inclusion criteria: * the participant is 6 months to \<6 years for parts 1, 2, and 3; 2 years to \<5 years for part 4a; and 6 months to \<2 years for part 4b at the time of consent (screening visit). note: for part 1, participant must be at least 6 months old and must not have reached 6 years of age at the time of administration of first dose. for part 4b, the participant must be at least 6 months old and must not have reached 2 years of age at the time of administration of first dose. * if the participant has a chronic disease (that is, asthma, diabetes mellitus, cystic fibrosis, human immunodeficiency virus \[hiv\] infection), the disease should be stable, per investigator assessment, so that the participant can be considered eligible for inclusion. stable diseases are those which have had no change in their status or in the medications required to control them in the 6 months prior to screening visit. note: a change in medication for dose optimization (that is, insulin dose changes, adjustments for age-related weight gain), change within class of medication, or reduction in dose are not considered signs of instability. * in the investigator's opinion, the parent(s)/legally authorized representative(s) (lar\[s\]) understand and are willing and physically able to comply with protocol-mandated follow-up, including all procedures and provide written informed consent. * the participant is 2 years or older and has a body mass index (bmi) at or above the second percentile according to world health organization (who) child growth standards at the screening visit. or * the participant is less than 2 years of age and the participant's height and weight are both at or above the second percentile according to who child growth standards at the screening visit. special inclusion criteria for participants aged 6 months to \< 12 months: * the participant was born at full-term (≥ 37 weeks gestation) with a minimum birth weight of 2.5 kilograms (kg). inclusion criteria for part 2: * the participant must have received 2 doses of mrna-1273, approximately 28 to 35 days apart, as 25-μg primary series, and second dose was given at least 4 months prior to enrollment. inclusion criteria for part 3 only: - the participant must have received an age-appropriate immunization series of an authorized/approved covid-19 vaccine, with the last dose given at least 4 months prior to enrolment (previous vaccines not allowed are: xbb.1.5-containing formulation).

inclusion criteria: * the participant is 6 months to \<6 years for parts 1, 2, and 3; 2 years to \<5 years for part 4a; and 6 months to \<2 years for part 4b at the time of consent (screening visit). note: for part 1, participant must be at least 6 months old and must not have reached 6 years of age at the time of administration of first dose. for part 4b, the participant must be at least 6 months old and must not have reached 2 years of age at the time of administration of first dose. * if the participant has a chronic disease (that is, asthma, diabetes mellitus, cystic fibrosis, human immunodeficiency virus \[hiv\] infection), the disease should be stable, per investigator assessment, so that the participant can be considered eligible for inclusion. stable diseases are those which have had no change in their status or in the medications required to control them in the 6 months prior to screening visit. note: a change in medication for dose optimization (that is, insulin dose changes, adjustments for age-related weight gain), change within class of medication, or reduction in dose are not considered signs of instability. * in the investigator's opinion, the parent(s)/legally authorized representative(s) (lar\[s\]) understand and are willing and physically able to comply with protocol-mandated follow-up, including all procedures and provide written informed consent. * the participant is 2 years or older and has a body mass index (bmi) at or above the second percentile according to world health organization (who) child growth standards at the screening visit. or * the participant is less than 2 years of age and the participant's height and weight are both at or above the second percentile according to who child growth standards at the screening visit. special inclusion criteria for participants aged 6 months to \< 12 months: * the participant was born at full-term (≥ 37 weeks gestation) with a minimum birth weight of 2.5 kilograms (kg). inclusion criteria for part 2: * the participant must have received 2 doses of mrna-1273, approximately 28 to 35 days apart, as 25-μg primary series, and second dose was given at least 4 months prior to enrollment. inclusion criteria for part 3 only: - the participant must have received an age-appropriate immunization series of an authorized/approved covid-19 vaccine, with the last dose given at least 4 months prior to enrolment (previous vaccines not allowed are: xbb.1.5-containing formulation).

