The first batch of an experimental Ebola vaccine is heading to Liberia and is expected to arrive Friday.

The shipment of 300 vials, produced by British drug maker GlaxoSmithKline (GSK) and the U.S. National Institutes of Health, is the first potentially preventative medicine to reach the country.

GSK hopes to immunise the first volunteer within the next few weeks. Healthcare workers tending Ebola patients will be among the first to receive it, and researchers hope to enroll up to 30,000 people, about a third of whom would receive the vaccine, the Guardian reports.

It’s being tested in trials involving 200 healthy volunteers in Britain, the U.S., Mali and Switzerland first and has been declared safe so far. However, it won’t be taken out of the development stage for mass immunisation until the World Health Organization and other regulators decide it’s both safe and effective. The real test will come when it’s used in affected countries.

Some experts have said that the fact Ebola cases are falling will make it difficult to establish whether the jab offers any protection.

“Because case numbers are starting to come down it will become harder and harder to show if the vaccine is having any impact,” Prof Jonathan Ball from Nottingham University said.

GSK’s chief executive, Sir Andrew Witty, told the BBC they had “delayed two other vaccine development programmes to free up the space to do this work.”

The World Health Organisation has reported over 8,000 deaths from the virus.

The latest WHO #Ebola situation report is available here http://t.co/QldaARcg3j pic.twitter.com/UC1rTfDdNl

— WHO (@WHO) January 22, 2015

Other vaccines, from Merck, as well as Zmapp, are also in development.

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