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"I am cautiously optimistic that we shall be able to bring it to an end soon," Doctor Tedros Adhanom Ghebreyesus told reporters at Kinshasa airport.
Earlier, he had tweeted that "it's far too early to declare victory in the #Ebola outbreak. But the signals are positive and we are cautiously optimistic."
His comments come after the WHO and NGOs since May 21 began helping the Democratic Republic of Congo immunise front-line workers with a vaccine that has shown to be highly effective in trials, though it still awaits regulatory approval.
A total of 1,369 people have received the experimental vaccine since May 21, the ministry said last Wednesday.