New tests with a candidate malaria vaccine have shown it to be less effective in weeks-old infants than older babies, a "frustrating" result in the fight against the killer disease.
The trial vaccine, RTS,S, was shown in Phase III tests to protect only a third of six- to 12-week-old infants innoculated, its developers announced at a teleconference on Friday.
The result from tests on 6,500 infants was "modest" compared to success rates of between 47 percent and 55 percent in children between the ages of five and 17 months, said a study report in the New England Journal of Medicine.
"We... would have liked to have seen higher efficacy than we have of course," said Andrew Witty, chief executive officer of vaccine developer GlaxoSmithKline, calling the results "a little frustrating".
But he stressed: "This is not a mission we should just walk away from," with the mosquito-borne disease killing hundreds of thousands of children per year.
"This remains the lead and still the most encouraging (malaria) candidate vaccine," said Witty.
A year ago, the RTS,S team announced that the candidate vaccine cut risk in half over a period of 12 months in African children between the ages of five and 17 months who had been innoculated.
The biggest trial of its kind, underway at 11 sites in seven African countries, seeks to create a vaccine to block the parasite that causes malaria.
Several vaccine development projects are under way around the world, with many of them in the clinical trial phase.
The disease kills an estimated 655,000 people every year, mainly children under five living in sub-Saharan Africa.
Dr Salim Abdulla, who leads the Tanzanian leg of the trial, said three doses of the drug had reduced clinical malaria in tiny babies by 31 percent, and severe malaria by 37 percent.
"We will continue to examine the different factors that may be behind the different levels of efficacy," between the two age groups, Abdulla said.
The researchers expect to have final trial results by 2014, including data on the vaccine's efficacy over a longer period of 30 months, and the potential benefits of a later, booster dose.
"These data simply confirm that the vaccine is potentially useful in significantly reducing the risk of malaria, but that it is not the complete solution," London School of Hygiene and Tropical Medicine immunologist Eleanor Riley told AFP of on the findings.
It can be effective if used with other prevention measures such as insecticide-treated bed nets.
University College London vaccine expert Jane Zuckerman added the outcome was not completely unexpected.
"Complex immunological responses are intrinsic in the development of a malaria vaccine," she said.
Abdulla said the positives include confirmation that the vaccine is safe and that it can be administered with other childhood vaccinations without any worrying side-effects.
"The efficacy came back lower than we had hoped, but developing a vaccine against a parasite is a very hard thing to do," said US billionaire Bill Gates, co-founder of the Bill & Melinda Gates Foundation which sponsors the trial.
"The trial is continuing and we look forward to getting more data to help determine whether and how to deploy this vaccine."