Humoral immune responses during a malaria vaccine trial in Tanzanian infants

The development of a malaria vaccine is a priority for improved and sustain ed malaria control. Optimal use of a vaccine in Africa Mill only be achieve d if it can be delivered through the Expanded Programme of Immunization (EP I). We have completed a trial of the peptide vaccine SPf66 in Tanzanian inf ants, given alongside the EPI vaccines. This paper describes the humoral re sponses to SPf66 and the EPI vaccines. A total of 1207 infants were recruit ed into a two-arm, double-blind, individually randomized placebo-controlled trial of SPf66. The objectives of the trial were to determine the safety, immunogenicity and efficacy of SPf66 and to assess interactions with EPI va ccines when three doses of SPf66 were delivered alongside the EPI vaccines. Cross-sectional surveys were carried out to asses seroconversion rates to the EPI vaccines and the antibody response to SPf66 (NANP)(50) and Plasmodi um falciparum lysates. Seroconversion rates to EPI vaccines were high and n o statistically, significant differences in prevalence or titres were found between SPf66 and placebo recipients. IgG antibodies against SPf66 (NANP)( 50) and whole P. falciparum lysate were present in high titres in mothers o f recruited children at the time of birth. Vaccination with SPf66 stimulate d a good anti-SPf66 IgG response which declined to preimmunization levels b y 2 pears of age and which was not associated with protection against clini cal malaria. The vaccine induced no IgG antibodies against (NANP)So or P. f alciparum lysates. SPf66 stimulated a humoral immune response when given to very young infants and did not interfere with seroconversion to EPI vaccin es. The response to SPf66 was qualitatively different from that seen in old er children, in whom SPf66 has been shown to be moderately efficacious. Thi s difference raises concerns about the difficulties of immunizing very youn g infants who need to be targeted by antimalarial vaccination programs.