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RTS,S/AS01 malaria vaccine in African children.

To the Editor: In the article by the RTS,S Clinical Trials Partnership (Nov. 17 issue)1 on the phase 3 trial of RTS,S (ClinicalTrials.gov number, NCT00866619), an analysis of the efficacy of RTS,S/AS01 against severe malaria was not reported for the vaccine’s target age group of infants who were 6 to 12 weeks of age and who received RTS,S with routine childhood vaccines. However, this efficacy can be calculated as 17% (P = 0.23) (see Table 1 in the Supplementary Appendix, available with the full text of this letter at NEJM.org). Therefore, in the age group for whom RTS,S is intended, the efficacy against severe malaria was minimal. This finding was unexpected,2 and the goal of 50% efficacy against severe malaria of the Malaria Vaccine Technology Roadmap (www.malariavaccine.org/files/Malaria_ Vaccine_TRM_Final.pdf) was not met. Moreover, in the reported 12-month data, there was already significant waning of vaccine efficacy against clinical malaria, and the 17% efficacy against severe malaria in young infants was measured over an average of 7 months. Thus, over longer periods, there will probably be less efficacy. Lower vaccine immunogenicity3,4 and the rapid waning of induced antibodies and vaccine efficacy5 shown in phase 2a and field trials1,4,5 probably contribute to the lower efficacy of RTS,S in young infants. By the time severe malaria develops in infants vaccinated at 2 to 4 months of age (for instance, 7 months after vaccination), vaccine efficacy may be minimal.