vaccine dose that, in the opinion of the investigator, might interfere with protocol compliance. 11. any other condition that, in the opinion of the investigator, would pose a health risk to the participant if enrolled or could interfere with evaluation of the trial vaccine or interpretation of study results (including neurologic or psychiatric conditions likely to impair the quality of safety reporting). 12. study team member or immediate family member of any study team member (inclusive of sponsor, clinical research organization \[cro\], and study site personnel involved in the conduct or planning of the study). 13. participants with a history of myocarditis or pericarditis.

participants meeting any of the following criteria will be excluded from the study. 1. received any additional covid-19 vaccine booster after the day 1 dose of nvx-cov2373 administered during participation in study 307. 2. history of laboratory-confirmed (by polymerase chain reaction \[pcr\] or rapid antigen test) covid-19 infection ≤ 4 months prior to day 1. 3. current participation in research involving receipt of an investigational product (drug/biologic/device). 4. any known allergies or history of anaphylaxis to the active substance or any of the other ingredients contained in the investigational product. 5. any autoimmune or immunodeficiency disease/condition (iatrogenic or congenital) or therapy that causes clinically significant immunosuppression. 6. received any vaccine ≤ 90 days prior to study vaccination, except for influenza vaccine which may be received \> 4 days prior to study vaccine, or rabies vaccine, which may be received at any time if medically indicated. 7. received immunoglobulin, blood-derived products, or immunosuppressant drugs within 90 days prior to study vaccination, except for rabies immunoglobulin which may be given if medically indicated. 8. active cancer (malignancy) on chemotherapy that is judged to cause significant immunocompromise within 1 year prior to first study vaccination (with the exception of malignancy cured via excision, at the discretion of the investigator). 9. participants who are breastfeeding, pregnant, or who plan to become pregnant prior to the eos visit. 10. suspected or known history of alcohol abuse or drug addiction within 3 months prior to the study vaccine dose that, in the opinion of the investigator, might interfere with protocol compliance. 11. any other condition that, in the opinion of the investigator, would pose a health risk to the participant if enrolled or could interfere with evaluation of the trial vaccine or interpretation of study results (including neurologic or psychiatric conditions likely to impair the quality of safety reporting). 12. study team member or immediate family member of any study team member (inclusive of sponsor, clinical research organization \[cro\], and study site personnel involved in the conduct or planning of the study). 13. participants with a history of myocarditis or pericarditis.

participants meeting any of the following criteria will be excluded from the study. received any additional covid-19 vaccine booster after the day 1 dose of nvx-cov2373 administered during participation in study 307. history of laboratory-confirmed (by polymerase chain reaction [pcr] or rapid antigen test) covid-19 infection ≤ 4 months prior to day 1. current participation in research involving receipt of an investigational product (drug/biologic/device). any known allergies or history of anaphylaxis to the active substance or any of the other ingredients contained in the investigational product. any autoimmune or immunodeficiency disease/condition (iatrogenic or congenital) or therapy that causes clinically significant immunosuppression. received any vaccine ≤ 90 days prior to study vaccination, except for influenza vaccine which may be received > 4 days prior to study vaccine, or rabies vaccine, which may be received at any time if medically indicated. received immunoglobulin, blood-derived products, or immunosuppressant drugs within 90 days prior to study vaccination, except for rabies immunoglobulin which may be given if medically indicated. active cancer (malignancy) on chemotherapy that is judged to cause significant immunocompromise within 1 year prior to first study vaccination (with the exception of malignancy cured via excision, at the discretion of the investigator). participants who are breastfeeding, pregnant, or who plan to become pregnant prior to the eos visit. suspected or known history of alcohol abuse or drug addiction within 3 months prior to the study vaccine dose that, in the opinion of the investigator, might interfere with protocol compliance. any other condition that, in the opinion of the investigator, would pose a health risk to the participant if enrolled or could interfere with evaluation of the trial vaccine or interpretation of study results (including neurologic or psychiatric conditions likely to impair the quality of safety reporting). study team member or immediate family member of any study team member (inclusive of sponsor, clinical research organization [cro], and study site personnel involved in the conduct or planning of the study). participants with a history of myocarditis or pericarditis.

participants meeting any of the following criteria will be excluded from the study. received any additional covid-19 vaccine booster after the day 1 dose of nvx-cov2373 administered during participation in study 307. history of laboratory-confirmed (by polymerase chain reaction [pcr] or rapid antigen test) covid-19 infection ≤ 4 months prior to day 1. current participation in research involving receipt of an investigational product (drug/biologic/device). any known allergies or history of anaphylaxis to the active substance or any of the other ingredients contained in the investigational product. any autoimmune or immunodeficiency disease/condition (iatrogenic or congenital) or therapy that causes clinically significant immunosuppression. received any vaccine ≤ 90 days prior to study vaccination, except for influenza vaccine which may be received > 4 days prior to study vaccine, or rabies vaccine, which may be received at any time if medically indicated. received immunoglobulin, blood-derived products, or immunosuppressant drugs within 90 days prior to study vaccination, except for rabies immunoglobulin which may be given if medically indicated. active cancer (malignancy) on chemotherapy that is judged to cause significant immunocompromise within 1 year prior to first study vaccination (with the exception of malignancy cured via excision, at the discretion of the investigator). participants who are breastfeeding, pregnant, or who plan to become pregnant prior to the eos visit. suspected or known history of alcohol abuse or drug addiction within 3 months prior to the study vaccine dose that, in the opinion of the investigator, might interfere with protocol compliance. any other condition that, in the opinion of the investigator, would pose a health risk to the participant if enrolled or could interfere with evaluation of the trial vaccine or interpretation of study results (including neurologic or psychiatric conditions likely to impair the quality of safety reporting). study team member or immediate family member of any study team member (inclusive of sponsor, clinical research organization [cro], and study site personnel involved in the conduct or planning of the study). participants with a history of myocarditis or pericarditis.