Malaria continues to be a major public health problem in Africa. An ongoing phase 3 study is investigating the efficacy, safety, reactogenicity, and immunogenicity of a candidate malaria vaccine, RTS,S/AS01, in two groups of children-those who are six to 12 weeks of age at enrollment and those who are five to 17 months of age at enrollment. Children enrolled in the study are randomized to receive either RTS,S/AS01 (with or without a booster dose) or a nonmalaria comparator vaccine (rabies or meningitis). This study reports the initial results of the trial, which enrolled 15,460 children at 11 centers in seven African countries.

During 12 months of follow-up in the first 6,000 children enrolled in the older age group, the rate of the first or only episode of clinical malaria was 0.44 per person-year in the group that received the RTS,S/AS01 vaccine, compared with 0.83 per person-year in the control group, an efficacy of 55.8%.

Among children in the combined age categories, the vaccine's efficacy against severe malaria was 34.8%: 149 (1.7%) of 8,597 children in the RTS,S/AS01 group had at least one episode of severe malaria, compared with 116 (2.7%) of those in the control group.

The frequency of serious adverse events was similar in the RTS,S/AS01 and control groups. Among the older children, serious adverse events occurred in 1,048 (17.6%) of 5,949 children who received the RTS,S/AS01 vaccine and in 642 (21.6%) of 2,974 children in the control group. Among the younger children, the corresponding rates were 569 (13.1%) of 4,358 and 293 (13.4%) of 2,179, respectively. In the older children, the rate of generalized convulsive seizures within seven days after RTS,S/AS01 vaccination was 1.04 per 1,000 doses compared with 0.57 per 1,000 doses in the control group receiving rabies vaccine (risk ratio, 1.8). In the younger children, the rates were 0.16 per 1,000 doses in the RTS,S/AS01 group and 0.47 per 1,000 doses in the control group receiving the meningitis vaccine (risk ratio, 0.3).

The authors concluded that the initial results of this study suggest that the RTS,S/AS01 vaccine could have an important effect on the burden of malaria in young African children.-KR

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