GENEVA (Reuters) - Democratic Republic of Congo’s Ebola outbreak has been “largely contained” and no new cases of the disease have been confirmed since the last known sufferer died on June 9, the World Health Organization said on Wednesday.

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The U.N. health body said it viewed the situation with “cautious optimism”, despite the difficulty of tracking down people who may have been exposed to the deadly virus in remote forests close to the Congo River.

The outbreak has killed 28 people since early April.

“Slightly over a month into the response, further spread of EVD (Ebola virus disease) has largely been contained,” the organization said in a report.

The outbreak triggered particular concern because it occurred in a remote northwestern area that was hard to reach but close to the Congo River, causing fears it could take root in a neighboring city or spread to the capital Kinshasa, a city of more than 10 million people.

There were also fears that river transport could help the virus spread to neighboring Central African Republic and the Republic of Congo.

This outbreak is Congo’s ninth, but the first to be countered with a “ring vaccination” strategy, where all contacts of known patients are vaccinated to try and stop the spread.

“The number of contacts requiring follow-up is progressively decreasing, with a total 1,417 completing the mandatory 21-day follow-up period. As of 17 June 2018, a total of 289 contacts were under follow up, of which 276 (96%) were reached on the reporting date,” the WHO report said.

“If no new cases are reported, the last contacts of the known confirmed or probable cases will complete follow-up on 27 June 2018.”

The disease has an incubation period of up to 21 days, meaning the risk declines the longer Congo goes without a new case. The vaccine takes about 10 days to work.

The risk of unidentified Ebola carriers traveling outside the affected province was now considered “very low”, but exit screening was being maintained as a precaution, the WHO report said.

An Ebola outbreak is normally declared over once 42 days have passed since blood samples from the last confirmed case tests negative for the second time.

The last laboratory-confirmed case of Ebola was a patient who died on June 9. By a Reuters calculation, if there are no more confirmed cases, the outbreak will be declared over around July 21.