But vaccines have historically proved the best technique for controlling infectious diseases, and researchers have high hopes for a new one called RTS,S/AS2A, or Mosquirix.
"This is a tremendous moment in the fight against malaria and the culmination of more than two decades of research, including 10 years of clinical trials in Africa," said Dr. Joe Cohen, a vice president of research and development at GlaxoSmithKline Biologicals. The company is producing the vaccine for the Phase 3 trial; Cohen is a co-inventor of it.
The first-generation vaccine is unlikely to reduce infections by more than 50%, but researchers believe that even that level of efficacy could make inroads against the disease and lay the foundation for the development of more-effective vaccines. The goal is to have a vaccine by 2025 that is 80% effective and that lasts for at least four years.
Producing a malaria vaccine has proved difficult because of the complicated life cycle of the mosquito-borne Plasmodium parasites that cause the disease. The cycle begins when infected mosquitoes bite humans, injecting them with a form of the parasite called a sporozoite. The sporozoites invade the liver and begin reproducing in a new form called merozoites. Some merozoites can remain dormant in the liver for years, but most escape into the bloodstream and infect red blood cells, where they continue to replicate.
Eventually, the blood cells burst, releasing more of the parasites into the bloodstream. The parasites are then ingested by mosquitoes when they bite an infected human.
The new vaccine uses two genetically engineered proteins from the surface of the Plasmodium sporozoites to induce immunity. It also contains an agent that stimulates the immune system to react more strongly to the proteins. Initial studies of the vaccine in children under 5 have shown that it can reduce infections by as much as 63%, but more often in the range of 50%. Its effects persist for at least a year.