inclusion criteria: 1. must provide documented informed consent prior to any study procedures being performed. 2. is capable of understanding and agrees to comply with planned study procedures and to be available for all study visits, including being willing and able to use electronic devices during the study. 3. has received at least 2 doses of pfizer-biontech (comirnaty) or moderna (spikevax) mrna covid-19 vaccine with the last dose of vaccine received at least 6 months prior to screening and has provided documentation of receiving the vaccination series. 4. negative for sars-cov-2 infection by rt-pcr test at screening. 5. is a male or nonpregnant female 18 to \<65 years of age (younger adult group) or ≥65 years of age (older adult group) at screening. 6. has a body mass index of 18 to 34.9 kg/m\^2, inclusive, at screening. 7. if the participant is a woman of child bearing potential the participant agrees to practice true abstinence or use at least 1 highly effective form of contraception for at least 30 days prior to study vaccination up to 3 months after study vaccination. 8. agrees to refrain from blood or plasma donation from screening and throughout the end of the study. 9. is healthy or medically stable as determined by medical history, clinical laboratory tests, vital sign measurements, and physical examination findings, as determined by investigator judgment.

inclusion criteria: 1. must provide documented informed consent prior to any study procedures being performed. 2. is capable of understanding and agrees to comply with planned study procedures and to be available for all study visits, including being willing and able to use electronic devices during the study. 3. has received at least 2 doses of pfizer-biontech (comirnaty) or moderna (spikevax) mrna covid-19 vaccine with the last dose of vaccine received at least 6 months prior to screening and has provided documentation of receiving the vaccination series. 4. negative for sars-cov-2 infection by rt-pcr test at screening. 5. is a male or nonpregnant female 18 to \<65 years of age (younger adult group) or ≥65 years of age (older adult group) at screening. 6. has a body mass index of 18 to 34.9 kg/m\^2, inclusive, at screening. 7. if the participant is a woman of child bearing potential the participant agrees to practice true abstinence or use at least 1 highly effective form of contraception for at least 30 days prior to study vaccination up to 3 months after study vaccination. 8. agrees to refrain from blood or plasma donation from screening and throughout the end of the study. 9. is healthy or medically stable as determined by medical history, clinical laboratory tests, vital sign measurements, and physical examination findings, as determined by investigator judgment.

inclusion criteria: must provide documented informed consent prior to any study procedures being performed. is capable of understanding and agrees to comply with planned study procedures and to be available for all study visits, including being willing and able to use electronic devices during the study. has received at least 2 doses of pfizer-biontech (comirnaty) or moderna (spikevax) mrna covid-19 vaccine with the last dose of vaccine received at least 6 months prior to screening and has provided documentation of receiving the vaccination series. negative for sars-cov-2 infection by rt-pcr test at screening. is a male or nonpregnant female 18 to <65 years of age (younger adult group) or ≥65 years of age (older adult group) at screening. has a body mass index of 18 to 34.9 kg/m^2, inclusive, at screening. if the participant is a woman of child bearing potential the participant agrees to practice true abstinence or use at least 1 highly effective form of contraception for at least 30 days prior to study vaccination up to 3 months after study vaccination. agrees to refrain from blood or plasma donation from screening and throughout the end of the study. is healthy or medically stable as determined by medical history, clinical laboratory tests, vital sign measurements, and physical examination findings, as determined by investigator judgment.

inclusion criteria: must provide documented informed consent prior to any study procedures being performed. is capable of understanding and agrees to comply with planned study procedures and to be available for all study visits, including being willing and able to use electronic devices during the study. has received at least 2 doses of pfizer-biontech (comirnaty) or moderna (spikevax) mrna covid-19 vaccine with the last dose of vaccine received at least 6 months prior to screening and has provided documentation of receiving the vaccination series. negative for sars-cov-2 infection by rt-pcr test at screening. is a male or nonpregnant female 18 to <65 years of age (younger adult group) or ≥65 years of age (older adult group) at screening. has a body mass index of 18 to 34.9 kg/m^2, inclusive, at screening. if the participant is a woman of child bearing potential the participant agrees to practice true abstinence or use at least 1 highly effective form of contraception for at least 30 days prior to study vaccination up to 3 months after study vaccination. agrees to refrain from blood or plasma donation from screening and throughout the end of the study. is healthy or medically stable as determined by medical history, clinical laboratory tests, vital sign measurements, and physical examination findings, as determined by investigator judgment.

inclusion criteria: must provide documented informed consent prior to any study procedures being performed. is capable of understanding and agrees to comply with planned study procedures and to be available for all study visits, including being willing and able to use electronic devices during the study. has received an authorized or licensed (only moderna or pfizer vaccines) covid-19 vaccine with the last dose administered between 6 and 18 months prior to screening and has provided documentation of receiving the vaccination series. negative for sars-cov-2 infection by rt-pcr test at screening. is a male or nonpregnant female 18 to <65 years of age (younger adult group) or ≥65 years of age (older adult group) at screening. has a body mass index of 18 to 32 kg/m2, inclusive, at screening. if the participant is a woman of child bearing potential the participant agrees to practice true abstinence or use at least 1 highly effective form of contraception for at least 30 days prior to study vaccination up to 3 months after study vaccination. agrees to refrain from blood or plasma donation from screening and throughout the end of the study. is healthy or medically stable as determined by medical history, clinical laboratory tests, vital sign measurements, and physical examination findings, as determined by investigator judgment.

inclusion criteria: must provide documented informed consent prior to any study procedures being performed. is capable of understanding and agrees to comply with planned study procedures and to be available for all study visits, including being willing and able to use electronic devices during the study. has received an authorized or licensed (only moderna or pfizer vaccines) covid-19 vaccine with the last dose administered between 6 and 18 months prior to screening and has provided documentation of receiving the vaccination series. negative for sars-cov-2 infection by rt-pcr test at screening. is a male or nonpregnant female 18 to <65 years of age (younger adult group) or ≥65 years of age (older adult group) at screening. has a body mass index of 18 to 32 kg/m2, inclusive, at screening. if the participant is a woman of child bearing potential the participant agrees to practice true abstinence or use at least 1 highly effective form of contraception for at least 30 days prior to study vaccination up to 3 months after study vaccination. agrees to refrain from blood or plasma donation from screening and throughout the end of the study. is healthy or medically stable as determined by medical history, clinical laboratory tests, vital sign measurements, and physical examination findings, as determined by investigator judgment.