inclusion criteria: the participant is 6 months to <6 years for parts 1, 2, and 3; 2 years to <5 years for part 4a; and 6 months to <2 years for part 4b at the time of consent (screening visit). note: for part 1, participant must be at least 6 months old and must not have reached 6 years of age at the time of administration of first dose. for part 4b, the participant must be at least 6 months old and must not have reached 2 years of age at the time of administration of first dose. if the participant has a chronic disease (that is, asthma, diabetes mellitus, cystic fibrosis, human immunodeficiency virus [hiv] infection), the disease should be stable, per investigator assessment, so that the participant can be considered eligible for inclusion. stable diseases are those which have had no change in their status or in the medications required to control them in the 6 months prior to screening visit. note: a change in medication for dose optimization (that is, insulin dose changes, adjustments for age-related weight gain), change within class of medication, or reduction in dose are not considered signs of instability. in the investigator's opinion, the parent(s)/legally authorized representative(s) (lar[s]) understand and are willing and physically able to comply with protocol-mandated follow-up, including all procedures and provide written informed consent. the participant is 2 years or older and has a body mass index (bmi) at or above the second percentile according to world health organization (who) child growth standards at the screening visit. or the participant is less than 2 years of age and the participant's height and weight are both at or above the second percentile according to who child growth standards at the screening visit. special inclusion criteria for participants aged 6 months to < 12 months: the participant was born at full-term (≥ 37 weeks gestation) with a minimum birth weight of 2.5 kilograms (kg). inclusion criteria for part 2: the participant must have received 2 doses of mrna-1273, approximately 28 to 35 days apart, as 25-μg primary series, and second dose was given at least 4 months prior to enrollment. inclusion criteria for part 3 only: - the participant must have received an age-appropriate immunization series of an authorized/approved covid-19 vaccine, with the last dose given at least 4 months prior to enrolment (previous vaccines not allowed are: xbb.1.5-containing formulation).

inclusion criteria: the participant is 6 months to <6 years for parts 1, 2, and 3; 2 years to <5 years for part 4a; and 6 months to <2 years for part 4b at the time of consent (screening visit). note: for part 1, participant must be at least 6 months old and must not have reached 6 years of age at the time of administration of first dose. for part 4b, the participant must be at least 6 months old and must not have reached 2 years of age at the time of administration of first dose. if the participant has a chronic disease (that is, asthma, diabetes mellitus, cystic fibrosis, human immunodeficiency virus [hiv] infection), the disease should be stable, per investigator assessment, so that the participant can be considered eligible for inclusion. stable diseases are those which have had no change in their status or in the medications required to control them in the 6 months prior to screening visit. note: a change in medication for dose optimization (that is, insulin dose changes, adjustments for age-related weight gain), change within class of medication, or reduction in dose are not considered signs of instability. in the investigator's opinion, the parent(s)/legally authorized representative(s) (lar[s]) understand and are willing and physically able to comply with protocol-mandated follow-up, including all procedures and provide written informed consent. the participant is 2 years or older and has a body mass index (bmi) at or above the second percentile according to world health organization (who) child growth standards at the screening visit. or the participant is less than 2 years of age and the participant's height and weight are both at or above the second percentile according to who child growth standards at the screening visit. special inclusion criteria for participants aged 6 months to < 12 months: the participant was born at full-term (≥ 37 weeks gestation) with a minimum birth weight of 2.5 kilograms (kg). inclusion criteria for part 2: the participant must have received 2 doses of mrna-1273, approximately 28 to 35 days apart, as 25-μg primary series, and second dose was given at least 4 months prior to enrollment. inclusion criteria for part 3 only: - the participant must have received an age-appropriate immunization series of an authorized/approved covid-19 vaccine, with the last dose given at least 4 months prior to enrolment (previous vaccines not allowed are: xbb.1.5-containing formulation).

inclusion criteria: if the participant has a chronic disease (that is, asthma, diabetes mellitus, cystic fibrosis, human immunodeficiency virus [hiv] infection), the disease should be stable, per investigator assessment, so that the participant can be considered eligible for inclusion. stable diseases are those which have had no change in their status or in the medications required to control them in the 6 months prior to screening visit. note: a change in medication for dose optimization (that is, insulin dose changes, adjustments for age-related weight gain), change within class of medication, or reduction in dose are not considered signs of instability. in the investigator's opinion, the parent(s)/legally authorized representative(s) (lar[s]) understand and are willing and physically able to comply with protocol-mandated follow-up, including all procedures and provide written informed consent. this includes inability to draw baseline blood samples (minimum amount needed). the participant is 2 years or older and has a body mass index (bmi) at or above the 2nd percentile according to world health organization (who) child growth standards at the screening visit. or the participant is less than 2 years of age and the participant's height and weight are both at or above the 2nd percentile according to who child growth standards at the screening visit. special inclusion criteria for participants aged 6 months to < 12 months: the participant was born at full-term (≥ 37 weeks gestation) with a minimum birth weight of 2.5 kilograms (kg). inclusion criteria for part 2: the participant must have received 2 doses of mrna-1273, approximately 28 to 35 days apart, as 25-μg primary series, and 2nd dose was given at least 4 months prior to enrollment.

