Malaria-infected mosquitoes are ready for dissection for vaccine production… (Robert Thompson / Sanaria…)
An experimental malaria vaccine is safe and effective, researchers said, performing better in early trials than any vaccine tested so far in the fight against the disease.
In a small clinical trial involving 40 U.S. adults, 12 of the 15 people who received doses of the PfSPZ vaccine were protected from malaria, according to a study published Thursday in the journal Science.
Though preliminary, the results represent hope in the battle against an illness contracted by hundreds of millions of people every year.
"It's an exciting result," said Dr. Robert Seder, an immunologist at the National Institutes of Health in Bethesda, Md., who led the study.
But he and others cautioned that more research, and possibly improvements in methods of delivering the vaccine, are needed before it would be practical to administer PfSPZ on a broad scale.
People contract malaria when they are bitten by mosquitoes infected with the parasite Plasmodium. The illness causes high fever, vomiting, anemia and sometimes death. In 2010, about 219 million cases of malaria and 660,000 deaths from it were reported. Most of those who died were African children, according to the World Health Organization.
For decades, scientists have sought to develop a vaccine to prevent malaria, but it has been tricky because the Plasmodium parasite changes significantly from one stage of its life to another, making it difficult to target.
When infected mosquitoes bite humans, they inject a form of the parasite called a sporozoite, which invades the liver and gives rise to another form, called a merozoite. Merozoites then spread through the bloodstream, and symptoms of the disease appear.
The team working on PfSPZ for the biotech company Sanaria figured out a way to home in on the shape-shifting parasite by developing a vaccine containing whole sporozoites that had been zapped with X-rays.
The radiation prevented the sporozoites from generating disease-triggering merozoites in study participants' bodies. The irradiated sporozoites stayed dormant in their livers, stimulating the production of sporozoite antibodies that protected against future infection.
Researchers tested the vaccine in 2010 with disappointing results. Back then, they injected it in subjects' skin or muscle. This time, they administered it in a vein instead.
The researchers gave the vaccine to 15 participants once every four weeks. Nine received a total of four doses, and six got five doses. Six unvaccinated subjects served as controls. The remaining 19 subjects were involved in other aspects of the study.
Three weeks later, the team brought in infected mosquitoes to bite the participants. Three of the subjects who received four doses of the vaccine developed malaria, but no one who received five doses got sick. All but one of the controls developed the disease.
Protected vaccinated subjects showed other signs of immunity too, including higher levels of sporozoite-specific antibodies and increases in several types of disease-battling immune cells. Participants who received five doses of the vaccine had a higher immune response than those who got four.
The vaccine didn't cause severe adverse effects or malaria infection in any of the subjects.
Dr. Aric Gregson, an infectious-disease specialist at UCLA who was not involved in the study, called PfSPZ one of the most effective malaria vaccines tested so far. But he said scientists would "need to replicate this over a larger population to verify the findings," and that delivering the vaccine intravenously would be difficult in the developing countries where malaria strikes hardest.
Other questions remain as well, including how long the vaccine's effects last and whether it can protect against different malaria strains, said Dr. Joseph Vinetz, an infectious-disease researcher at UC San Diego, who also was not part of the study.
He also said that getting five doses of the vaccine isn't practical. "People want to get a single shot and be done with it," he said.