A malaria vaccine was 67 percent protective against infection in an early-stage trial involving adults in Kenya, according to a study published in Science Translational Medicine today. The results are encouraging, as many malaria vaccines that work well in the lab have failed to show the same efficacy in the field. But low malaria infection rates in the region at the time of the study are putting a damper on the results.
Whether the vaccine would work well in regions with lots of malaria is unclear
The vaccine "shows promise," says Philip Bejon, an infectious disease researcher at the University of Oxford and a co-author of the study. But we can't know if the vaccine would perform as well in areas where the risk for malaria is high, he says.
Increased bed net use and mosquito control measures have helped reduce the number of malaria infections by 47 percent since 2000, but a lot of people around the world are still affected by this potentially deadly disease. There were about 198 million malaria cases in 2013, and approximately 584,000 malaria-related deaths, according to the World Health Organization. That's why developing a vaccine that can protect people against malaria — or the parasite that causes the disease — remains vital.
In the study, researchers recruited 121 healthy adult Kenyan men. Half of them received the malaria vaccine, while the other half received a control rabies vaccine. All the participants were also given antimalarial drugs to clear any previous infections that could have confused the results. Then, the researchers monitored the men for eight weeks.
The vaccine used by the researchers was made from two different viruses that were genetically tailored to produce a protein found on the surface of the malaria parasite. Using two viruses allows for a "prime-boost strategy." When the vaccine is introduced into the body, the first virus "primes" the immune cells by exposing them to the malaria protein. Then, the second virus — the "booster" — strengthens the immunity that's being developed by re-stimulating the immune system.
Blood tests showed that the men built up a strong immune response
The researchers found that the vaccine was safe overall. Blood tests showed that the men built up a strong immune response against infection. In addition, the vaccine provided partial protection — it was 67 percent effective — against infection with the malaria parasite for at least two weeks following vaccination. But because of weather-related fluctuations, the transmission rate of malaria was "much lower than expected" during the study period, the researchers note. "All the malaria was outside the monitoring period that we planned for the study," Bejon says. It's possible that the vaccine would have worked just as well when confronted with a higher malaria risk. But, it's also possible that it would have been a total flop. "It might be harder to overcome more frequent inoculations of malaria from mosquitoes," he says. This adds an extra layer of difficulty for researchers trying to determine if the vaccine works.
The researchers "consistently demonstrated vaccine efficacy at eight weeks," says Matthew Laurens, a malaria researcher at the University of Maryland who didn't participate in this study. "But this is a brief period for efficacy evaluation" in an area where malaria is found regularly. The WHO Malaria Vaccine Technology Roadmap, for instance, targets vaccine efficacy of 75 percent over a period of two years, Laurens notes. And similar early trials looked at efficacy over a number of months. Ideally, the researchers would have to show that the vaccine works beyond the eight-week period, during a time when study participants would be more at risk for infection, he says. "This may increase enthusiasm for [this] vaccine approach."
"The target population for protection are children."
In response, Bejon explains that the short trial period was meant to show that there is "a faster way of testing vaccine efficacy in the field" before doing larger, more expensive trials. "The larger and more expensive a trial is, the slower it is to set up and run," he says. A smaller trial such as this one can yield valuable results before moving on to something bigger.
Now that the study is complete, Bejon and his team would like to test the vaccine in other populations — including in kids. "The target population for protection are children," he says; a study involving kids "would often require several more months follow-up."