Congo's Ebola outbreak is the world's second largest, and still accelerating

Congo's eastern Ebola outbreak is now the second-largest ever recorded, and the fastest-growing on any continent. The government in Kinshasa reported 3,802 cases and 1,707 deaths as of Tuesday. The virus behind it is not the strain the world has spent the last decade learning to fight.

The outbreak is caused by Bundibugyo ebolavirus, a rarer form of the Ebola virus for which no approved vaccine or treatment exists. The post-2014 toolkit that defined the global response after West Africa (the Ervebo vaccine, the Inmazeb antibody cocktail, ring vaccination) was built around Zaire ebolavirus, the species that drove the 2014–2016 epidemic and the Kivu outbreak that followed. None of those countermeasures are licensed for Bundibugyo. That mismatch is now the binding constraint on the response.

Two post-exposure prophylaxis drug trials are running in Ituri province, and two separate vaccine candidates are being tested in the UK and Canada. None of the candidate vaccines are being trialed inside the outbreak zone. "The speed of the outbreak is outpacing what is available to us," Africa CDC Director-General Jean Kaseya said on his second visit to Bunia, the provincial capital. "This is the second largest Ebola outbreak ever, and it is still the fastest-growing."

WHO is monitoring more than 17,000 potential contacts and seeing roughly 80 percent of them each day. That sounds operational. In practice, 60 to 70 percent of new cases are now spreading inside the community rather than being captured through contact tracing, the standard method of tracking an outbreak by following each confirmed case to the people they infected. The patients showing up at treatment centers are, increasingly, people no tracer ever reached.

Ituri province accounts for nearly 90 percent of cases, with five other provinces affected, including Kisangani, where the virus has now been confirmed. The outbreak was declared on May 15. The first identified case in the chain, patient zero, has still not been found. Armed-group violence, illegal mining, and population displacement have made parts of the outbreak zone inaccessible to contact tracers, and community mistrust has slowed work in areas that are reachable.

WHO Director-General Tedros Adhanom Ghebreyesus was expected in Kinshasa on Tuesday and was due to travel to Bunia later in the week. WHO's Africa regional office, led by Mohamed Janabi, is running more than 2,000 lab tests a day. Uganda, which borders Ituri, declared itself Ebola-free after discharging its last patient in mid-June. Uganda's outcome is a counterfactual sitting next to the Congolese outbreak: a country with fewer cases, a known index patient, and no Bundibugyo-specific countermeasures still managed to interrupt transmission.

The West Africa and Kivu outbreaks were stopped by tools matched to the virus in front of them. The current outbreak is being asked to be stopped by tools that do not exist for the virus in front of it. The two Ituri drug trials and the two UK and Canada vaccine trials are the only on-ramp to that toolkit. The curve is moving faster than they are.