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Congo’s Ebola outbreak has surpassed 11,000 confirmed cases, making it the second-largest Ebola outbreak on record, as aid groups warn the virus remains unchecked.

Getting into it: Congo’s health ministry said Wednesday that 11,042 people have been infected since the outbreak was declared in mid-May, and 5,387 of them have died, a fatality rate close to 50%. Only the 2014 to 2016 West Africa epidemic, with more than 28,600 cases and over 11,000 deaths, has been bigger.

The rare Bundibugyo strain driving the outbreak has no approved vaccine or treatment. Eight of Congo’s 26 provinces have now reported cases, with most cases concentrated in the conflict-hit eastern provinces of Ituri and North Kivu. The virus has also reached Uganda, Kenya and Nigeria. On Thursday, Doctors Without Borders said many cases are going undetected, with about two-thirds of recent deaths occurring in communities rather than treatment centers. The group also said frontline workers are not being paid and ambulances are running out of fuel.

Violence has also hampered the response, with youths burning down an Ebola treatment center in Ituri on Sunday. The WHO said last week’s attack by Islamic State-linked militants on the village of Cantine in North Kivu killed 66 people, twice what local officials first reported, and left more than 1,000 Ebola contacts and three confirmed patients unaccounted for.

Neil Ferguson, an epidemiologist at Imperial College London who is modeling the outbreak, estimated that only about half of cases are being detected and that transmission will likely still be ongoing a year from now, meaning the outbreak could eventually surpass West Africa’s.

The WHO has approved trials of two experimental Ebola vaccines and is testing whether an existing vaccine for a more common strain offers any protection, with results expected in early 2027.