key inclusion criteria: * for participants with chronic diseases (such as, asthma, diabetes mellitus, cystic fibrosis, human immunodeficiency virus \[hiv\] infection), the disease should be stable, per investigator assessment. * investigator assessment that the parent(s)/legally acceptable representatives understand and are willing and physically able to comply with protocol mandated follow-up, including all procedures, written informed consent is provided, and participants provide assent. * for children 2 years of age or older has a body mass index at or above the third percentile according to world health organization (who) child growth standards at the screening visit. * for children 6 months to \<12 months of age: born at full-term with a minimum birth weight of 2.5 kilograms (kg). * 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, agreement to continue adequate contraception or abstinence through 3 months following the second injection (day 29) and the third dose in part 3 (day 149/booster dose day 1), and not currently breastfeeding. key

key inclusion criteria: * for participants with chronic diseases (such as, asthma, diabetes mellitus, cystic fibrosis, human immunodeficiency virus \[hiv\] infection), the disease should be stable, per investigator assessment. * investigator assessment that the parent(s)/legally acceptable representatives understand and are willing and physically able to comply with protocol mandated follow-up, including all procedures, written informed consent is provided, and participants provide assent. * for children 2 years of age or older has a body mass index at or above the third percentile according to world health organization (who) child growth standards at the screening visit. * for children 6 months to \<12 months of age: born at full-term with a minimum birth weight of 2.5 kilograms (kg). * 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, agreement to continue adequate contraception or abstinence through 3 months following the second injection (day 29) and the third dose in part 3 (day 149/booster dose day 1), and not currently breastfeeding. key

key inclusion criteria: for participants with chronic diseases (such as, asthma, diabetes mellitus, cystic fibrosis, human immunodeficiency virus [hiv] infection), the disease should be stable, per investigator assessment. investigator assessment that the parent(s)/legally acceptable representatives understand and are willing and physically able to comply with protocol mandated follow up, including all procedures, written informed consent is provided, and participants provide assent. for children 2 years of age or older has a body mass index at or above the third percentile according to world health organization (who) child growth standards at the screening visit. for children 6 months to <12 months of age: born at full-term with a minimum birth weight of 2.5 kilograms (kg). 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, agreement to continue adequate contraception or abstinence through 3 months following the second injection (day 29) and the third dose in part 3 (day 149/booster dose day 1), and not currently breastfeeding. key

key inclusion criteria: for participants with chronic diseases (such as, asthma, diabetes mellitus, cystic fibrosis, human immunodeficiency virus [hiv] infection), the disease should be stable, per investigator assessment. investigator assessment that the parent(s)/legally acceptable representatives understand and are willing and physically able to comply with protocol mandated follow up, including all procedures, written informed consent is provided, and participants provide assent. for children 2 years of age or older has a body mass index at or above the third percentile according to world health organization (who) child growth standards at the screening visit. for children 6 months to <12 months of age: born at full-term with a minimum birth weight of 2.5 kilograms (kg). 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, agreement to continue adequate contraception or abstinence through 3 months following the second injection (day 29) and the third dose in part 3 (day 149/booster dose day 1), and not currently breastfeeding. key

key inclusion criteria: - for participants with chronic diseases (such as, asthma, diabetes mellitus, cystic fibrosis, human immunodeficiency virus [hiv] infection), the disease should be stable, per investigator assessment. - investigator assessment that the parent(s)/legally acceptable representatives understand and are willing and physically able to comply with protocol mandated follow up, including all procedures, written informed consent is provided, and participants provide assent. - for children 2 years of age or older has a body mass index at or above the third percentile according to world health organization (who) child growth standards at the screening visit. - for children 6 months to <12 months of age: born at full-term with a minimum birth weight of 2.5 kilograms (kg). - 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, agreement to continue adequate contraception or abstinence through 3 months following the second injection, and not currently breastfeeding. key

key inclusion criteria: - for participants with chronic diseases (such as, asthma, diabetes mellitus, cystic fibrosis, human immunodeficiency virus [hiv] infection), the disease should be stable, per investigator assessment. - investigator assessment that the parent(s)/legally acceptable representatives understand and are willing and physically able to comply with protocol mandated follow up, including all procedures, written informed consent is provided, and participants provide assent. - for children 2 years of age or older has a body mass index at or above the third percentile according to world health organization (who) child growth standards at the screening visit. - for children 6 months to <12 months of age: born at full-term with a minimum birth weight of 2.5 kilograms (kg). - 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, agreement to continue adequate contraception or abstinence through 3 months following the second injection, and not currently breastfeeding. key

key inclusion criteria: - for participants with chronic diseases (such as, asthma, diabetes mellitus, cystic fibrosis, human immunodeficiency virus [hiv] infection), the disease should be stable, per investigator assessment. - investigator assessment that the parent(s)/legally acceptable representatives understand and are willing and physically able to comply with protocol mandated follow up, including all procedures, written informed consent is provided, and participants provide assent. - body mass index at or above the third percentile according to world health organization (who) child growth standards at the screening visit. - 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, agreement to continue adequate contraception or abstinence through 3 months following the second injection, and not currently breastfeeding. - for children 6 months to <2 years of age: born at full-term with a minimum birth weight of 2.5 kilograms (kg). key

key inclusion criteria: - for participants with chronic diseases (such as, asthma, diabetes mellitus, cystic fibrosis, human immunodeficiency virus [hiv] infection), the disease should be stable, per investigator assessment. - investigator assessment that the parent(s)/legally acceptable representatives understand and are willing and physically able to comply with protocol mandated follow up, including all procedures, written informed consent is provided, and participants provide assent. - body mass index at or above the third percentile according to world health organization (who) child growth standards at the screening visit. - 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, agreement to continue adequate contraception or abstinence through 3 months following the second injection, and not currently breastfeeding. - for children 6 months to <2 years of age: born at full-term with a minimum birth weight of 2.5 kilograms (kg). key