The vaccine dose and administration plan for Part B was selected based on Part A and Phase 1 data. Vaccine efficacy will be evaluated in Part B by comparing incidence of symptomatic ZIKV infections between vaccine and placebo groups. During the study, when participants exhibit any possible symptom of ZIKV infection, they will be evaluated by blood and urine ZIKV polymerase chain reaction (PCR). Stored blood and urine samples will also be assessed retrospectively by ZIKV PCR to identify possible asymptomatic cases. A Data and Safety Monitoring Board (DSMB) will oversee the study.