key inclusion criteria: part 1 (phase 1/2) * adults ≥18 to \<80 years of age at the time of consent. * body mass index (bmi) of 18 kilograms (kg)/square meter (m\^2) to 35 kg/m\^2 (inclusive) at the screening visit. * healthy as determined by medical evaluation, including medical history, physical examination, and laboratory tests. * 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 day 1, agreement to continue adequate contraception through 90 days following vaccine administration, and not currently breastfeeding. * fully vaccinated for covid-19 primary series according to the locally authorized or approved regimen, and their last covid-19 vaccine (primary series or booster) was ≥120 days prior to day 1. part 2 (phase 2 extension) * adults ≥18 to \<50 years of age at the time of consent. * bmi of 18 kg/m\^2 to 35 kg/m\^2 (inclusive) at the screening visit. * healthy as determined by medical evaluation, including medical history, and physical examination. * 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 day 1, agreement to continue adequate contraception through 90 days following vaccine administration, and not currently breastfeeding. * have received at least 2 doses of locally authorized or approved covid-19 vaccines and last dose was ≥90 days prior to day 1. key

key inclusion criteria: part 1 (phase 1/2) * adults ≥18 to \<80 years of age at the time of consent. * body mass index (bmi) of 18 kilograms (kg)/square meter (m\^2) to 35 kg/m\^2 (inclusive) at the screening visit. * healthy as determined by medical evaluation, including medical history, physical examination, and laboratory tests. * 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 day 1, agreement to continue adequate contraception through 90 days following vaccine administration, and not currently breastfeeding. * fully vaccinated for covid-19 primary series according to the locally authorized or approved regimen, and their last covid-19 vaccine (primary series or booster) was ≥120 days prior to day 1. part 2 (phase 2 extension) * adults ≥18 to \<50 years of age at the time of consent. * bmi of 18 kg/m\^2 to 35 kg/m\^2 (inclusive) at the screening visit. * healthy as determined by medical evaluation, including medical history, and physical examination. * 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 day 1, agreement to continue adequate contraception through 90 days following vaccine administration, and not currently breastfeeding. * have received at least 2 doses of locally authorized or approved covid-19 vaccines and last dose was ≥90 days prior to day 1. key

key inclusion criteria: part 1 (phase 1/2) adults ≥18 to <80 years of age at the time of consent. body mass index (bmi) of 18 kilograms (kg)/square meter (m^2) to 35 kg/m^2 (inclusive) at the screening visit. healthy as determined by medical evaluation, including medical history, physical examination, and laboratory tests. 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 day 1, agreement to continue adequate contraception through 90 days following vaccine administration, and not currently breastfeeding. fully vaccinated for covid-19 primary series according to the locally authorized or approved regimen, and their last covid-19 vaccine (primary series or booster) was ≥120 days prior to day 1. part 2 (phase 2 extension) adults ≥18 to <50 years of age at the time of consent. bmi of 18 kg/m^2 to 35 kg/m^2 (inclusive) at the screening visit. healthy as determined by medical evaluation, including medical history, and physical examination. 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 day 1, agreement to continue adequate contraception through 90 days following vaccine administration, and not currently breastfeeding. have received at least 2 doses of locally authorized or approved covid-19 vaccines and last dose was ≥90 days prior to day 1. key

key inclusion criteria: part 1 (phase 1/2) adults ≥18 to <80 years of age at the time of consent. body mass index (bmi) of 18 kilograms (kg)/square meter (m^2) to 35 kg/m^2 (inclusive) at the screening visit. healthy as determined by medical evaluation, including medical history, physical examination, and laboratory tests. 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 day 1, agreement to continue adequate contraception through 90 days following vaccine administration, and not currently breastfeeding. fully vaccinated for covid-19 primary series according to the locally authorized or approved regimen, and their last covid-19 vaccine (primary series or booster) was ≥120 days prior to day 1. part 2 (phase 2 extension) adults ≥18 to <50 years of age at the time of consent. bmi of 18 kg/m^2 to 35 kg/m^2 (inclusive) at the screening visit. healthy as determined by medical evaluation, including medical history, and physical examination. 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 day 1, agreement to continue adequate contraception through 90 days following vaccine administration, and not currently breastfeeding. have received at least 2 doses of locally authorized or approved covid-19 vaccines and last dose was ≥90 days prior to day 1. key

key inclusion criteria: body mass index (bmi) of 18 kilograms (kg)/m2 to 35 kg/square meter (m^2) (inclusive) at the screening visit. healthy as determined by medical evaluation, including medical history, physical examination, and laboratory tests. 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 day 1, agreement to continue adequate contraception through 90 days following vaccine administration, and not currently breastfeeding. fully vaccinated for covid-19 primary series according to the locally authorized or approved regimen, and their last covid-19 vaccine (primary series or booster) was ≥120 days prior to day 1. key

key inclusion criteria: body mass index (bmi) of 18 kilograms (kg)/m2 to 35 kg/square meter (m^2) (inclusive) at the screening visit. healthy as determined by medical evaluation, including medical history, physical examination, and laboratory tests. 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 day 1, agreement to continue adequate contraception through 90 days following vaccine administration, and not currently breastfeeding. fully vaccinated for covid-19 primary series according to the locally authorized or approved regimen, and their last covid-19 vaccine (primary series or booster) was ≥120 days prior to day 1. key