Right now, somewhere in the world—in a petri dish in Baltimore, maybe, or in the salivary glands of a laboratory-bred mosquito in Seattle, or in the bloodstream of a villager in Ghana—resides a chemical compound that could help eradicate human history’s biggest killer. Scientists have many promising malaria vaccine candidates in the works, and for the first time one has reached advanced human trials. If it or another candidate is even partly effective in people, it could save the lives of millions of children and pregnant women. It would be the only vaccine yet developed against a human parasite, an achievement of Nobel caliber. And it could, in its first-generation form, be distributed in Africa as soon as 2015.

“If all goes well, five years from today, a vaccine could start being implemented in a wide way in six- to 12-week-old children,” says Joe Cohen, a scientist who is leading some of the most promising research. “It is a fantastic achievement. We are all very proud of that.” This is an extraordinary moment for malaria vaccine research. So why isn’t Regina Rabinovich singing from the rooftops?

Rabinovich is a formidable dark-haired woman with an M.D., an M.P.H. and a résumé that includes a stint as director of the PATH Malaria Vaccine Initiative, as well as her current job as head of infectious disease programs at the Bill & Melinda Gates Foundation. But ask her about all the progress that scientists have made in the past few years, and she pauses.

Rabinovich administers one of the world’s largest funding programs for malaria vaccine R&D, but she will go only as far as to say there are “some things percolating.” Pressed about those things, she cautions that some of them, especially the ones that are still in the early stages of research, are “just doomed to break your heart.” Her painstaking caution makes sense. For all the challenges malaria researchers have overcome, a new one looms now. As they move closer to the first vaccine for the disease, they must prevent their hopes from tipping over into hype.

The malaria community is all too accustomed to cycles of excitement and heartbreak. In the 1960s an enormous campaign wiped out the disease in many parts of the world and drove its numbers down in others. But that success ultimately bred its own end. As malaria came to be perceived as less of a threat, global health agencies became complacent; their chief tool, DDT, was found to be toxic to birds, and they largely abandoned their efforts. Malaria numbers roared back more fiercely than before. Meanwhile scientists left the field, and vaccine research stagnated.

It is surprising—and shameful—that for so long malaria was neglected by funders and thus by scientists who could not get grants to study it. On the other hand, it is easy to see why people lost hope. Malaria was, after all, a particularly tough organism to fight. Its complex parasitic life cycle—which starts in the salivary glands of mosquitoes, moves to the human bloodstream, shifts to the human liver for a sort of adolescence, comes back to the human bloodstream, and finally moves back into the body of a new mosquito—was not well understood until recently. A small group of researchers at GlaxoSmithKline (GSK) made a serious attempt to start up momentum for a vaccine in the mid-1980s, working with a protein from the surface of the common and deadly Plasmodium falciparum strain of the parasite. But their first try failed, and the parasite kept on killing a million people every year.

The circumstances could not be more different today. Thanks to a string of innovations and a huge infusion of cash (largely from the Gates Foundation, which has given $4.5 billion to general vaccine development since 1994 and recently upped its pledge to $10 billion for the next 10 years), dozens of malaria vaccine–related projects are now under way, albeit mostly in early stages. And GSK scientists kept reinventing the candidate that began in the 1980s until they got something more promising that has now reached late-stage human trials. It has been proved safe and is now being tested in a large series of randomized clinical trials at 11 sites in Africa in which one group will receive the vaccine and another will be injected with a placebo only. This is the only vaccine to get this far—ever—but promising preliminary clinical research is moving forward on other candidates.

Some researchers are pursuing an unorthodox strategy in phase I (which mostly involves safety testing): culturing genetically weakened parasites inside the bodies of mosquitoes and delicately dissecting the creatures out of the insects’ spit glands to fashion them into a vaccine. A third category of vaccines would immunize the mosquitoes that transport the malaria parasite to its human victims, using the human body to deliver antibodies. “We’re talking about using people to passively immunize insects,” says Rhoel Dinglasan, one of the pioneers of this approach. “It’s a bit crazy.”

