A steep fall in Ebola cases in Liberia will make it hard to prove whether experimental vaccines work in a major clinical trial about to start in the country, the head of the U.S. National Institutes of Health (NIH) said on Saturday.

The NIH might have to move some testing to neighboring Sierra Leone, while regulators could end up approving Ebola shots based on efficacy data from animal tests backed by only limited human evidence, Francis Collins told Reuters.

Liberia, once the epicenter of West Africa's deadly Ebola epidemic, has just five remaining confirmed cases of the disease, a senior health official has said.

The sharp decrease in cases is clearly good news, but it poses a problem for scientists from the NIH, GlaxoSmithKline and Merck, who want to enroll 27,000 people at risk of infection in the pivotal Phase III Liberian study.

"It's going to be a hard trial," Collins said on the sidelines of the World Economic Forum in Davos. "It's possible we may have to move some of the effort to Sierra Leone, which is unfortunately in not quite such a good position as Liberia."

The big Liberian trial, the first of several planned for West Africa, aims to enroll at-risk people such as healthcare staff, family members and burial workers. It will test a GSK vaccine, a rival one from Merck and NewLink, and a placebo.

"It may, at this point, be hard to find 27,000 people at risk," Collins said. "It is going to be challenging."

Nonetheless, vaccines could still be submitted to regulators using efficacy data from non-human primate experiments, plus proof of safety and immune system response in humans.

"That is the default and certainly the FDA (U.S. Food and Drug Administration) has that particular pathway available. If it is not possible to get the rigorous human data, it is still possible a vaccine could be approved," Collins said.

Healthcare experts meeting in Davos this week have stressed the need to keep up the fight against Ebola until there are zero cases in West Africa, where more than 8,600 people have died from the disease.

Jeremy Farrar, director of Britain's Wellcome Trust health charity, said vaccines and drugs were still needed for the current epidemic and to fight future ones.

Johnson & Johnson, working with Bavarian Nordic , also has an Ebola vaccine in earlier-stage clinical tests.