inclusion criteria: * female participants of childbearing potential may be enrolled in the study if the participant has a negative pregnancy test at the screening visit and on the day of vaccination prior to vaccine dose being administered on day 1; has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first dose (day 1); and has agreed to continue adequate contraception through 90 days following vaccine administration. * participant has received 2 prior doses of one of the following approved/authorized covid-19 vaccines: moderna, pfizer/biontech, oxford/astrazeneca, janssen. a heterologous vaccine regimen is acceptable. * participants who will receive the 4th dose as part of the study must have previously received a mrna vaccine (moderna or pfizer/biontech) as the 3rd dose of a covid-19 vaccine. participants who will receive the 3rd dose as part of the study may have previously received 2 doses of an approved/authorized mrna or a non-mrna covid-19 vaccine (a heterologous vaccine regimen is acceptable).

inclusion criteria: * female participants of childbearing potential may be enrolled in the study if the participant has a negative pregnancy test at the screening visit and on the day of vaccination prior to vaccine dose being administered on day 1; has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first dose (day 1); and has agreed to continue adequate contraception through 90 days following vaccine administration. * participant has received 2 prior doses of one of the following approved/authorized covid-19 vaccines: moderna, pfizer/biontech, oxford/astrazeneca, janssen. a heterologous vaccine regimen is acceptable. * participants who will receive the 4th dose as part of the study must have previously received a mrna vaccine (moderna or pfizer/biontech) as the 3rd dose of a covid-19 vaccine. participants who will receive the 3rd dose as part of the study may have previously received 2 doses of an approved/authorized mrna or a non-mrna covid-19 vaccine (a heterologous vaccine regimen is acceptable).

inclusion criteria: female participants of childbearing potential may be enrolled in the study if the participant has a negative pregnancy test at the screening visit and on the day of vaccination prior to vaccine dose being administered on day 1; has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first dose (day 1); and has agreed to continue adequate contraception through 90 days following vaccine administration. participant has received 2 prior doses of one of the following approved/authorized covid-19 vaccines: moderna, pfizer/biontech, oxford/astrazeneca, janssen. a heterologous vaccine regimen is acceptable. participants who will receive the 4th dose as part of the study must have previously received a mrna vaccine (moderna or pfizer/biontech) as the 3rd dose of a covid-19 vaccine. participants who will receive the 3rd dose as part of the study may have previously received 2 doses of an approved/authorized mrna or a non-mrna covid-19 vaccine (a heterologous vaccine regimen is acceptable).

inclusion criteria: female participants of childbearing potential may be enrolled in the study if the participant has a negative pregnancy test at the screening visit and on the day of vaccination prior to vaccine dose being administered on day 1; has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first dose (day 1); and has agreed to continue adequate contraception through 90 days following vaccine administration. participant has received 2 prior doses of one of the following approved/authorized covid-19 vaccines: moderna, pfizer/biontech, oxford/astrazeneca, janssen. a heterologous vaccine regimen is acceptable. participants who will receive the 4th dose as part of the study must have previously received a mrna vaccine (moderna or pfizer/biontech) as the 3rd dose of a covid-19 vaccine. participants who will receive the 3rd dose as part of the study may have previously received 2 doses of an approved/authorized mrna or a non-mrna covid-19 vaccine (a heterologous vaccine regimen is acceptable).

inclusion criteria: female participants of nonchildbearing potential may be enrolled in the study. female participants of childbearing potential may be enrolled in the study if the participant has a negative pregnancy test at the screening visit and on the day of vaccination prior to vaccine dose being administered on day 1; has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first dose (day 1); and has agreed to continue adequate contraception through 90 days following vaccine administration. participant has received 2 prior doses of one of the following approved/authorized covid-19 vaccines: moderna, pfizer/biontech, oxford/astrazeneca, janssen. a heterologous vaccine regimen is acceptable. participants who will receive the 4th dose as part of the study must have previously received a mrna vaccine (moderna or pfizer/biontech) as the 3rd dose of a covid-19 vaccine. participants who will receive the 3rd dose as part of the study may have previously received 2 doses of an approved/authorized mrna or a non-mrna covid-19 vaccine (a heterologous vaccine regimen is acceptable).

inclusion criteria: female participants of nonchildbearing potential may be enrolled in the study. female participants of childbearing potential may be enrolled in the study if the participant has a negative pregnancy test at the screening visit and on the day of vaccination prior to vaccine dose being administered on day 1; has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first dose (day 1); and has agreed to continue adequate contraception through 90 days following vaccine administration. participant has received 2 prior doses of one of the following approved/authorized covid-19 vaccines: moderna, pfizer/biontech, oxford/astrazeneca, janssen. a heterologous vaccine regimen is acceptable. participants who will receive the 4th dose as part of the study must have previously received a mrna vaccine (moderna or pfizer/biontech) as the 3rd dose of a covid-19 vaccine. participants who will receive the 3rd dose as part of the study may have previously received 2 doses of an approved/authorized mrna or a non-mrna covid-19 vaccine (a heterologous vaccine regimen is acceptable).