New results from a phase III clinical trial for a malaria vaccine for African infants have been described as "frustrating" after its efficacy proved lower than expected.
The RTS,S/AS01 vaccine (RTS,S) reduced the incidence of clinical malaria by 31 per cent and of severe malaria by 37 per cent, when given to infants aged 6-12 weeks. The results were presented at the International African Vaccinology Conference, taking place in Cape Town, South Africa, this week (8-11 November), and are published online in the New England Journal of Medicine today.
The trial, conducted on more than 6,500 babies, has seen a significant drop in efficacy, compared with results published last year on the same vaccine for children aged 5-17 months.
The data from the older children showed a 56 per cent drop in cases of clinical malaria, and a 47 per cent drop in cases of severe malaria, when three doses of the RTS,S vaccine were given at one-month intervals.
"It is frustrating seeing different levels of protection in different age groups," said Andrew Witty, CEO of GlaxoSmithKline, which has been developing the vaccine together with the PATH Malaria Vaccine Initiative.
"If we had seen … a repeat of last year, we would have seen this in a more enthusiastic light," Witty said. "However, it is very encouraging that the vaccine works."
The researchers speculated that the lower efficacy could be caused by differing immune responses in children of different age groups or differences in malaria transmission rates across the 11 research centres, based in seven countries in Sub-Saharan Africa, where the trial is being conducted.
"We will continue to study the factors affecting efficacy of the vaccine, including the duration of efficacy and the effects of a booster dose," said Salim Abdulla, chief executive director of the Ifakara Health Institute, Tanzania, who led the new trial.
However, the trial did confirm the safety of the vaccine, which has been designed to protect against the malaria parasite Plasmodium falciparum.
Although vaccinated infants were more likely to develop a non-malarial fever than the control group, the researchers described it as "very low and not requiring medical assistance".
This is in contrast to the initial trial, which found that some 5-17 month-old children vaccinated with RTS,S developed more serious side effects such as convulsions and meningitis.
The trial caused controversy last year when trial results from the first age group were published early, prompting critics to question why interim findings were announced at the 12-month mark. Further efficacy and safety data are expected in 2014, when the effects of the vaccine over 30 months will be published.
David Kaslow, director of the PATH Malaria Vaccine Initiative, said: "One size does not fit all in combating malaria: different combinations of tools will be needed. There's still a lot we don't know and we are still two years away from the final results."
New England Journal of Medicine doi:10.1056/NEJMoa1208394 (2012)