“Crazy,” of course, given enough time and luck and hard work, can turn into something “innovative.” But for any of these vaccine candidates—or the many others in even earlier stages
of development—to succeed, it will have to meet a number of challenges first. The time has arrived to take those challenges on, Rabinovich says: “You don’t progress by hiding your head in the sand.”

Yes, It Works. But How Well?
The candidate made by GSK, called RTS,S, still relies on the same P. falciparum protein as before. But now it has a helper. If all had gone as hoped in the 1980s, this so-called circumsporozoite, or CS, protein, would have served alone as an antigen, the part of a vaccine that provokes the immune system to produce antibodies or other immune responses that attempt to kill the parasite. The approach had worked with a similarly constructed vaccine for hepatitis B. The immune system, however, did not react as planned to the CS protein, and researchers embarked on a 20-year quest to reformulate the vaccine. To elicit a strong enough response from the body, they first had to assemble many copies of the protein onto a chemical scaffold with the aim of eliciting the production of sufficient antibodies. “The idea was to make it look more like the actual pathogen,” says GSK’s Cohen, the scientist who has spearheaded the work on RTS,S.

The body did respond more robustly to this reformulation but still not strongly enough to yield any real protection against the disease (a common problem with many vaccine candidates for all kinds of diseases). Boosting the response further required another breakthrough. After 15 years, investigators succeeded in adding a chemical that increased the numbers of antibody-making B cells. This adjuvant also roped in T cells, which play many important roles in maintaining the body’s defenses against disease.

Today researchers are injecting that formulation in late-stage trials, constituting the largest test of a malaria vaccine ever conducted. A target group of 16,000 children—some between six and 12 weeks old and some a little older, at five to 17 months—have started to receive their vaccinations. By December the researchers will have completed all their injections, and results will begin to roll in during the middle of next year. If those data and a follow-up set seem promising, says Christian Loucq, current director of the PATH Malaria Vaccine Initiative, which is a key organizer of new studies, it will be time to see “the impact of the vaccine in real life.”

That impact could be enormous, saving hundreds of thousands of lives every year—provided that the vaccine is widely distributed. But two possible hurdles exist. The first is expense. All told, developing RTS,S and getting it to market will end up costing hundreds of millions of dollars, so it could be too pricey for practical use in the developing world. But GlaxoSmithKline has said that it will set the price very low, with a small profit of 5 percent and that it hopes that international consortia and organizations such as UNICEF and the Global Alliance for Vaccines and Immunization will buy the vaccine and distribute it to developing countries in Africa where it is most needed.

Second, it is exceedingly unlikely that RTS,S will work as well as most vaccines for other diseases, which generally need to be at least 80 percent effective before they are approved for wide use. So far the best data from phase II suggest that RTS,S reduces cases of malaria by as much as half. Most vaccines with that kind of statistic would never be considered effective enough for widespread use. But malaria is enough of a killer, and the vaccine is so far advanced compared with other candidates, that 50 percent actually looks pretty good—that is still 500,000 lives potentially saved every year.

Very young children face the greatest susceptibility to severe malaria. They have little natural protection against the disease, unlike older victims, who acquire some level of immunity after repeated infections and tend to get milder cases as they age. Unprotected, children may be left with lifelong neurological disabilities. Even if some youngsters become infected after being vaccinated, they may contract milder, nonfatal cases.

The World Health Organization and UNICEF already immunize African infants against other diseases such as polio and diphtheria around the same very young age (three months, give or take) that they would ideally get RTS,S. “We want the malaria vaccine to be integrated into that delivery system,” Cohen says. “We can piggyback on the fact that they’re already giving other vaccines.” That built-in delivery infrastructure might speed up the process of getting the vaccine from bench to bedside—and it would mean that the costs of delivering injections would be fairly low, because it would be possible to forgo new distribution networks. Still, Cohen says, “we need to make sure now that preparedness on the ground is ready. It’s not going to be a trivial thing.” And it is as yet unclear how often those kids would need to come back to the clinic to get booster shots.

