A nurse administers an Ebola vaccine as part of a clinical trial on February 2, 2015 in Monrovia, Liberia. John Moore/Getty The non-profit organization Gavi, the Vaccine Alliance, bought 300,000 doses of the most promising Ebola vaccine from Merck in January. But the shot hasn't been approved by any country yet.

Why would Gavi — whose members include the World Health Organization and the World Bank — stockpile something that can't be administered to patients yet, and maybe ever?

Well, they're betting the world will need the vaccine if the slow trickle of Ebola cases we're seeing today turns back into a full-fledged outbreak. And part of the $5 million agreement is that Merck will submit the vaccine for approval by 2017. If and when it's approved, Gavi would presumably purchase many more doses.

The vaccine in question, currently called VSV-EBOV for short, has been remarkably successful in clinical trials at preventing people in outbreak regions from getting Ebola, at least so far.

In the first Phase III trial, which published preliminary results in July 2015, the vaccine was 100% effective in several thousand patients, though it only monitored them for a few months. The long-term effectiveness of the vaccine is not yet known.

The researchers expect to publish the final results this spring, study lead author Ana Maria Henao-Restrepo said at the American Association for the Advancement of Science conference on February 12.

But for a highly contagious disease like Ebola, the end of the outbreak presents a difficult challenge for vaccine development.

Dr. Cliff Lane, deputy director for clinical research at the National Institute of Allergy and Infectious Diseases, was the principal investigator of the US arm of the Phase II trial testing VSV-EBOV against another vaccine candidate. He said in a November interview that they had to stop recruiting volunteers in September 2015 for the planned Phase III trial he was leading.

A woman has her temperature taken as part of Ebola prevention in Freetown, Sierra Leone on January 21, 2016. A second case of Ebola emerged in Sierra Leone a day after after health officials declared the epidemic over on January 14. Aurelie Marrier d'Unienville/AP Lane said this was because, thankfully, there weren’t enough cases of Ebola circulating to enroll more patients and test the true efficacy of the vaccine. Scientists can accurately evaluate the protection a vaccine offers only when people are being inoculated in the midst of a real and present threat. Otherwise, it's hard to say how it would perform under the dangerous circumstances where it is needed most.

In order to do proper Phase III studies, Lane said, they have to wait until the next Ebola outbreak.

"To demonstrate clinical efficacy in humans has been very challenging because of the paucity of cases," he said. "Scientifically, it's pretty straightforward to develop a candidate vaccine, but operationally extremely difficult to prove that vaccine is clinically effective because of the few cases."

So Gavi's move to stockpile 300,000 doses of the VSV-EBOV vaccine may be the best move for getting a future outbreak under control. It could also be the ticket to getting the world's first Ebola vaccine approved.