As the Zika virus continues to spread through the Americas, researchers have been scrambling to develop a vaccine—an achievement some say we could reach within the year.
That's exciting news, but it raises a question. Why is it we're able to whip up a vaccine for one mosquito-carried infection in just a few months but we still have no vaccine for one of the deadliest mosquito-borne illnesses on the planet: malaria?
Malaria kills over half a million people a year and causes as many as hundreds of millions of cases of infection annually, many of them children. It's an old disease that has been killing humans since Ancient Egypt, so we've had a lot of time to study, understand, and (theoretically) eliminate it. But one of our best hopes for curbing the spread of malaria—vaccination—has eluded us. Why have we been able to develop vaccines for other mosquito-borne illnesses like Japanese encephalitis, yellow fever, and maybe soon dengue and Zika, but not malaria?
Unlike many mosquito-borne diseases, malaria is not caused by a virus, but a parasite. Though we've got a lot of vaccines for bacterial and viral infections, we don't have any vaccines for human parasitic infections. That's because it's just a lot harder to do, according to Dr. Fidel Zavala, a microbiology and immunology professor at the Johns Hopkins Malaria Research Institute.
"Influenza, for example, has something like eight genes. Eight. And malaria has about 5,000 genes," Zavala told me over the phone. "We're talking about a completely different level of biological complexity."
With malaria in particular, it gets even more complicated because the parasite matures and changes throughout the infection. At each stage, the parasite produces a different kind of antigen—the molecule that can stimulate an immune response—so by the time the body has recognized it, it's changed.
There are also five different kinds of parasites that can cause malaria, and different populations of these parasites can produce unique antigens, making it a moving target for developing a vaccine.
"People can get malaria and the next year get another infection," said Jun Li, a malaria researcher at the University of Oklahoma. "So even the natural malaria infection cannot stimulate an immune response strong enough to provide full protection against a future infection."
That said, individuals who live in malaria-endemic areas and have survived multiple malaria infections do eventually build up a natural immunity over time, Zavala said. They may still get infections, but it becomes much milder, and often these people are asymptomatic even if they do get malaria.
With enough exposure, our body can do naturally what we haven't yet been able to do in a lab: create an immunity. The trouble is, many people never reach that point, and those that do have to suffer immensely throughout the years. Still, Zavala said this is a signal that a vaccine is possible.
And we've had some success in the past. In the 1960s and 70s, researchers showed that having people bit by mosquitoes that carried an attenuated version of the malaria parasite effectively vaccinated them—similar to how vaccines like the flu shot work. But this technique was considered too impractical, since you needed mosquitoes to do the inoculation.
In more recent years, scientists have developed malaria vaccines that target specific antigens produced by the parasite. These vaccines are partially effective, but not at the rates that get financial donors excited. For one vaccine candidate, which has shown to be about 30 percent effective in infants, the efficacy waned after a few years, leading the World Health Organization to not recommend going forward with it. In a paper published in PLOS Medicine Tuesday, the researchers argued that though not perfect, the vaccine could still save lives, especially for vulnerable newborns.
Zavala told me another challenge is that there are few researchers working in this field, and those in it don't have enough financial backing to make a malaria vaccine a reality. Non-profit organizations like the Gates Foundation invest in this kind of research, but he said there's still a funding gap.
"The evidence has been with us for 40 years, there is simply not a sufficient effort," he said.
"For us, it's so clear that a vaccine is entirely possible. It's just a matter of doing the work, but we can't do it because we are too few, and there is not sufficient financial support."
Correction: An earlier version of this story said malaria kills 1 million people annually. While that used to be true, in recent years the number has dropped to about half a million.