The vaccine has other drawbacks, too. First, RTS,S is designed to work in African strains of P. falciparum, not the other malarial strains that circulate around the rest of the globe. Second, that 50 percent efficacy statistic means the vaccine could never be used by itself to completely wipe out malaria—and eradication is the rationale for developing a vaccine in the first place.

To use RTS,S for eradication, researchers will ultimately have to reformulate it—again—or else administer it alongside a different compound. GSK scientists are now contemplating a “prime boost” approach—the two-stage strategy that has shown some hints of effectiveness in early HIV vaccine trials—for their next iteration of RTS,S. Both the “prime” and the “boost” arms of this new vaccine would present the CS protein to the body, but in different ways, possibly yielding a stronger immune response. At least that is how the thinking goes. But the research has taken place only in lab animals so far. If this new attempt to reformulate RTS,S lasts as long as the previous one did, it could be another 15 years before a fully effective version of RTS,S becomes a fixture of public health. “In that time,” Cohen asks, “who knows what else scientists will find?”

Mosquitoes as Living Labs
Short of taking RTS,S and being in the lucky 50 percent for whom it works, there is just one other well-established way to make yourself immune to malaria without actually catching the disease: First, find a swarm of mosquitoes that are carrying weak, genetically damaged parasites. Then let 1,000 or more of them bite you. The parasites may sail down your bloodstream into your liver, but instead of developing into their adult form as they usually would, they will get stuck there and die, unable to mature past adolescence. Meanwhile your body will manufacture antibodies against them, and you will be set for life. U.S. Navy researchers discovered this phenomenon in the 1970s, and two decades later several scientists picked it up and ran with it. Two of them—Stefan H. Kappe of Seattle BioMed and Stephen L. Hoffman, chief executive of Sanaria in Rockville, Md.—now operate mosquito-breeding labs where gloved technicians sit all day extracting weakened parasites from the spit glands of mosquitoes and crushing them into a solution that may be suitable for a vaccine.

Researchers can injure the malaria parasite’s DNA before culturing the microorganism in the bodies of mosquitoes in two different ways. Seattle BioMed’s approach is a precise one: it deletes only the genes that help the parasite mature past adolescence in the human liver. Without these genes, the parasite does not develop further. “It can check into the liver, but it can’t check out,” Kappe says.

Currently Kappe’s team knocks out only two genes, which normally help the parasite build a membrane around itself while it takes up residence in the liver cells. The membrane seems to somehow keep the liver cells from realizing they are infected. Parasites without membranes promptly cause a liver cell to commit suicide rather than playing host to it. In many hundreds of mice, including some genetically engineered to carry human liver cells. Kappe’s team has administered its parasite vaccine and shown 100 percent protection against malaria. There is no damage to the liver in the process—only 10,000 or so parasites enter the body, so even if they all make it to the liver, the maximum amount of lost cells is fairly small compared with the many millions of cells that make up the organ (which, of course, can also regenerate).

This spring, as part of an early-stage clinical trial, 20 people who have received multiple doses of the vaccine will roll up their sleeves and offer their arms to five mosquitoes apiece—all of them infected with what Kappe calls “real malaria,” a strain that would probably need treating if it took hold in the body. Then they will go about their daily lives for a week, checking into a hotel on the seventh day to be thoroughly examined by a clinical team. If they are malaria-free, Kappe says, the team will consider it a sign that the vaccine has worked. If instead they have the parasites in their bloodstream, researchers will remove them by giving them antimalarial medications. “It’s a very powerful tool to be able to actually give people malaria,” Kappe says. “This is a very unique model. You obviously cannot do it with HIV or anything else that is untreatable. But even in the worst-case scenario [if patients get sick], we can treat the infection. How enthusiastic people are to participate in these studies is truly amazing. They are not scared at all.”

