Interim results from a large clinical trial show that RTS, S/AS01vaccine cut the incidence of malaria by half among children aged 5 to 17 months.
Although this efficacy falls short of the 90% expected from viral and bacterial vaccines, the Plasmodium falciparum parasite is a much more complicated pathogen and RTS/AS01 is only a first-generation vaccine, according to the international team of investigators evaluating it. Details appear in the 18 October 2011 New England Journal of Medicine (10.1056/NEJMoa1102287).
Its 56% protection against all episodes of malaria after 12 months among 6,000 toddlers who were each inoculated three times was at the upper end of expectations. However, the 35% reduction in severe malaria in RTS, S/AS01-vaccinated infants and toddlers was slightly less than anticipated. "While we still need more information on how long protection lasts, we do seem to have the first effective vaccine against a parasitic disease in humans," says Nicholas J. White of Mahidol University in Bangkok, Thailand, who was not part of the vaccine study team. In any case, if and when it is deployed, RTS, S/AS01 will be used with other interventions such as insecticide-treated bed nets and drug treatments of patients.
Infants aged 5-12 weeks are the primary target population for the vaccine, and its efficacy in this age group will not be unveiled for another year. "Even then only short-term data will be available," White says. Nonetheless, officials of the World Health Organization might launch the vaccine in some African countries as early as 2015 if the anticipated findings involving newborns match efficacy rates among older children.
Also on the positive side, the RTS, S/AS01 vaccine appears generally safe, despite some surprises, according to White. "There were significantly more instances of meningitis among children immunized with RTS, S/AS01 than those vaccinated with the rabies comparator vaccine," he says. "This may have occurred by chance, but it cannot be ignored. [In contrast,] the increased risk of fevers and seizures among RTS, S/AS01 recipients looks real and might reflect the vaccine's extreme immunogenicity."
In the vaccine pipeline at Glaxo- SmithKline (GSK) since 1987, the RTS, S/AS01construct combines part of a surface protein from P. falciparum with a protein in the hepatitis B vaccine (HBsAg). The immune response it triggers keeps parasites from colonizing the host liver, where they would normally replicate before invading red blood cells. GSK is currently collaborating with Crucell in Leiden, the Netherlands, on an improved version of RTS, S using an adenoviral vector for antigen delivery.
"These initial results show, for the first time, that a malaria vaccine can provide significant benefit to children living in malaria-infested regions in Africa, a real-world setting," says Joe Cohen, co-inventor of the vaccine at GSK, in Rixensart, Belgium. "This is a true testimony to perseverance in public health, and it doesn't get any more important than this," adds Peter Hotez, President of the American Society of Tropical Medicine and Hygiene. "It underscores the importance of a longterm investment in research."
RTS, S/AS01 was developed by GSK in partnership with the Program for Appropriate Technology in Health (PATH) Malaria Vaccine Initiative and supported by the Bill and Melinda Gates Foundation. GSK pledges to keep the vaccine price low, covering manufacturing costs plus about 5%, which will be used to support further vaccine research.
Marcia Stone
Marcia Stone is a science writer based in New York City.
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