A highly protective malaria vaccine would greatly facilitate the prevention and 
elimination of malaria and containment of drug-resistant parasites. A high level 
(more than 90%) of protection against malaria in humans has previously been 
achieved only by immunization with radiation-attenuated Plasmodium falciparum 
(Pf) sporozoites (PfSPZ) inoculated by mosquitoes; by intravenous injection of 
aseptic, purified, radiation-attenuated, cryopreserved PfSPZ ('PfSPZ Vaccine'); 
or by infectious PfSPZ inoculated by mosquitoes to volunteers taking chloroquine 
or mefloquine (chemoprophylaxis with sporozoites). We assessed immunization by 
direct venous inoculation of aseptic, purified, cryopreserved, non-irradiated 
PfSPZ ('PfSPZ Challenge') to malaria-naive, healthy adult volunteers taking 
chloroquine for antimalarial chemoprophylaxis (vaccine approach denoted as 
PfSPZ-CVac). Three doses of 5.12 × 104 PfSPZ of PfSPZ Challenge at 28-day 
intervals were well tolerated and safe, and prevented infection in 9 out of 9 
(100%) volunteers who underwent controlled human malaria infection ten weeks 
after the last dose (group III). Protective efficacy was dependent on dose and 
regimen. Immunization with 3.2 × 103 (group I) or 1.28 × 104 (group II) PfSPZ 
protected 3 out of 9 (33%) or 6 out of 9 (67%) volunteers, respectively. Three 
doses of 5.12 × 104 PfSPZ at five-day intervals protected 5 out of 8 (63%) 
volunteers. The frequency of Pf-specific polyfunctional CD4 memory T cells was 
associated with protection. On a 7,455 peptide Pf proteome array, immune sera 
from at least 5 out of 9 group III vaccinees recognized each of 22 proteins. 
PfSPZ-CVac is a highly efficacious vaccine candidate; when we are able to 
optimize the immunization regimen (dose, interval between doses, and drug 
partner), this vaccine could be used for combination mass drug administration 
and a mass vaccination program approach to eliminate malaria from geographically 
defined areas.