The final stages of clinical trials of a vaccine against malaria are ongoing, which, if successful, could help immunise millions of children from the disease which can be deadly.
However tests conducted on 16,000 children in Burkina Faso, Gabon, Ghana, Kenya, Malawi, Mozambique and Tanzania, showed that booster doses were not as effective as the initial dose, and immunisation waned over time.
Prof Brian Greenwood, study author and professor of clinical tropical medicine at the London School of Hygiene and Tropical Medicine, said to the BBC he was "a little disappointed" by the results.
"I hoped the vaccine would be more effective, but we were never going to end up with the success seen in measles vaccines with 97% efficacy."
Prof Mike Turner, head of infection at the Wellcome Trust, said the vaccine was better than anything else available.
"While the levels of protection the vaccine offers against clinical malaria may seem relatively low, they are better than any other potential vaccine we currently have.
"The findings are not only important in their own right but also in signposting a road to developing better vaccines in the future."