The efficacy of an investigational malaria vaccine waned from 36% to 2.5% over a period of 7 years in Kenya, at which time it was even associated with an excess of cases in areas of high malaria transmission.
The overall vaccine efficacy during the entire follow-up period was 4.4%, Ally Olotu, PhD, and colleagues wrote in the June 29 issue of the New England Journal of Medicine (2016;374:2519-29. doi: 10.1056/NEJMoa1515257). But after 7 years, in areas of high malaria risk, the vaccine was associated with 141 excess cases per 1,000 vaccinated children.
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“We found that RTS,S/AS01 provided protective efficacy in the first year after vaccination but that the efficacy subsequently waned,” wrote Dr. Olotu of the Kenya Medical Research Institute in Africa and associates. “Efficacy was close to zero in the fourth year and may have been negative in the fifth year. This result eroded the benefits that were seen in early years.”
The rebound in cases probably occurred because the vaccine targets only a particular early-stage form of Plasmodium falciparum, they noted. “[It] protects against malaria sporozoites but does not induce clinical immunity against blood-stage parasites. We and others have previously found lower levels of antibodies against blood-stage parasites in children who have been immunized with the RTS,S/AS01 vaccine than in those given the control vaccine. The reduced exposure to blood-stage parasites among persons who have received the vaccine may lead to a slower acquisition of immunity to blood-stage parasites, leading to an increase in episodes of clinical malaria in later life.”
The phase II study comprised 447 children aged 5-17 months who were randomized to three doses of RTS,S/AS01 or a control rabies vaccine, administered at baseline and at 1 and 2 months. Children were followed for 7 years. The study had a large attrition rate, with 312 children completing all the follow-up visits.
In an intent-to-treat analysis, 150 incident cases of malaria developed among 223 children in the active group, and 157 cases among 224 in the control group. This equated to a vaccine efficacy of 27% against a first episode of malaria.
Overall, there were 1,002 episodes of malaria in the vaccine group and 992 in the control group. When the investigators grouped the children according to the endemic potential of their environment (high- vs. low-risk exposure), they observed that efficacy was consistently better in the low-risk group than the high-risk group (16.6% vs. –2.4%)
They then examined this finding year by year. Vaccine efficacy declined from 36% in year 1 to 3.6% in year 7. At year 5, there was significant negative efficacy in the high-risk exposure cohort (–56.8%, P = .008).
Overall, the vaccine averted 317 cases of clinical malaria per 1,000 children vaccinated, but the investigators noted that this finding was nonsignificant. In the low-exposure cohort, the vaccine did better, averting 718 cases per 1,000 vaccinated children. “However, in the high-exposure cohort … there were more cumulative cases among participants in the RTS,S/AS01 group than among those in the control group … which more than offset the cases that were averted in earlier years,” leading to an excess of 141 cases per 1,000 vaccinated children over baseline levels.
The rate of serious adverse events was similar between the vaccine and control groups (17.9% vs. 25.4%). No cases of meningitis occurred.
The investigators said a larger, phase III trial is being conducted in several areas with varying levels of transmission, and with different dosing schedules. “It will be essential to monitor efficacy in longer-term follow-up for year 5 and beyond to accurately measure the benefit and potential risk of vaccination with the RTS,S/AS01 vaccine,” they wrote.
The study was funded by grants from the PATH Malaria Vaccine Initiative, GlaxoSmithKline Biologicals, the Bill and Melinda Gates Foundation, and the Wellcome Trust. Two of the investigators, Amanda Leach and Marc Lievens, reported receiving personal fees and other support from GlaxoSmithKline Vaccines.
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