A second Ebola vaccine is to be offered to around 50,000 people in the Democratic Republic of Congo, as part of a major clinical trial.
The Johnson & Johnson (J&J) vaccine will be used alongside a vaccine made by Merck, which has already been given to around 250,000 people.
Merck's jab has now been approved by the World Health Organization.
More than 2,100 people have died in DR Congo, in the second largest Ebola outbreak on record.
The WHO's approval of Merck's one-dose vaccine is based on it being satisfied of its safety and efficacy. The European Commission has also approved the vaccine.
J&J's vaccine, which requires two doses given 56 days apart, will be available to adults and children over one, living in two areas of the DR Congo city of Goma, where there is no active transmission of the deadly disease.
Goma, which has a population of one million people, is on the border with Rwanda and has a major international airport.
Prof Daniel Bausch, director of the UK Public Health Rapid Support Team, is one of the scientists leading the trial.
He said there was "no contest" between the two vaccines, and both had their advantages and disadvantages.
Merck's, which is given to those who come into direct and indirect contact with an Ebola patient, could be best used in the middle of an outbreak - while the J&J vaccine could be used to protect people not yet exposed to Ebola.
Prof Bausch said: "The J&J vaccine is not ideally suited to an outbreak setting, primarily because it requires two doses to provide the optimal immunity."
But he said the vaccine "may provide longer-term immunity, and may be associated with fewer side-effects than a live virus vaccine, like the Merck one".
The number of new cases in DR Congo's Ebola outbreak has dropped significantly since its height in the spring, but there are still around 20 new infections reported every week.
Dr Jeremy Farrar, director of the Wellcome Trust, which is helping to fund the J&J trial was a "critical step forward".
"Previous trials have shown that the J&J vaccine produces an immune response, a good indication it will be protective against the Ebola virus."
Dr Eteni Longondo, Minister of Public Health in DR Congo, said: "It's vital that we intensify our efforts. That's why we're working with international partners to provide our response teams with another tool to fight and ultimately stop the spread of this terrible disease."
The outbreak has been fuelled by misinformation and rumour, along with an extremely difficult security situation.
Around 200 health facilities have been attacked since the outbreak began in August 2018, so introducing a new vaccine into already sceptical communities is a major task.
Immunisation teams from the medical charity Médecins Sans Frontières (MSF) will administer the vaccine.
John Johnson, MSF's project lead on Ebola vaccination, said: "Using two different vaccines in nearby areas might result in misconceptions, which means that community engagement will be crucial before and during the deployment of the second investigational vaccine."