• Signed informed consent or thumb-printed and witnessed informed consent obtained from the parent/legal guardian of the infant.

• Infants must have been born full-term (at ≥37 weeks of gestation) and \> 2500 grams at birth.

• Immunization schedule Cohorts 1, 2, and 3: : Male and female infants 42-49 days (inclusive) of age at time of enrollment. For infants in Cohort 1, randomization to receive vaccine dose 1 (Groups 1 and 2 of R21/MM or placebo, respectively) will occur at 42-49 days of age. For infants in Cohort 2, randomization to receive vaccine dose 1 (Groups 3 and 4 of R21/MM or placebo, respectively) will occur at 2 months (56-63 days of age). For infants in Cohort 3, randomization to receive vaccine dose 1 (Groups 5 and 6 of R21/MM or placebo, respectively) will occur at 3 months (84-91 days of age).

• The participant's parent/guardian must be willing to avoid travel, particularly in the 28 days after each study vaccination, must confirm willingness to contact the study team in the event of unexpected/unavoidable travel and, for the safety cohort, must confirm availability for the home visits to be conducted by a field worker to collect solicited AEs over the 7 days (day of vaccination and 6 subsequent days) following each study vaccine.

• The participant's parent/guardian must confirm willingness to bring their child to the study clinic / local health care clinic, and capacity to contact the study team in the event the subject has any illnesses or other health concerns during the study.

• Participants who the investigator believes that their parent/guardian can and will comply with the requirements of the protocol (e.g. return for follow-up visits) may be enrolled in the study.