Story highlights More than 400 people have been vaccinated so far in the current outbreak

Five experimental drugs to treat Ebola virus disease will be trialed

(CNN) More than 400 people have been vaccinated and more than 800 contacts traced in the city of Mbandaka in the Democratic Republic of Congo.

Use of the experimental vaccine, known as rVSV-ZEBOV, will now spread to the more rural regions of Iboko and Bikoro where the majority of cases have occurred.

Vaccination began in the urban setting of Mbandaka to "prevent an urban outbreak" as well as further spread along the nearby Congo river, to reduce "risk through the interior of DRC and surrounding countries," Dr. Peter Salama, the deputy director-general of the World Health Organization's Health Emergencies Program said Tuesday.

He confirmed the outbreak spread to Mbandaka after two brothers visited Bikoro to attend a funeral and traveled back to the city.

"Mbandaka is relatively safeguarded at the moment," he said, adding that officials "haven't seen an explosive increase in cases" and the teams have reason to "cautiously optimistic."

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