Phase 1 safety and immunogenicity trial of malaria vaccine RTS,S/AS02A in adults in a hyperendemic region of western Kenya.

We conducted a phase 1 trial of candidate malaria vaccine RTS,S/AS02A in western Kenya to determine its safety and immunogenicity in healthy adults in an area hyperendemic for malaria. Twenty adults were enrolled and received RTS,S/AS02A (50 microg of RTS,S in 0.5 mL of AS02A) by intramuscular injection on a 0-, 28-, and 178-day schedule. All 60 scheduled immunizations were given, and 18 of 20 volunteers completed the last study visit on day 210. The vaccine was safe and well-tolerated. There were no vaccine-related severe adverse events. The most common solicited adverse events associated with immunization were injection site pain and headache. The geometric mean concentration of antibodies to circumsporozoite protein was 1.9 microg/mL at baseline and it increased 2-4 weeks after each dose to 16, 17.8, and 36.6 microg/mL, respectively. These safety and immunogenicity data from adults in hyperendemic Kenya are comparable to data reported earlier from two trials in west African adults in hypo-endemic and meso-endemic areas of The Gambia. We conclude that in this small study, RTS,S/AS02A is safe and similarly immunogenic in malaria-exposed African adults of different ethnicity in different transmission settings.