The World Health Organization said the Democratic Republic of Congo’s Ebola outbreak began in February, months before authorities officially declared it on May 15, as health responders struggle to contain rapidly rising infections.
WHO regional director for Africa Mohamed Yakub Janabi said sequencing established the earlier start, while some initial cases had been incorrectly attributed to malaria and typhoid.
Government figures now show 4,200 confirmed cases and more than 1,900 deaths.
The outbreak involves the rare Bundibugyo strain of Ebola, which has no approved vaccines or treatments, unlike the more common Zaire strain.
Early testing focused on the more common form of Ebola, contributing to delays in identifying the outbreak.
Health authorities have said between 60% and 70% of new cases are being detected among people outside the contacts already under monitoring.
The response has also been complicated by strikes among unpaid health workers, threats from rebel groups, community anger and misinformation denying Ebola’s existence.
Teams must travel along remote, unpaved roads while facing shortages of protective equipment.
Large numbers of displaced people also lack reliable access to water needed for handwashing.
WHO Director-General Tedros Adhanom Ghebreyesus said last week that Ebola was spreading faster than efforts to contain it, with new infections doubling in some hotspots.
The outbreak has already killed people at a faster rate than previous Ebola outbreaks, including the 2014-2016 West Africa epidemic, which recorded more than 11,000 deaths among at least 28,000 cases.
The WHO on Friday recommended full-scale human trials of the licensed Ervebo vaccine after early data indicated it could provide some protection against Bundibugyo.
Ervebo is proven against the Zaire strain, while two vaccines specifically targeting Bundibugyo are in early human trials and a third is being developed toward that stage.
The outbreak is unfolding in a region where years of rebel conflict have already weakened healthcare access, while some people, including pregnant women, are avoiding health centers because they fear exposure to the virus.