BACKGROUND: A live-attenuated Plasmodium falciparum sporozoite (SPZ) vaccine 
(PfSPZ Vaccine) has shown up to 100% protection against controlled human malaria 
infection (CHMI) using homologous parasites (same P. falciparum strain as in the 
vaccine). Using a more stringent CHMI, with heterologous parasites (different P. 
falciparum strain), we assessed the impact of higher PfSPZ doses, a novel 
multi-dose prime regimen, and a delayed vaccine boost upon vaccine efficacy 
(VE).
METHODS: We immunized 4 groups that each contained 15 healthy, malaria-naive 
adults. Group 1 received 5 doses of 4.5 x 105 PfSPZ (Days 1, 3, 5, and 7; Week 
16). Groups 2, 3, and 4 received 3 doses (Weeks 0, 8, and 16), with Group 2 
receiving 9.0 × 105/doses; Group 3 receiving 18.0 × 105/doses; and Group 4 
receiving 27.0 × 105 for dose 1 and 9.0 × 105 for doses 2 and 3. VE was assessed 
by heterologous CHMI after 12 or 24 weeks. Volunteers not protected at 12 weeks 
were boosted prior to repeat CHMI at 24 weeks.
RESULTS: At 12-week CHMI, 6/15 (40%) participants in Group 1 (P = .04) and 3/15 
(20%) participants in Group 2 remained aparasitemic, as compared to 0/8 
controls. At 24-week CHMI, 3/13 (23%) participants in Group 3 and 3/14 (21%) 
participants in Group 4 remained aparasitemic, versus 0/8 controls (Groups 2-4, 
VE not significant). Postboost, 9/14 (64%) participants versus 0/8 controls 
remained aparasitemic (3/6 in Group 1, P = .025; 6/8 in Group 2, P = .002).
CONCLUSIONS: Administering 4 stacked priming injections (multi-dose priming) 
resulted in 40% VE against heterologous CHMI, while dose escalation of PfSPZ 
using single-dose priming was not significantly protective. Boosting unprotected 
subjects improved VE at 24 weeks, to 64%.
CLINICAL TRIALS REGISTRATION: NCT02601716.