Healthcare workers and others at high risk from Ebola in Liberia could be given an experimental vaccine as early as next week following the shipment of the first doses to Monrovia, according to the manufacturer GlaxoSmithKline.

The potential vaccine has been developed with unprecedented speed, but the rapid decline in Ebola cases in Liberia may make it hard to find out whether it works. Last week, there were only eight cases in the country, the World Health Organisation said on Thursday, down from a high of more than 300 a week in September.

GSK’s vaccine is the first of several under development to enter trials in west Africa. Trials of healthy volunteers in Europe, the US and unaffected African nations have shown it is safe and produces a response from the immune system. The question is whether that response will protect vaccinated health workers and burial teams from infection or enable them to fight off the disease.

The trial is being carried out by the National Institutes of Health in the US, which will vaccinate 30,000 people. Only one-third will get the candidate vaccine; others will get a routine vaccine against another disease, such as measles. The investigators will be looking to see whether there are further cases of infection in the control group than among those who were given the potential Ebola vaccine.

Dr Moncef Slaoui, chairman of global vaccines at GSK, said getting to the point of shipping the vaccine, due to arrive in Liberia on Friday, was a major achievement.

He said: “The initial phase I data we have seen are encouraging and give us confidence to progress to the next phases of clinical testing, which will involve the vaccination of thousands of volunteers, including frontline healthcare workers. If the candidate vaccine is able to protect these people, as we hope it will, it could significantly contribute to efforts to bring this epidemic under control and prevent future outbreaks.

“It is important to remember that this vaccine is still in development and any potential future use in mass vaccination campaigns will depend on whether the WHO [World Health Organisation], regulators and other stakeholders are satisfied that the vaccine candidate provides protection against Ebola without causing significant side-effects and how quickly large quantities of vaccine can be made.”

The company acknowledged that the declining number of cases in Liberia will make it harder to find out whether the vaccine works.

“Clearly it is good news that new cases of Ebola appear to be falling and we hope this trend continues,” said a spokesperson. “Lower incidence of Ebola does increase the chance that the planned trials may take longer to complete, and if cases drop very low, that they may be not be able to answer the question about whether the investigational vaccines are able to offer protection from the disease. However, we are absolutely committed to going ahead as planned to see if we can gain the information we need to help with this outbreak or prevent future outbreaks.”

The chair of GSK, Sir Andrew Witty, said on BBC Radio 4’s Today programme that the company had disrupted the production of other vaccines in order to speed through the vaccine against the Ebola virus. “We have delayed two other vaccine programmes to do this work,” he said.

Dr Jeremy Farrar, director of the Wellcome Trust, which is funding a trial of the vaccine in UK and Mali and parallel studies of other vaccines in Geneva, Gabon, Kenya and Guinea, said: “This week we’ve heard encouraging news from west Africa, indicating that we may at last have reached a turning point in what has been the worst Ebola outbreak in history. Mali has been declared free from the disease and schools in Guinea reopened, restoring some sense of normality for those communities.

“But the disease continues to take a terrible toll elsewhere in the region, particularly in Sierra Leone. This is certainly not the time for the national and international efforts to be reduced. If anything, efforts need to be redoubled to bring the epidemic to a complete end. Which is why it’s so important and heartening that the first doses of a potential vaccine are now making their way to west Africa in preparation for large-scale trials. There is no doubt that we need vaccines and therapeutics for this epidemic and to try to prevent and respond to the inevitable future epidemics.

“The unprecedented speed at which the vaccine preparation has progressed would not have been possible without sheer determination and global partnership between national governments, funders, researchers and pharma companies, and agencies on the ground who have worked tirelessly to get this crisis under control.”