The other way to wreck a malaria parasite’s DNA and make it safe fodder for a vaccine is the old-fashioned one: irradiate it. This is the approach taken by Sanaria, Hoffman’s biotech company. It may have advantages, he says. Because radiation scrambles the genetic code in many more sites than two, it may be a safer, more complete way to ensure that the parasite cannot reproduce once it gets to the liver. But Hoffman remains unconvinced that radiation can outdo the precise approach of knocking out specifically targeted genes, and he is experimenting with the latter, too. He will not know which is better until both approaches are “empirically tested,” he says. “There’s no substitute for that.”

Hoffman knows all too well the value of empirical testing. This past summer, during a phase I trial of his irradiated-sporozoite vaccine, he got a reality check. The Food and Drug Administration had given Sanaria the signal to run a trial with 100 subjects. At the time he obtained approval, Hoffman believed that a mosquito transmitting malaria to a human injected somewhere between five and 10 parasites. He based the strength of his doses—how many parasites his vaccine would contain—on that figure. It was only later that scientists realized the mosquito injects a lot more parasites when it bites, somewhere between 300 and 500. “What all that means is by the time we got into the trial, we were probably 10-fold low with our dose,” Hoffman says. “We figured this out in the middle of the study. You can’t really change your design at that point.” Even the too-low doses provided some protection against malaria, Hoffman says, but they were not as effective as RTS,S.

Hoffman, like Rabinovich, remains a realist. For a while, he became depressed, until he talked to vaccinologists and biotechnology mavens, all of whom reacted with enthusiasm. “This is fantastic what you’ve done,” they told Hoffman. “Did you think you were a magician, that you could come in and put this stuff into people and walk out with 100 percent protection? It doesn’t work that way in real life.” Hoffman now hopes to start a new phase I trial. He has increased the concentration of parasites in his vaccine and changed the way it is administered.

The “Immunological Bed Net”
If it does prove too difficult to immunize people against malaria, what about vaccinating the third organism in this unholy trinity: the mosquito? Any vaccine needs to break the cycle of transmission, and until now one step in that cycle has been largely ignored: the point at which the mosquito bites an infected human, picks up the malaria parasite from its victim’s bloodstream and becomes infected itself. If the parasite’s development could be halted at that point, inside the mosquito’s body, it would die, unable to infiltrate a human host. Case numbers would plummet. Dinglasan, a young molecular biologist at Johns Hopkins University (and a native of the Philippines who has seen plenty of malaria in his homeland), has an idea for how to make that happen. He is working in mice only thus far and is appropriately circumspect. “I’m not going to be like a car salesman about it if it doesn’t work,” he says. If it does, though, it will mark a genuine shift in combating malaria.

When the malaria parasite enters a mosquito’s body, it immediately tries to make itself at home in the insect’s gut by seeking out a specific enzyme in the digestive tissue, an aminopeptidase. If it does not find that enzyme to establish a beachhead in the gut in the first 24 hours, it gets digested, and the mosquito fails to become an incubator for the parasite. (At least people assume that is what happens, adds Dinglasan with a laugh: “No one’s really looked at the mosquito poo to check.”) If mosquitoes receive a meal of antibodies against the aminopeptidase, they come away with protection against malaria. The theory: the antibodies mask the enzyme, hanging around and preventing the parasite from targeting it. Dinglasan has isolated a specific fragment of the enzyme unique to mosquitoes and injected mice with that fragment only, causing them to make antibodies against it. Mosquitoes that bite those mice then pick up the antibodies, which do not appear to degrade significantly in the digestive tract. The insects, in effect, become immunized by unwittingly eating a vaccine—and because the parasite dies inside of them, it does not get transmitted to mammalian hosts. If the concept works in humans, too, voilà, Loucq says: “It’s like an immunological bed net.”

The approach has downsides, of course, chief among them the challenge of getting people to accept a vaccine that protects mosquitoes but not people—not directly, anyway. (You could get the vaccine and still be infected by a mosquito that first picked up malaria from someone who was not immunized.) Yes, eventually, the disease burden will drop as there are fewer infected vectors buzzing around, but at first a lot of people might get vaccinated and still get sick, and there could be side effects in people who had previously felt fine. That would in some sense violate the first rule of medicine: “do no harm.”

