The experimental Ebola vaccine to be deployed in an outbreak in the Democratic Republic of Congo has conquered some major scientific hurdles in giving high protection, but it now faces extreme real-world tests including heat, humidity, language barriers and lack of roads.
Because it is not yet licensed, the Merck & Co vaccine has been offered to the DRC under a "compassionate use" protocol agreed upon by national and international health and ethics authorities.
This means fully informed, signed consent is needed from every person who wants the shot. And in the current Ebola outbreak, that makes logistical, cultural and language barriers the ultimate challenges, global health specialists say.
The hurdles illustrate how hard it can be to move from laboratory to real life, especially in remote communities with no functioning health systems. The DRC outbreak is a chance to reality-test a vaccine against a disease epidemic that cannot be replicated in controlled environments.
"This is going to need a highly sophisticated operation in one of the most difficult places on earth," said Peter Salama, the World Health Organisation's deputy director-general for emergency preparedness and response.
"It's very hot and very humid, and we're talking about hundreds of kilometres of densely forested areas."
The shot is designed for use in a so-called "ring vaccination". When a new Ebola case is diagnosed, all people who might have been in recent contact with them are traced and vaccinated to try and prevent the disease's spread.
The vaccine supplies so far will be enough to vaccinate 50 rings of 150 people, according to the WHO. It said that as of May 15, 527 contacts of Ebola cases and suspected cases had been identified and were being followed up.
Health workers will need to use translators for several local languages and explain the vaccine to leaders from different communities, Mr Salama said. Limited communications, health facilities and electricity, as well as the need to keep the vaccine in a "cold chain" at -60 to -80 degrees Celsius will also present challenges.
If any of these elements fail, the vaccine's potential to protect 100 per cent of those immunised will go unrealised.
"These are make or break issues," said Mr Salama, who visited the DRC last week. "There's a lot of complex logistics and social science here."
Results of a trial using the ring vaccination technique with the Merck shot, which is known as VSV-EBOV, in Guinea in West Africa in 2015 showed 100 per cent protection in those vaccinated immediately.
Health experts working to contain the outbreak in DRC's Equateur province, which was first reported on May 8, say a cold chain will be in place to get the vaccines from the capital Kinshasa to the affected areas within a few days. Then the shot could be given to local frontline medical, burial and hospital workers who volunteer for it as early as next week.
Experts advising the WHO said on Friday that they believe the outbreak can be brought under control and is not an international public health emergency. The UN health body had said earlier on Friday that there was a "very high" public health risk to the country and a "high" risk to the region after the first confirmed Ebola case in Mbandaka, a city of about about a million people. Three cases were later confirmed in the city, which is connected to Kinshasa by the Congo River.
There have been more than 40 suspected, probable or confirmed Ebola cases in this outbreak, and 23 people have died.
Jeremy Farrar, a specialist in infectious diseases and director of the Wellcome Trust global health charity, said the epidemic now had "all the features of something that could turn really nasty".
"You can't over-respond in this scenario," he said. "But the vaccine must be seen in the context of an overarching public health response. Critically that means early diagnosis, early isolation, safe burials and understanding the social context. The vaccine can only be a part of the solution."
Experts also caution that acting too hastily could jeopardise the potential success of a vaccine deployment.
Micaela Serafini, a medical director for the international charity Medecins Sans Frontieres who is helping coordinate the response to the DRC outbreak, said its teams are planning for at least 45 minutes of discussion and information-sharing with each person, with a translator present, before signed consent would be obtained.
She said medical teams would then probably return the following day to administer the vaccine.
"What we need to avoid at all costs is an uncontrolled situation in the communities affected," she said.
Even though the vaccine has yet to get a licence, the emergency response teams say its safety and efficacy data is strong. And despite lingering suspicions in some of the more remote parts of Africa of western medicines, experts anticipate widespread public acceptance.
The DRC's health minister Oly Ilunga signalled on Thursday that his government was fully behind using the vaccine.
"The vaccine will help us save lives in the Equateur province, in the DRC and in neighbouring countries. The vaccine allows us to limit the virus, so we must use it," he said.
Merck and the GAVI vaccine alliance have said a stockpile of more than 300,000 doses of the shot is available for emergency use in an epidemic.
Mr Salama and Ms Serafini said one tricky task may be managing a scenario where larger groups of people not identified as high-risk contacts of a Ebola case might demand the vaccine for themselves or their family.
"We want to make sure we are engaging whole communities so that the broader community understands what we are doing and why," said Mr Salama. "That is particularly important when you're not targeting everyone, because naturally people will ask: 'How come you're vaccinating that person but not me?'"
