The study is a randomized, double-blind placebo-controlled trial assessing the order of vector priming and boosting (Ad26 versus Ad35), the timing of boost (3 versus 6 months) and the homologous versus heterologous regimen at the 3-month time interval. Groups A-D will be enrolled in Boston, MA, USA, Groups E-H will be enrolled at the East African Clinical Research Centres and Groups I-L will be enrolled at the South African Clinical Research Centres.
Volunteers will be screened up to 56 days before vaccination and will be followed for 12 months after the second vaccination.
Approximately 212 volunteers will be randomized to receive either vaccine or placebo within a group (A-L); Groups A-D, Groups E-H and Groups I-L will be randomized separately. Up to 7% over-enrolment (approximately 15 volunteers) will be allowed to facilitate enrolment.