On 2 September, a 39-year-old woman in Bethesda, Maryland, received a novel Ebola vaccine never given to humans before. In as little as 2 months, this same vaccine may go into the arms of thousands of health care workers and other first-line responders to the Ebola epidemic now wreaking havoc in West Africa. No experimental vaccine has ever been on a faster track toward widespread use. “It’s absolutely unprecedented,” said Marie-Paule Kieny, an assistant director-general at the World Health Organization (WHO).

On 5 September, more than 200 “technical experts” convened by WHO recommended bypassing the usual time-consuming regulatory pathways for potential Ebola vaccines and therapies. “We have to change the sense that there is no hope in this situation to a realistic hope,” Kieny said. No one expects a vaccine will slow the epidemic directly, yet it could help keep and attract workers leading containment efforts.

The Bethesda woman is taking part in a phase I trial at the U.S. National Institutes of Health (NIH), one of several small studies getting under way in four countries to assess this vaccine’s safety and the immune responses it triggers. The genetically engineered vaccine she received contains genes for a surface protein from two different strains of Ebola stitched into a harmless chimpanzee adenovirus that serves as a vector. Pharmaceutical giant GlaxoSmithKline (GSK), which is developing the vaccine in collaboration with NIH’s National Institute of Allergy and Infectious Diseases (NIAID), may produce as many as 10,000 doses by the end of the year.

A second Ebola vaccine starting human tests this month uses a similar Trojan horse strategy. Initially engineered by the Public Health Agency of Canada and now being developed by NewLink Genetics of Ames, Iowa, this vaccine contains the Ebola surface protein gene inside a weakened version of vesicular stomatitis virus (VSV), which commonly infects farm animals. Most of the 1500 doses of this vaccine that exist will be used in clinical studies, a company representative says.

Both vaccines have protected monkeys and smaller animals in lab experiments. A study published online on 7 September in Nature Medicine shows that monkeys primed with the NIAID/GSK preparation and boosted by yet another vaccine that uses a vector known as MVA—a modified version of the smallpox vaccine virus—were protected from an Ebola virus “challenge” 10 months after their last vaccination. The researchers, led by NIAID’s Nancy Sullivan, claim this is the first demonstration of “durable” protection from Ebola virus.

If the ongoing safety trials are encouraging—and results will be in as soon as November—health care workers and first responders would be the first to get the vaccines, following the recommendation of another WHO panel in August. Many of these responders have been killed and sickened by the virus, and recruiting people to fight the epidemic has been difficult. But putting the vaccines on such a fast track raises a host of perplexing issues “that the field has to think through very carefully,” says John Mascola, who heads NIAID’s Vaccine Research Center (VRC).

Typically, vaccines that pass phase I safety trials move into expanded phase II trials meant to answer the same questions in more people, finally moving into phase IIb and phase III tests to determine whether they actually work and should go to market. Mascola and other Ebola vaccine developers have long contended that traditional efficacy trials could not be done because past outbreaks have ended quickly with the help of standard infection control procedures such as isolating confirmed cases, testing their contacts, and making sure that all health care workers wear personal protective equipment.

Instead, researchers had planned to take advantage of what’s known as the Animal Rule at the U.S. Food and Drug Administration (FDA). The rule says when efficacy studies “are not ethical or feasible,” FDA will license vaccines for diseases if they work in two animal models and if large-scale phase II studies conducted in humans prove they are safe and trigger immune responses that mirror those in protected animals.

But with today’s fast-moving epidemic, which has spread to five West African countries and killed more than half of the roughly 4000 cases, talk about large phase II studies in unaffected populations has gone out the window. Now, the push is to vaccinate people at high risk and try to gain interpretable data, says Adrian Hill, director of the Jenner Institute at the University of Oxford in the United Kingdom. Hill’s group will soon launch a small safety study of the NIAID/GSK vaccine, and he is confident it will pass muster. “Then it’s decision time,” he says. “Not just whether we go forward but how to go forward.”

Traditionally, efficacy trials randomly assign participants to receive the vaccine or a dummy shot. That’s clearly not ethical here, so some researchers are calling for a “step-wedge” trial, which analyzes what happens to people at similar risk who receive the vaccine at different times. That way those who have been vaccinated can be compared with others who have yet to receive their shots. “You can’t give everyone the vaccine the same day,” Hill notes.

But a step-wedge design could face serious limitations. The rate of spread can differ between sites because one might have, say, better or worse personal protection measures, which could cloud analyses of a vaccine’s shortcomings or strengths. Researchers might also have trouble detecting efficacy if a vaccine offers only partial protection, NIAID’s Mascola says. “If there’s a 50% mortality with Ebola and you use the vaccine and it’s 40% protective, what does that mean?” he asks.

Impassioned debates also surround which experimental vaccine to deploy. As the Nature Medicine study shows, the chimp adenovirus vaccine worked best when boosted with the MVA-Ebola construct. But that’s not being assessed in the initial human trials. Mascola says a company has made the MVA preparation in bulk but it’s not ready for testing.

Thomas Geisbert, a researcher at the University of Texas Medical Branch in Galveston, who helped develop several VSV Ebola vaccines, including the one made by NewLink Genetics, contends “it’s a much stronger vaccine system.” The VSV replicates, unlike the chimpanzee adenovirus vector, stimulating an immune response that Geisbert argues is as good as the one achieved by the prime-boost approach NIAID has championed. “In the context of an outbreak, where you are going to put first responders on an airplane, you don’t have time for a prime-boost,” he says. “You need a single injection.”

But even though this VSV is weakened, safety concerns remain about the possibility that it could cause neurological disease, as it has in some animal studies, or infect livestock. And Anthony Fauci, who heads NIAID, says that the chimpanzee adenovirus vaccine may work well enough without the extra boost, noting that a single injection protected 100% of monkeys challenged with Ebola virus at 5 weeks. “The vaccine was developed to be able to induce immunity really fast,” he says.

Fauci and his NIAID colleagues say it makes sense to test different vaccine strategies in parallel. “We’ve been fooled by trying to translate monkey findings to humans before,” says Fauci, who notes that an AIDS vaccine worked well in monkeys and actually increased HIV infection rates when tested in humans.

Gary Nabel, who headed NIAID’s VRC before leaving in 2012 to join Sanofi in Cambridge, Massachusetts, says manufacturers should massively scale up production now, before trial results are in. “If it were me personally, I’d err on the side of caution, and I’d think carefully about stockpiling doses,” Nabel says. “If it isn’t safe, you throw away those lots. In the worst case you’ve wasted some money.”

If the virus spreads even farther and one of these vaccines does prove safe and effective but is not available, “I think there’ll be a lot of finger-pointing,” Nabel says. “Extraordinary times call for extraordinary measures, and this is an extraordinary time.”

*The Ebola Files: Given the current Ebola outbreak, unprecedented in terms of number of people killed and rapid geographic spread, Science and Science Translational Medicine have made a collection of research and news articles on the viral disease freely available to researchers and the general public.