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.