An experimental vaccine that attacks the malaria parasite in its early stages prevents a significant number of malaria cases, and should move closer to licensing and widespread use, according to a new review of recent studies.
Among children in Mozambique, where malaria is common, the new vaccine -- called RTS,S -- reduced the number of clinical malaria episodes by 26 percent for up to 18 months after vaccination. There were 58 percent fewer severe episodes among the children over the same time period.
The vaccine also reduced the number of clinical malaria episodes in partially immune men in Gambia by 63 percent after they received a booster shot a year later, say co-authors Patricia Graves, Ph.D., and Hellen Gelband.
The RTS,S vaccine, which is still being tested, "showed extremely promising results," said Graves, of EpiVec Consulting, a research firm in Atlanta.
Four other trial vaccines that target early-stage malaria did not have any significant effect on the parasite, the researchers said.
The review appears in the latest issue of The Cochrane Library, a publication of The Cochrane Collaboration, an international organization that evaluates medical research. Systematic reviews draw evidence-based conclusions about medical practice after considering both the content and quality of existing medical trials on a topic.
In a second Cochrane review, Graves and Gelband analyzed studies of a vaccine that targets malaria at a later stage of development, after the parasite has entered the bloodstream.
The experimental "blood-stage" vaccine called MSP/RESA or Combination B "shows promise as a way to reduce the severity of malaria episodes," but was most effective against one particular strain of falciparum malaria in Papua New Guinea," Graves said.
"In light of this proof of principle, the vaccine should undergo further development to improve its effectiveness," she added.
By targeting two different stages in the malaria parasite's lifestyle, the two types of vaccines may accomplish different ends, the Cochrane reviewers say.
Vaccines such as RTS,S "usually aim to completely prevent infection, while blood-stage vaccines aim to reduce, and preferably eliminate, the parasite load once a person has been infected," Graves said.
Many researchers think multistage vaccines that recognize and latch on to different proteins produced by the parasite during its life cycle "are likely to be most effective," Graves said.
The October 8 issue of the Washington Post Magazine details the efforts of scientists at Walter Reed Army Research Institute, who codeveloped the RTS,S vaccine with the drug company GlaxoSmithKline.
"The idea is that RTS, S would work like a Patriot missile system," said Col. Gray Heppner in the Post article. "All those parasites will be coming in like missiles, and RTS,S will be shooting them down."
There is a range of support for different vaccine types among malaria researchers, said Walter Brandt, Ph.D., a senior program officer at the Malaria Vaccine Initiative, a vaccine development program of the nonprofit global health organization PATH.
While some researchers think malaria will be prevented eventually with the right combination of early-stage vaccines, "others believe there is no 100 percent vaccine, which means we have to have some kind of blood-stage vaccine in the mix," Brandt said.
However, MVI is "neutral" in this debate, Brandt said, adding that his organization is more concerned with identifying and promoting effective vaccine candidates of any type.
The Cochrane review of early-stage malaria vaccines included 11 studies and more than 3,000 participants. The blood stage review included five studies and 217 people. Arm pain and swelling were the most common side effects reported in both vaccine reviews.
"Serious side effects were very rare with either of the effective vaccines," Graves said.
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