Just three days before Congo planned to declare an end to the second-worst Ebola outbreak in modern history, public health officials say they have identified a new case.

Few details were immediately available, but World Health Organization (WHO) officials said at a news briefing Friday that they had remained on alert for the possibility that new cases would emerge, even weeks after the last Ebola patient left a medical center with a clean bill of health.

The new patient is a 26-year old man in Beni, according to Congo's health ministry. Beni was one of the hardest-hit cities at the heart of the outbreak, suggesting that the virus continues to spread undetected in the region.

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"The positive news is that the government of Congo, the teams, the partners on the ground and WHO were alert," said Mike Ryan, executive director of the WHO's Health Emergencies Program.

The Ebola outbreak that began in North Kivu and Ituri provinces in August 2018 has claimed more than 2,200 lives, according to the latest figures from Congo's health department. It infected at least 3,450 people and posed steep challenges for health care workers who faced near constant attacks in an unstable region dominated by armed rebel groups.

The outbreak waned through 2019 and into the early months of this year. The last known Ebola patient recovered and tested negative for the virus early last month.

An outbreak is declared over when a country goes through 42 days without identifying a new case. The Congolese government had planned to make such an announcement on Monday, 42 days since the last patient left the medical center.

Ryan said WHO teams on the ground are still testing thousands of potential Ebola patients, and that vaccinations have continued as well. But he added the new case would reset the clock for declaring the outbreak over.

"We will continue that active surveillance right the way through. We'll just have to go again for another 42 days," Ryan said. "There is no exit strategy until you're in control of the strategy, and you must always be ready to go back again and start again."