A new Ebola vaccine shows promise. But there's good reason for caution.

Exactly a year ago, West Africa was battling the most devastating Ebola outbreak in human history.

Now, after nearly 30,000 cases and 12,000 deaths, there's finally some good news: Researchers think they might have a vaccine that can protect people from the deadly virus.

The rVSV-ZEBOV vaccine candidate, discovered by researchers at the Public Health Agency of Canada and now under development by the drug company Merck, is currently being studied in Guinea. Promising preliminary results from the trial were published in The Lancet on Friday: Everyone who got the shot immediately after contact with an Ebola victim didn't get the virus.

The initial results seems shockingly good

A woman gets vaccinated during the first clinical trials of the VSV-EBOV vaccine. (Cellou Binani/AFP/Getty Images)

This study, which was led by the World Health Organization, suggests the vaccine is 100 percent effective — a finding that had the global health community excited.

As Jeremy Farrar, director of the Wellcome Trust, wrote in the Guardian today, "I normally like to avoid superlatives when describing the interim results of a medical trial, but it is difficult to talk about the report of the experimental Ebola vaccine in the Lancet as being anything less than spectacular." He continued: "It is in fact incredibly unusual for any vaccine to show such efficacy, and so swiftly, too."

Here's how the trial worked: Between April and July this year, researchers vaccinated more than 4,000 people — all contacts of individuals who got the Ebola virus.

Because the number of Ebola cases in Guinea had declined, they had to be creative about how to design their study and decided on a "ring vaccination" approach. Used to develop the smallpox shot in the 1970s, ring vaccination involves immunizing the immediate contacts — friends, family, housemates, neighbors — of a person who falls ill with a virus with the aim of creating a protective ring around them to stop transmission.

For the study, 48 clusters (or rings) were vaccinated soon after an Ebola case was detected. Another 42 rings were given the shot three weeks later — considered a control group to compare with those who immediately got immunized. The researchers then looked at how many people in each group tested positive for the virus 40 days after vaccination.

Of the 2,014 people who were vaccinated immediately, no one got Ebola

Of the 2,014 people who were vaccinated immediately, no one got Ebola. In the other group — 2,380 individuals who got shot three weeks later — 16 people fell ill. These results, researchers think, show that a single shot of the vaccine can protect people from the Ebola virus.

The results were so positive, in fact, the researchers changed the trial design going forward so that everyone gets the vaccine immediately with no delay.

What's more, the vaccine also seemed to be safe: Of the thousands vaccinated, people had 43 serious adverse health events, but only one — a bad fever — was determined to be caused by the vaccine.

There are reasons for optimism — and caution

But there is reason to be cautious about the findings. Again, these are only preliminary results. The trial isn't even over, and it can't yet tell us how long the vaccine's protection lasts. It's not clear, for example, that those who got immunized will still be safe from Ebola six months or a year from now, and that there are no long-term side effects. So researchers will need to follow up to study the vaccine's efficacy and safety over time.

There's also some concern about limitations of the ring trial design. The randomized control trial — where you take two groups of people, give one the medical intervention, and see how the two groups fare — is the gold standard for clinical trials. Researchers at the US Food and Drug Administration expressed concern that the results from a ring design may not be valid, according to the Wall Street Journal.

"A positive first step. ... These results are encouraging but not definitive" David Heymann, the director of the Chatham House Centre on Global Health Security, said he's simply looking at this Lancet publication as "a positive step." He added: "These results are encouraging but not definitive. No agency would license a vaccine based on this study."

What's more, he pointed out, in its current form the vaccine has to be kept very cold, at minus 60°C. This is not a practical vaccine for the hot climate of Ebola endemic regions, which also lack basic health infrastructure.

"If it continues to show promising results," said Heymann, "it would be proof of concept that a vaccine could be developed. Then there would need to be optimization of the vaccine so that it could be transported easily and used effectively in humans."

What it would take to get the vaccine on the market

For now, a lot of hope is riding on this vaccine; it's the only candidate that's gotten this far into clinical trials. (Other treatment and vaccine candidates have failed because Ebola cases in West Africa dwindled to numbers that were too small to study.)

But we're still a long way from actually getting an Ebola vaccine on the market. When the trial is finished, researchers will analyze the findings and write them up, and their results will need to pass scientific peer review and publication.

If the vaccine continues to show promise after that, Merck would need to figure out how to produce and optimize their product, as Heymann noted, in a way that makes sense for the developing countries that suffer most with Ebola. Merck would then need to decide whether to apply for marketing approval and present its data to health regulators, who may or may not accept it.

If accepted, regulators would need to figure out how to best license the vaccine. If the shot seems to protect people only for a short while, it might be a good vaccine for an outbreak. If the immunity is long term, the vaccine might be licensed for prevention and administered outside of outbreaks to those most at risk for Ebola, such as health workers.

Whatever happens next, this Ebola trial is historic

According to John-Arne Røttingen, director of the division of infectious disease control at the Norwegian Institute of Public Heath who worked on the study, "This is the only vaccine trial that will demonstrate effectiveness in this outbreak, and it's because of the ring vaccination design."

In other words, if the researchers relied on traditional study methods, the trial would have taken much longer and needed many more participants than would have been feasible in Guinea — a key reason other Ebola vaccine and drug candidates have already failed.

So, he said, "The weaknesses in this study design are also what made it possible to do the trial." This means the health community will be watching the results of this trial very closely — not only for the public health implications, but also for what they can tell us about how to best search for a vaccine or cure during an outbreak.

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