Scientists have found a genetic explanation for why the world's first malaria vaccine is not very effective in protecting some African children against the deadly mosquito-borne disease.
The analysis, published in the New England Journal of Medicine on Wednesday, comes at a critical time for the experimental shot from GlaxoSmithKline, when global experts are assessing how it might be deployed.
GSK's Mosquirix, also known as RTS,S, is specifically designed for use by African babies and toddlers. While it has the potential to help prevent many cases of malaria, researchers have been disappointed by its limited efficacy.
Hopes it could wipe out malaria were dampened when trial data in 2011 and 2012 showed it reduced episodes of the disease in babies aged 6-12 weeks by only 27 percent, and by about 46 percent in children aged 5-17 months.
Now scientists have discovered that genetic variability in a protein found on the surface of malaria parasites may help explain the patchy response because, while the protein comes in different forms, GSK's vaccine incorporates only one variant.
The new study, funded by the U.S. National Institutes of Health, looked at blood from 5,000 youngsters and found that the vaccine gave less protection when toddlers were infected with parasites with a different protein variant to the vaccine.
GSK said the research was interesting but early-stage.
"This single study is too small to offer conclusive evidence for how RTS,S should be deployed," a spokeswoman said.
"However, the work underscores how a full and comprehensive catalog of the genetic diversity of key pathogens could inform the design of clinical studies in the future," she said.
Experts advising the World Health Organization (WHO) have been meeting in Geneva this week to draw up recommendations on how the vaccine could be used. The U.N. agency, which normally follows such advice, will make a recommendation by November.
Even if the WHO view is positive, the vaccine is unlikely to be rolled out widely before 2017 since it still needs to win funding from international donors and be approved by governments in the African countries where it will be used.
GSK has said it will not make any profit from the vaccine, since it will be priced at the cost of manufacture plus a five percent margin, which will be reinvested in research on malaria and other neglected tropical diseases.