With major questions still remaining unanswered, WHO says the vaccine tested in Guinea could be registered in 2018.

A major trial has shown a prototype vaccine to be “very effective” against the deadly Ebola virus, according to the World Health Organization that co-funded the study.

Not one of the 6,000 people in Guinea who were given the test vaccine last year contracted the disease within 10 days, WHO said on Friday announcing the final results, without elaborating on what happens after 10 days as Ebola has an incubation period of up to three weeks.

In a comparison group of similar size, not given the vaccine, there were 23 cases in the 10 days, researchers reported in The Lancet medical journal.

“If we compare zero to 23, this strongly suggests that the vaccine is very effective, that it could be up to 100 percent effective,” Marie-Paule Kieny, WHO’s assistant director-general and lead author of the study, told AFP news agency.

Her team of three dozen researchers calculated a 90 percent likelihood during a full-fledged epidemic that the vaccine, known as rVSV-ZEBOV, would work in more than 80 percent of cases.

The new vaccine – initially developed in Canada by public health authorities before being taken over by pharmaceutical giant Merck – is scheduled to be submitted to health authorities in the United States and Europe sometime next year under a fast-track approval process.

“We may have a vaccine which is registered in 2018,” Kieny said at a press conference on Thursday, noting that the standard approval process for a new drug takes a decade, if not more.

Major questions

However, it is still unknown how long the efficacy of an inoculation lasts.

“With the Canadian Merck vaccine, you have a protection very early after vaccination, but we don’t know if it will last after six months,” Kieny said.

Also, initial tests last year did not include children, while the most recent trials covered those over six years old.

Of the more than 6,000 people injected with the Ebola vaccine only two showed serious adverse effects, the study reported. Both recovered fully.

But it is still unknown if the vaccine is safe for children six and under, pregnant women, or people with the Aids virus – all groups that were excluded from the most recent trials.

READ MORE: What is Ebola?

Other Ebola vaccines under development – some of which have been tested in humans – could prove more effective over a longer period.

British firm Glaxosmithkline and Johnson & Johnson, based in the US, each have experimental products in the pipeline.

China and Russia have also developed vaccines, with the Russian one having just finished the second phase of three-step clinical trials.

Some of these vaccines require two doses three weeks apart, and may confer a longer immunity.

“That might be better suited to immunise health workers in advance of an outbreak,” Kieny said.

Health officials also point to the fact that other strains of the virus – including one in Sudan – will require the development of separate vaccines.

First identified in 1976 in what is now the Democratic Republic of Congo, the Ebola virus erupted periodically in outbreaks of up to a couple hundred cases, mainly across west and east Africa.

In early 2014, however, a handful of infections in southern Guinea mushroomed rapidly into an epidemic.

Over the next two years, more than 28,000 people fell ill, mainly in Guinea, Liberia and Sierra Leone. Some 11,300 died.

With a mortality rate above 40 percent, the disease – one of a category of so-called haemorrhagic fevers – has an incubation period of up to three weeks.

It causes violent and painful symptoms, including vomiting, diarrhoea, organ failure and internal bleeding.