The efficacy of the malaria vaccine candidate RTS,S/AS01E against episodes of malaria diminishes over time and also varies with exposure to the Plasmodium falciparum parasite, according to data from a long-term follow-up of a phase 2 trial of the vaccine in Africa (Olotu A et al. N Engl J Med. 2013;368:1111-1120).
After 4 years of follow-up of 320 of 447 children in Kenya who had been vaccinated at 5 to 17 months of age, protection declined each year to almost 0 by year 4. The vaccine efficacy varied with exposure to malaria. In areas of high malaria transmission, protection faded even faster than it did in areas with lower transmission. The authors suggest that children in the areas of higher transmission who were vaccinated may have had less exposure to blood-stage parasites and therefore developed natural immunity more slowly than the control group.
Friedrich MJ. Waning Efficacy of Experimental Malaria Vaccine. JAMA. 2013;309(19):1985. doi:10.1001/jama.2013.5881