Last August, scientists injected a potential vaccine for Zika virus into a human being — just 3 1/2 months after they had decided exactly what molecular recipe to use.
In the world of vaccine development, 3 1/2 months from design to injection is “warp speed,” says vaccine researcher Nelson Michael of the Walter Reed Army Institute of Research in Silver Spring, Md. Clinical trials can take years and epidemics can burn out before vaccines make it to doctors’ shelves. Even vaccine creation is typically sluggish.
But in this case, the vaccine is a bit of DNA, which means scientists can get moving fast. Unlike some traditional methods, DNA vaccines don’t use dead or weakened viruses. Instead, they rely on a snippet of genetic material. This “naked” DNA carries, for example, the blueprints for Zika proteins. It’s just a long sequence of DNA blocks.
With DNA vaccines, “it’s easy to move very quickly,” says Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases in Bethesda, Md. “All you need to do is get the right sequence, and Bingo! — you’re there.”
Historically, though, DNA vaccines have been deviled with drawbacks. “They work absolutely fantastically in mice,” says infectious diseases physician Anna Durbin of Johns Hopkins Bloomberg School of Public Health. But “they fail miserably when we use them in humans.”
Researchers at the infectious diseases institute will soon begin the second phase of human clinical trials for a DNA vaccine candidate for Zika, vaccine clinical researcher Julie Ledgerwood said February 6 in Washington, D.C., at an American Society for Microbiology meeting on biothreats. The virus made headlines last year as it continued its tear through the Americas, and scientists confirmed its link to birth defects, including microcephaly (SN: 12/24/16, p. 19). Ledgerwood hopes to see efficacy data on the vaccine by the end of 2018.
“Ultimately, we want a vaccine that can prevent congenital Zika infection,” she said. “We think the DNA vaccine platform is an opportunity to do things safely and very quickly.”
Government researchers aren’t betting everything on DNA, though, Fauci points out. “We’ve got multiple shots on goal here,” he says. A slew of other vaccine candidates, based on both traditional and new techniques, are also in the works. But the DNA vaccine has stepped up to the plate first, and the world will soon see if it can deliver.
“If it works,” Durbin says, “we’ve hit a home run.”
Making a DNA vaccine is simple, in principle. Scientists synthesize genes from a pathogen, insert them into a circular strand of DNA called a plasmid, make lots of copies and then inject the purified plasmid into a person. “You can literally build a DNA vaccine in weeks,” says Dan Barouch, an immunologist at Beth Israel Deaconess Medical Center and Harvard Medical School. The approach is flexible, too, he adds. Researchers can tinker with the DNA building blocks in the plasmid, adding bits from other viruses that might ultimately enhance the immune response.
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For a DNA vaccine against Zika, scientists insert genes for Zika proteins into a circular piece of DNA called a plasmid. Many copies of the plasmid are injected into muscle. Some of the DNA travels into cells’ nuclei, where it is used to make messenger RNA. After exiting the nucleus, mRNA helps build Zika proteins, which can form viruslike particles that trigger the immune system to make antibodies.
Barouch’s team was the first to report a Zika DNA vaccine that offered protection in mice — in a study published last June in Nature. Five weeks later, he and colleagues reported in Science that the vaccine, and two others created via different strategies, worked well in monkeys, too. And in September, a team led by government scientists, and including Barouch as a coauthor, came out with two additional DNA vaccine candidates, described in Science.
It’s one of those additional candidates, called VRC 5283, that the infectious diseases institute plans to test in a Phase II trial. The trial will help researchers figure out the precise dose and number of injections to use. VRC 5283 includes the blueprints for making two Zika virus proteins, as well as DNA from Japanese encephalitis virus.
When the vaccine is injected into the body, a small amount of DNA makes its way into cells and on into the cell nucleus. There, molecular machinery reads the DNA and writes a message in RNA. When the message leaves the nucleus, it serves as a how-to guide for making Zika proteins. The proteins assemble into viruslike particles that trigger alarm bells in cells, which marshal their defenses. Cells then know the face of the enemy and are ready to fight if Zika invades.
