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Experimental Malaria Vaccines Attack the Parasite in New Ways

Malaria is transmitted to humans through mosquitoes. Photo by the World Health Organization.

The malaria parasite is one of the most widely studied disease-causing organisms, yet there is still no effective vaccine available to prevent the deadly illness.

Two new experimental vaccine studies indicate some progress towards that goal, one that uses whole, live parasites in the vaccine production process, and one that stalks the parasite as it begins to spread through a patient’s blood.

Research out Thursday in the New England Journal of Medicine found for the first time that a vaccine targeting the malaria parasite at the blood stage, which is when the parasite is emerging from the liver into the patient’s bloodstream, could provide protection against specific strains of the disease.

The vaccine would have limitations — the Phase II trial did not find broad-based protection from clinical malaria, but did find patients were well protected against parasites genetically similar to those used in producing the vaccine. But if several strains of the vaccine could be combined, much like a flu vaccine, the protection could be multiplied.

Researchers immunized 400 children in Mali with either the experimental vaccine or the control and found that the test vaccine was 64 percent effective at preventing infection from malaria from similar genetic strains.

“This gives us hope that we could create an effective blood-stage vaccine by using a precise combination of just the right strains to protect against most types of malaria,” said lead author Dr. Christopher Plowe, who heads the Malaria Group at the University of Maryland School of Medicine’s Center for Vaccine Development.

The university was also involved in new research on the first whole-parasite malaria vaccine approved by the FDA to test in humans, published in the journal Science earlier this month. That vaccine uses a live, but weakened, malaria parasite to elicit an immune response.

A team of researchers from the biotech firm Sanaria, the U.S. Military Malaria Vaccine Program and the University of Maryland developed and tested of the vaccine, which is being prepared for clinical trials now in the United States in Africa.

The vaccine was found to be safe and well-tolerated by humans, and triggered unprecedented high levels of malaria-parasite specific immune system cells when injected intravenously into animals. The same vaccine had shown disappointing levels of immune response in humans when applied through a vaccine on the skin, so researchers were relieved to see the higher response with the intravenous approach. The team is aiming for protection rates of 90 percent or higher.

CEO Stephen Hoffman told Science staff members have to pick apart mosquitoes to create the vaccine, making it very time intensive.

“Our goal has always been to show that this vaccine is highly protective,” he said. “Once we have done that, we’ll figure out how to make it practical.”

Results from the only malaria vaccine candidate in large-scale Phase III human trials are expected by the end of 2011. That candidate, called RTS,S from GlaxoSmithKline, showed protection between 50 and 60 percent in previous trials. The less-than-ideal level of protection for that vaccine is part of why other options are being explored so vigorously.

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