Group 1 volunteers will receive a low dose ChAdOx1 LS2 vaccination on day 0. Group 2 volunteers will receive a high dose ChAdOx1 LS2 vaccination on day 0 and a dose of MVA LS2 on day 56, followed by a CHMI on day 77. Volunteers exhibiting sterile protection will undergo a repeat CHMI 5-7 months later.