A second Ebola vaccine will be administered to about 50,000 people in the Democratic Republic of the Congo, as part of a major clinical trial.
The Johnson & Johnson (J&J) vaccine will be used next to a Merck vaccine, which has already been given to nearly 250,000 people.
Merck's jab is now approved by the World Health Organization.
More than 2,100 people have died in DR Congo, the second largest Ebola outbreak ever recorded.
The WHO approval of a Merck dose vaccine is based on its perceived safety and effectiveness. The European Commission also approved the vaccine.
The J&J vaccine, which requires two doses given 56 days apart, is available to more than one adult and child, living in two areas of the Gongo city of Gongo, where there is no active delivery of lethal disease.
Rubber, with a population of one million people, borders Rwanda and has a major international airport.
Prof Daniel Bausch, director of the UK Public Health Rapid Support Team, is one of the leading scientists. the test.
He stated that there was no "contest" between the two vaccines, and that both had their advantages and disadvantages.
Merck's, provided in direct and indirect contact with an Ebola patient, may be best used in the midst of an outbreak – while the J&J vaccine can be used to protect people who have never been exposed to Ebola.
Prof Bausch said: "The J&J vaccine is not suitable for an outbreak setting, especially since it requires two doses to provide optimal safety in sick. "
But he said the vaccine "may provide longer-term immunity, and may be associated with fewer side-effects than a live virus vaccine, such as the Merck one".
The number of new cases of DR Congo's Ebola outbreak has dropped significantly since its height in the spring, but there are still about 20 new infections reported each week.
Dr Jeremy Farrar, director of the Wellcome Trust, helping fund the J&J trial is a "critical step forward".
"Previous tests have shown that the J&J vaccine produces an immune response, a good indication of it being protective against the Ebola virus."
Dr Eteni Longondo, Minister of Public Health in DR Congo, said: "It is important that we improve our efforts. So we are working with international partners to provide our team responses to another tool to fight and to ultimately preventing the spread of this deadly disease. "
The outbreak was fueled by misinformation and gossip, along with a very difficult security situation.
Around 200 health facilities have been attacked since the outbreak began in August 2018, so the introduction of a new vaccine in skeptical communities is a major task.
Immunization teams from the medical charity Médecins Sans Frontières (MSF) will administer the vaccine.
John Johnson, lead of the MSF project on Ebola vaccination, said: "The use of two different vaccines in the vicinity may result in misunderstanding, which means that community engagement will be important before and during the expansion of the second investigation vaccine. "