Traditionally, health officials have largely relied on something called contact tracing to contain Ebola outbreaks.
The WHO statement said there had been 21 suspected, 20 probable and 3 confirmed cases of Ebola between April 4 and May 15, a total of 44 cases, including 15 deaths.
"This all will, as you can imagine in this challenging environment, take some time to function optimally".
And if the WHO and other organizations are able to overcome the logistical hurdles and prove that V920 is effective at preventing an outbreak from escalating, Ebola's reign of terror over the world might finally come to an end.
Ebola is lethal and highly contagious, which makes it hard to contain - especially in urban environments where people are mobile and come into more contact with others.
As the outbreak continues to grow, fears that it may spread overseas linger as many remember the West African Epidemic in 2014.
Other cases in this outbreak "have been detected in more isolated areas, giving authorities a better chance of ring-fencing the virus", The Guardian reports. Last May, a small outbreak resulted in five confirmed cases and four deaths in a province neighboring Équateur.
"With the new case confirmed in Mbandaka, the scenario has changed, and it has become more serious and worrying, since the disease is now affecting an urban area", said Henry Gray, MSF emergency coordinator in Mbandaka.
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Salama, the WHO's deputy director-general for emergency response, said Mbandaka's location on the Congo river raised the prospect of Ebola spreading to Congo-Brazzaville and the Central African Republic, as well as downstream to Kinshasa, which has a population of 10 million.
"This vaccine is no longer experimental".
AFP factfile on how the Ebola virus attacks. Someone sick with Ebola might not be able to provide a comprehensive list of everyone they've come in contact with.
The aid group Medecins Sans Frontieres said 514 people who are thought to have been in contact with infected people were now being monitored.
"This puts a whole different lens on this outbreak and gives us increased urgency to move very quickly into Mbandaka to stop this new first sign of transmission", he said.
Nine Ebola outbreaks have occurred in DRC since the virus was first identified in 1976.
The average fatality rate among those infected with the virus is about 50 percent, according to the WHO.
The outbreak in West Africa that started in 2014 reached epidemic proportions and was the worst ever recorded, infecting more than 28,000 and killing more than 11,000. Symptoms include fever, vomiting, diarrhea, muscle pain and at times internal and external bleeding.
Eleven people were treated for Ebola in the United States in 2014, two of whom died, according to the Center for Disease Control and Prevention.