Still, precedent exists for taking vaccines to protect other people (such as men who get immunized against the human papillomavirus: their risk of catching it is low to begin with, but by getting the vaccine, they protect their sexual partners). And in the long term, a transmission-blocking vaccine could be equally or more effective than a traditional one that immunizes the person to whom it is given. “People say there’s no direct benefit,” Dinglasan says. “The fact of the matter is that there is a benefit. It’s just delayed.”

The aminopeptidase approach also has benefits that no other vaccination strategy can boast. For technical reasons, it would probably be more “scalable”—translation, cheaper to mass-produce—than RTS,S or the mosquito-cultivated vaccines at Sanaria and Seattle BioMed. The antigen, it turns out, shows up in all 40-some mosquito species that transmit malaria, so the vaccine should work in all of them. (“Is that lucky?” Dinglasan says. “Yes. Completely lucky.”) And the antibodies seem to work against both P. falciparum, the common African type of malaria parasite, and P. vivax, more commonly found in Asia. RTS,S does not work against P. vivax, because the CS protein targeted by the vaccine differs between the two parasite species.

Tests by Dinglasan so far against P. falciparum in mice have shown 100 percent effectiveness and a tally of 98 percent in combating the P. vivax found in Thailand. That matters for practical reasons, because ideally, a malaria vaccine needs to be universally useful. “The truth of the matter is, it’s too expensive to make many separate vaccines,” Dinglasan points out. “The coffers of the donors, the Warren Buffets and the Bill Gateses—people may think they’re infinite, but they’re not.”

Dinglasan has a long way to go yet before the coffers open wide for him. At the moment, he is only in the “feasibility stage,” trying to see how much antigen his lab can make. He says he should have some solid results by February. Then he will allow himself to think about practical applications. Could this vaccine be used in combination with RTS,S? How much of it would a mosquito need to suck from the human bloodstream to become immunized? How long might it take to move it from mice to people?

Dinglasan will be answering these questions as a much larger one looms: What, really, is needed to eradicate malaria around the world, once and for all? Without a strong vaccine, it is already possible to control the disease with bed nets and medications that can either prevent or treat it, such as chloroquine, artemisinin and malarone. But it is probably impossible to eliminate it entirely. Bed nets fail; mosquitoes may develop resistance to the insecticides the nets are treated with, and users may not want to sleep under a malaria-resistant curtain all the time. “The bed net story is beautiful, and if they’re used in a highly controlled environment, they can prevent transmission,” Dinglasan says. “But humans tend to be poor at following directions. Have you ever lived in these countries? I have. Yes, most of the kids are under the bed nets. But the adults are drinking outside of the hut, and alcohol makes you more attractive to mosquitoes. We’ve also seen that the insecticide-treated nets kill the indoor mosquitoes, but then the outdoor mosquitoes take over the niche.”

As for preventive meds, they are better for travelers than they are for people in the developing world: they can be unpleasant and expensive, and they would have to be taken constantly. Some other steps that helped to rid the developed world of malaria—draining swamps, for instance, or widespread spraying of DDT—would be impractical in the developing world.

Then again, those are only the options on the table today. Scientists are busy studying the malaria parasite’s genome and certain aspects of the human genome that may offer some resistance; new and surprising leads may well come from those projects. And there is even talk of malaria-control strategies that sound outlandish now, such as releasing genetically engineered, malaria-resistant mosquitoes into nature to compete with the wild type. Ten years ago, of course, the idea that we would be this close to even a partly effective malaria vaccine would have sounded outlandish, too.

The key, Dinglasan says, is making sure that the global health community stays engaged for the long haul. “The current leaders of the malaria community have told me they don’t even know if my generation will see [the vaccination effort] through,” he says. “It might be the next generation. That’s how long we’re thinking. Will the world stay interested? Will it hang in there for that long?” One thing, at least, is sure—the malaria parasite will.