inclusion criteria: if the participant has a chronic disease (that is, asthma, diabetes mellitus, cystic fibrosis, human immunodeficiency virus [hiv] infection), the disease should be stable, per investigator assessment, so that the participant can be considered eligible for inclusion. stable diseases are those which have had no change in their status or in the medications required to control them in the 6 months prior to screening visit. note: a change in medication for dose optimization (that is, insulin dose changes, adjustments for age-related weight gain), change within class of medication, or reduction in dose are not considered signs of instability. in the investigator's opinion, the parent(s)/legally authorized representative(s) (lar[s]) understand and are willing and physically able to comply with protocol-mandated follow-up, including all procedures and provide written informed consent. this includes inability to draw baseline blood samples (minimum amount needed). the participant is 2 years or older and has a body mass index (bmi) at or above the 2nd percentile according to world health organization (who) child growth standards at the screening visit. or the participant is less than 2 years of age and the participant's height and weight are both at or above the 2nd percentile according to who child growth standards at the screening visit. special inclusion criteria for participants aged 6 months to < 12 months: the participant was born at full-term (≥ 37 weeks gestation) with a minimum birth weight of 2.5 kilograms (kg). inclusion criteria for part 2: the participant must have received 2 doses of mrna-1273, approximately 28 to 35 days apart, as 25-μg primary series, and 2nd dose was given at least 4 months prior to enrollment.

inclusion criteria: if the participant has a chronic disease (that is, asthma, diabetes mellitus, cystic fibrosis, human immunodeficiency virus [hiv] infection), the disease should be stable, per investigator assessment, so that the participant can be considered eligible for inclusion. stable diseases are those which have had no change in their status or in the medications required to control them in the 6 months prior to screening visit. note: a change in medication for dose optimization (that is, insulin dose changes, adjustments for age-related weight gain), change within class of medication, or reduction in dose are not considered signs of instability. in the investigator's opinion, the parent(s)/legally authorized representative(s) (lar[s]) understand and are willing and physically able to comply with protocol-mandated follow-up, including all procedures and provide written informed consent. this includes inability to draw baseline blood samples (minimum amount needed). the participant is 2 years or older and has a body mass index (bmi) at or above the 3rd percentile according to world health organization (who) child growth standards at the screening visit. or the participant is less than 2 years of age and the participant's height and weight are both at or above the 3rd percentile according to who child growth standards at the screening visit. special inclusion criteria for participants aged 6 months to < 12 months: the participant was born at full-term (≥ 37 weeks gestation) with a minimum birth weight of 2.5 kilograms (kg). inclusion criteria for part 2, all age groups based on study mrna-1273-p204: must have received 2 doses of mrna-1273, approximately 28 days apart, as 25 μg primary series, and 2nd dose was given at least 4 months prior to enrollment.

inclusion criteria: if the participant has a chronic disease (that is, asthma, diabetes mellitus, cystic fibrosis, human immunodeficiency virus [hiv] infection), the disease should be stable, per investigator assessment, so that the participant can be considered eligible for inclusion. stable diseases are those which have had no change in their status or in the medications required to control them in the 6 months prior to screening visit. note: a change in medication for dose optimization (that is, insulin dose changes, adjustments for age-related weight gain), change within class of medication, or reduction in dose are not considered signs of instability. in the investigator's opinion, the parent(s)/legally authorized representative(s) (lar[s]) understand and are willing and physically able to comply with protocol-mandated follow-up, including all procedures and provide written informed consent. this includes inability to draw baseline blood samples (minimum amount needed). the participant is 2 years or older and has a body mass index (bmi) at or above the 3rd percentile according to world health organization (who) child growth standards at the screening visit. or the participant is less than 2 years of age and the participant's height and weight are both at or above the 3rd percentile according to who child growth standards at the screening visit. special inclusion criteria for participants aged 6 months to < 12 months: the participant was born at full-term (≥ 37 weeks gestation) with a minimum birth weight of 2.5 kilograms (kg). inclusion criteria for part 2, all age groups based on study mrna-1273-p204: must have received 2 doses of mrna-1273, approximately 28 days apart, as 25 μg primary series, and 2nd dose was given at least 4 months prior to enrollment.