Uganda has begun an emergency vaccine trial to halt an outbreak of a deadly Ebola virus strain, only days after the first case was spotted.
The World Health Organization (WHO) said the trial had been set up with "unprecedented speed" in what could be a test run for quickly dealing with future disease outbreaks.
Some 40 people linked to a nurse who died with the haemorrhagic fever last week will be the first to receive the new vaccine, in the hope it will create a firebreak that halts any further spread.
They include people who have been in direct contact with the dead man and 'contacts of contacts'.
Contacts of two other infected people will also be vaccinated in the coming days, in what experts call a series of 'vaccination rings' around cases.
Uganda late last week identified an outbreak of the Sudan strain of Ebola, for which there is currently no licenced vaccine.
The strain kills as many as half of those infected and has struck Uganda five times before.
The most recent previous outbreak, in September 2022, killed more than 70 people before it was halted.
Full trials of potential vaccines have in the past been difficult to organise because the virus flares up, often in remote areas, before disappearing.
Uganda had access to the same candidate vaccine during the last outbreak, but trials could not be turned around in time.
Vaccine stocks were put in place ready after the last flare up, meaning researchers have this time managed to get the trial ready only four days after a case was first identified.
Dr Tedros Adhanom Ghebreyesus, WHO's director general, said: "This is a critical achievement towards better pandemic preparedness, and saving lives when outbreaks occur."
Preliminary trials have found the vaccine is safe and it creates a strong immune response, but it has not been tested in the field on an outbreak.
Christophe Fraser, Moh Family Foundation Professor of Infectious Disease Epidemiology at Oxford's Pandemic Sciences Institute, said setting up vaccine rings was a standard response to outbreaks of emerging infections that are relatively slow moving.
Ebola is transmitted person-to-person through contact with infected bodily fluids, such as blood and vomit.
Infections start with fever, fatigue, aches and pains, diarrhoea and vomiting. They can then go on to haemorrhage and bleed from bodily orifices.
Healthcare staff or relatives who come into contact with bodily fluids while they are tending to the sick, or preparing the dead, are often most at risk.
Prof Fraser, who is helping model the trial, said: "It's a real challenge to get vaccine trials started quickly enough."
The vaccine candidate is being provided by the International Aids Vaccine Initiative (IAVI).
Mark Feinberg, IAVI president, said: "Critically, having vaccine doses readily available in country made it possible for us all to mobilise in just days and to be able to incorporate evaluation of a promising Ebola Sudan vaccine clinical trial into the public health response very soon after the first reports of the current outbreak."
The WHO, Uganda's ministry of health, the Uganda Virus Research Institute, and the Makerere University Lung Institute are all working on the trial.
Experts say that accurately finding and tracking those who might have been exposed is key to stopping the outbreak.
The 32-year-old male nurse who died had sought treatment at a hospital just outside the capital, Kampala, and later travelled to Mbale in the country's east, where he was admitted to a public hospital.
Health authorities said he also sought the services of a traditional healer.
A vaccine has already been licenced for the separate 'Zaire strain' of Ebola, but that vaccine is not effective with the Sudan strain.