At least, that’s the idea. DNA vaccines are hardly a new concept, Barouch says. “People have been calling them the ‘vaccines of the future’ for decades.” But they haven’t yet lived up to the hype.
Scientists have created DNA vaccines for dozens of pathogens, but so far, not one has been licensed for use in humans. One problem is that scientists need massive doses of DNA to provoke an immune response — a few milligrams or so. “That is a god-awful amount of plasmid DNA,” Michael says. It’s so much DNA that the liquid of each dose is viscous, he says. “It’s like syrup.”
Naked DNA doesn’t readily travel into the nucleus, so scientists dump a lot in the bloodstream to ensure that some winds up inside. The Phase II trial of VRC 5283 will test both four and eight milligrams of DNA, and people will receive three immunizations, each spaced weeks apart, Ledgerwood said. The best dosing regimen then will be used in the second part of the trial — a test to see how VRC 5283 performs in thousands of participants in regions likely to see Zika outbreaks.
But even if the vaccine eventually ends up in clinics, ensuring that patients come back for multiple doses won’t be easy, says University of Pennsylvania immunologist Drew Weissman. Giving people one shot is hard enough, he says. “Giving them two more immunizations is an absolute nightmare.”
Weissman and colleagues at the infectious diseases institute and elsewhere are working on a different kind of vaccine that could make multiple doses moot. Like the DNA vaccine for Zika, Weissman’s uses genetic material. But instead of DNA, his vaccine relies on modified versions of messenger RNA — that how-to guide for making proteins.
Unlike DNA vaccines, those made of messenger RNA don’t have to stop in the nucleus first. That makes these vaccines more efficient, Fauci says. The modified Zika RNA vaccine was enough to protect monkeys from the virus five weeks after vaccination, Weissman and colleagues reported online February 2 in Nature. The dose was just 50 micrograms — roughly a hundredth as much as a single dose of the DNA vaccine.
On February 17, a different team of researchers reported online in Cell even more RNA vaccines for Zika. The vaccines protected mice from the virus, and some even reduced the severity of a subsequent dengue infection.
Scientists still need to test RNA vaccines in humans to gauge how they stack up against other candidates, Michael says. “But the bottom line is this: If a single shot works and lasts a long time, that would be a game changer.”
One of the RNA vaccines reported in Cell began a clinical trial in December, but trials for Weissman’s vaccine are still 12 to 18 months away. In the meantime, other vaccines are charging forward. The biotech company Inovio Pharmaceuticals, for example, has begun human trials with yet another DNA vaccine for Zika. (It comes with a little zap of electricity, which blasts tiny holes in cell membranes to help DNA slip in, researchers reported November 10 in NPJ Vaccines.)
And Michael’s team at Walter Reed has partnered with Sanofi Pasteur on a more traditional approach. Researchers grow vats of virus, kill it, purify it and then use the killed virus in the vaccine. It’s the same way Jonas Salk tackled polio in the 1950s. These “inactivated virus” vaccines are generally very safe, Nelson says, because the virus is “as dead as a doornail.” Nelson expects data from three Phase I clinical trials for the vaccine, called ZPIV, in early April.
But for a vaccine with both durability and efficacy, Fauci says, the gold standard is a live-attenuated vaccine. Such vaccines, like the one for measles, mumps and rubella, use weakened rather than killed viruses to rile up the immune system.
“It’s the broadest, best type of protection — lifelong, we think,” says Durbin, who is part of a team developing a live-attenuated vaccine for Zika. The downside is that scientists have to make sure that the weakened vaccine is harmless. Even then, Durbin says, “we would never consider giving a live-attenuated vaccine to a pregnant woman.”
When exactly scientists have a working Zika vaccine ready for use “is totally dependent on the outbreak situation in South America and Puerto Rico,” Fauci says. If new infections don’t crop up over the coming spring and summer, scientists may have to wait years to collect the efficacy data needed for vaccine approval.
But the race to make a Zika vaccine probably won’t come down to just one winner, he says. Having several kinds of vaccines in play would give public health officials flexibility: more weapons to fight the virus and an opportunity to tailor the response to different populations.
In fact, the fevered quest for a Zika vaccine isn’t really a race at all, Barouch says. “We’re all working together.”