key inclusion criteria: * participant is male or female, between 2 and \<6 months of age at the time of consent (screening visit), who is in good general health, in the opinion of the investigator, based on review of medical history and screening physical examination. 1. participant must be at least 12 weeks completed age and must not have completed 6 months at the time of administration of first dose. 2. if the participant has a chronic, stable disease, they may be eligible to enroll in part 2, but ineligible for part 1. the chronic condition (for example, gastroesophageal reflux disease) should be stable, per investigator assessment, so that the participant can be considered eligible for inclusion in part 2. * participant was born at ≥37 weeks gestation (part 1) or ≥34 weeks gestation (part 2), with a minimum birth weight of 2.5 kilograms (kg), without fetal growth restriction, and the participant's height and weight are both at or above the second percentile for age according to the centers for disease control and prevention/world health organization child growth standard at the screening visit. * in the investigator's opinion, the parent(s)/legally authorized representative(s) understand and are willing and physically able to comply with protocol-mandated follow-up, including all procedures, and provide written informed consent. key

key inclusion criteria: * participant is male or female, between 2 and \<6 months of age at the time of consent (screening visit), who is in good general health, in the opinion of the investigator, based on review of medical history and screening physical examination. 1. participant must be at least 12 weeks completed age and must not have completed 6 months at the time of administration of first dose. 2. if the participant has a chronic, stable disease, they may be eligible to enroll in part 2, but ineligible for part 1. the chronic condition (for example, gastroesophageal reflux disease) should be stable, per investigator assessment, so that the participant can be considered eligible for inclusion in part 2. * participant was born at ≥37 weeks gestation (part 1) or ≥34 weeks gestation (part 2), with a minimum birth weight of 2.5 kilograms (kg), without fetal growth restriction, and the participant's height and weight are both at or above the second percentile for age according to the centers for disease control and prevention/world health organization child growth standard at the screening visit. * in the investigator's opinion, the parent(s)/legally authorized representative(s) understand and are willing and physically able to comply with protocol-mandated follow-up, including all procedures, and provide written informed consent. key

key inclusion criteria: participant is male or female, between 2 and <6 months of age at the time of consent (screening visit), who is in good general health, in the opinion of the investigator, based on review of medical history and screening physical examination. participant must be at least 12 weeks completed age and must not have completed 6 months at the time of administration of first dose. if the participant has a chronic, stable disease, they may be eligible to enroll in part 2, but ineligible for part 1. the chronic condition (for example, gastroesophageal reflux disease) should be stable, per investigator assessment, so that the participant can be considered eligible for inclusion in part 2. participant was born at ≥37 weeks gestation (part 1) or ≥34 weeks gestation (part 2), with a minimum birth weight of 2.5 kilograms (kg), without fetal growth restriction, and the participant's height and weight are both at or above the second percentile for age according to the centers for disease control and prevention/world health organization child growth standard at the screening visit. in the investigator's opinion, the parent(s)/legally authorized representative(s) understand and are willing and physically able to comply with protocol-mandated follow-up, including all procedures, and provide written informed consent. key

key inclusion criteria: participant is male or female, between 2 and <6 months of age at the time of consent (screening visit), who is in good general health, in the opinion of the investigator, based on review of medical history and screening physical examination. participant must be at least 12 weeks completed age and must not have completed 6 months at the time of administration of first dose. if the participant has a chronic, stable disease, they may be eligible to enroll in part 2, but ineligible for part 1. the chronic condition (for example, gastroesophageal reflux disease) should be stable, per investigator assessment, so that the participant can be considered eligible for inclusion in part 2. participant was born at ≥37 weeks gestation (part 1) or ≥34 weeks gestation (part 2), with a minimum birth weight of 2.5 kilograms (kg), without fetal growth restriction, and the participant's height and weight are both at or above the second percentile for age according to the centers for disease control and prevention/world health organization child growth standard at the screening visit. in the investigator's opinion, the parent(s)/legally authorized representative(s) understand and are willing and physically able to comply with protocol-mandated follow-up, including all procedures, and provide written informed consent. key