key inclusion criteria: * investigator's assessment that the participant understands and is willing and physically able to comply with protocol-mandated follow-up, including all procedures. * for female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first injection, and agreement to continue adequate contraception or abstinence through 90 days following the vaccine administration. * part 1: has previously received a primary series of an authorized/approved covid-19 vaccine. for participants ≥18 years of age, at least 1 booster dose must have also been received. proof of prior vaccination is required. a heterologous vaccine regimen is acceptable. part 2: no prior vaccination is required. for participants who have been previously vaccinated, proof of vaccination is required. key

key inclusion criteria: * investigator's assessment that the participant understands and is willing and physically able to comply with protocol-mandated follow-up, including all procedures. * for female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first injection, and agreement to continue adequate contraception or abstinence through 90 days following the vaccine administration. * part 1: has previously received a primary series of an authorized/approved covid-19 vaccine. for participants ≥18 years of age, at least 1 booster dose must have also been received. proof of prior vaccination is required. a heterologous vaccine regimen is acceptable. part 2: no prior vaccination is required. for participants who have been previously vaccinated, proof of vaccination is required. key

key inclusion criteria: * investigator's assessment that the participant understands and is willing and physically able to comply with protocol-mandated follow-up, including all procedures. * for female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first injection, and agreement to continue adequate contraception or abstinence through 90 days following the vaccine administration. * has previously received a primary series of an authorized/approved covid-19 vaccine. for participants ≥18 years of age, at least 1 booster dose must have also been received. proof of prior vaccination is required. a heterologous vaccine regimen is acceptable. key

key inclusion criteria: * investigator's assessment that the participant understands and is willing and physically able to comply with protocol-mandated follow-up, including all procedures. * for female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first injection, and agreement to continue adequate contraception or abstinence through 90 days following the vaccine administration. * has previously received a primary series of an authorized/approved covid-19 vaccine. for participants ≥18 years of age, at least 1 booster dose must have also been received. proof of prior vaccination is required. a heterologous vaccine regimen is acceptable. key

key inclusion criteria: investigator's assessment that the participant understands and is willing and physically able to comply with protocol-mandated follow-up, including all procedures. for female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first injection, and agreement to continue adequate contraception or abstinence through 90 days following the vaccine administration. has previously received a primary series of an authorized/approved covid-19 vaccine. for participants ≥18 years of age, at least 1 booster dose must have also been received. proof of prior vaccination is required. a heterologous vaccine regimen is acceptable. key

key inclusion criteria: investigator's assessment that the participant understands and is willing and physically able to comply with protocol-mandated follow-up, including all procedures. for female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first injection, and agreement to continue adequate contraception or abstinence through 90 days following the vaccine administration. has previously received a primary series of an authorized/approved covid-19 vaccine. for participants ≥18 years of age, at least 1 booster dose must have also been received. proof of prior vaccination is required. a heterologous vaccine regimen is acceptable. key

key inclusion criteria: investigator's assessment that the participant understands and is willing and physically able to comply with protocol-mandated follow-up, including all procedures. for female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first injection, and agreement to continue adequate contraception or abstinence through 90 days following the vaccine administration. has previously received a primary series of an authorized/approved covid-19 vaccine. for participants ≥18 years of age, at least 1 booster dose must have also been received. a heterologous vaccine regimen is acceptable. key

key inclusion criteria: investigator's assessment that the participant understands and is willing and physically able to comply with protocol-mandated follow-up, including all procedures. for female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first injection, and agreement to continue adequate contraception or abstinence through 90 days following the vaccine administration. has previously received a primary series of an authorized/approved covid-19 vaccine. for participants ≥18 years of age, at least 1 booster dose must have also been received. a heterologous vaccine regimen is acceptable. key