ABSTRACT
A phase III malaria vaccine trial in 13 villages in an endemic area, South Venezuela, compared incidence rates of Plasmodium falciparum and Plasmodium vivax infections in 1422 vaccinated and 938 nonvaccinated subjects over 18 months. The SPf66 vaccine was given in three doses, on days 0, 20, and 112. Vaccination was complete in 976 subjects (68.7%). Minor side effects requiring no treatment were reported by 123 (12.6%), with an apparent increase in frequency from the first to the third vaccine dose. No autoimmune evidence was observed in a sample of subjects. Antibodies against SPf66 were present at low titers in 24.7% of tested subjects before vaccination, increasing to 53.6% after the second dose and to 73.6% after the third dose; 26.4% of subjects initially seronegative never seroconverted. The SPf66 malaria vaccine showed a protective efficacy of 55% (95% confidence interval, 21%-75%) against P. falciparum and of 41% (19%-57%) against P. vivax malaria.