In a clinical trial, the experimental vaccine was shown to be only 30 percent effective.
LONDON - A GlaxoSmithKline experimental malaria vaccine touted as a new weapon in the fight to eradicate the deadly disease proved only 30 percent effective when given to African babies in a crucial clinical trial.
The surprisingly poor result leaves uncertain whether the vaccine will have a useful role to play in fighting the mosquito-borne disease that kills hundreds of thousands of children a year.
Philanthropist Bill Gates, who has helped fund its development, said further data was needed to determine whether and how the vaccine might be used.
"The efficacy came back lower than we had hoped, but developing a vaccine against a parasite is a very hard thing to do," he said in a statement.
Results from the final-stage trial with 6,537 babies aged six to 12 weeks showed the vaccine provided "modest protection", reducing episodes of the disease by 30 percent compared to immunisation with a control vaccine, researchers said on Friday.
That efficacy rate one year after vaccination is less than half the 65 percent reported in a smaller mid-stage trial in 2008 that followed babies of a similar age for six months.
It is also a lot less than the 50 percent seen last year in a large Phase III trial involving children aged five to 17 months.
Vaccinating babies, rather than toddlers, is the preferred option, since the new vaccine could then be added to other routine infant immunisations. A separate programme for older children would involve a lot of extra costs.
Despite the limited success, Britain's top drugmaker said it would push ahead with developing the vaccine, called RTS,S or Mosquirix, and Chief Executive Andrew Witty said he still believed it would be an important tool in fighting malaria.
GSK does not expect to make any significant profit from the vaccine, which would only be sold in poor countries.
Given the target market, it is governments and international groups that will have to fund the vaccine's roll-out and they will need more evidence before deciding that it is worth buying.
"We will have to have more information to give us a clearer idea as to how useful this vaccine will be," said Seth Berkley, CEO of the GAVI Alliance, which funds bulk-buy vaccination programmes for poorer nations.
In particular, Berkley told Reuters he wanted to see longer-term data, including the effect of booster shots, and an analysis of how the vaccine performed in different settings, which might show if it was suited for particular locations.
The setback comes two months after disappointing results with a vaccine against dengue fever, another mosquito-borne disease that is proving a formidable enemy.
Details of the malaria trial were presented at a medical meeting in Cape Town and published online by the New England Journal of Medicine.
Malaria is caused by a parasite carried in the saliva of mosquitoes. It is endemic in more than 100 countries worldwide and infected around 216 million people in 2010, killing around 655,000 of them, according to the World Health Organisation.
Control measures such as insecticide-treated bednets, indoor spraying and the use of combination anti-malaria drugs have helped cut the numbers of malaria cases and deaths significantly in recent years, but experts say an effective vaccine is vital to complete the fight against the disease.
The RTS,S vaccine is designed to kick in when the parasite enters the human bloodstream. By stimulating an immune response, it can prevent the parasite from maturing and multiplying in the liver. Without that immune response, the parasite gets back into the bloodstream and infects red blood cells, leading to fever, body aches and in some cases death.
Other teams of scientists around the world are working on other potential malaria vaccines which work in different ways, but RTS,S is by far the furthest ahead in development.