Health officials in eastern Congo have documented the first relapse in the current Ebola epidemic, the World Health Organization said on Friday.

Axar.az reports citing Reuters that the Ebola outbreak in the Democratic Republic of Congo has so far infected more than 3,300 people and killed more than 2,200 since the middle of last year, making it the second-worst on record.

In early December, Congolese health authorities reported that a survivor in Mabalako, North Kivu province, had fallen ill with the virus again.

Preliminary tests have since classified it as a relapse, the WHO said in a weekly report.

“We had a massive problem (in Butembo) only 6 months ago, so there is a real concern that any continued transmission in Mabalako may potentially re-infect Butembo.”

Despite the development of an effective vaccine and treatments, a recent surge in violence by rebel militias and criminal bands near Congo’s borders with Uganda and Rwanda has hampered efforts to contain the outbreak.

“That has created the perfect storm that has allowed the virus to get away from us and go on the underground,” Ryan said.