The Ebola outbreak in Congo passed a grim milestone this week — 10,000 confirmed cases — and spurred questions about whether health authorities are losing their ability to contain it.

MIKE STOBBE NEW YORK — The Ebola outbreak in Congo passed a grim milestone this week — 10,000 confirmed cases — and spurred questions about whether health authorities are losing their ability to contain it.

Nearly half the infected people in this outbreak have died, and some experts say there are reasons to worry this could grow into the worst Ebola outbreak in history. Others don’t think it will reach that level. Researchers are currently working on updated projections.

Current circumstances suggest it’s unlikely to spark significant problems in most other countries, some researchers said. But about 30 confirmed cases have already popped up in other nations. And the risk that more will appear outside Congo will grow if the outbreak isn’t tamed, said Justin Lessler, a University of North Carolina researcher who studies the dynamics of infectious disease outbreaks.

Experts also are hopeful that a vaccine will become available next year that could help control it.

At a virtual summit of African health ministers on Wednesday, Africa CDC Director-General Dr. Jean Kaseya said the continent is “not winning” the fight against Ebola and needs to do more after it spread to multiple African countries“Together, we agreed to prepare every country, coordinate across our borders and keep those borders open while watching them closely,” Kaseya wrote on Steady case numbers raise concerns about spread beyond Congo About 400 to 500 new confirmed cases have been reported in Congo each week, and that number seems to be holding steady.

It’s good news that “cases are not rising exponentially,” at least for the moment, said Dr. Boghuma Titanji, an Emory University infectious disease specialist.

“But they’re not coming down, either,” she said. “It’s at a plateau. And the plateau is at a very, very high set point.”A major challenge in Congo is limited contact tracing and collaboration with affected communities; health workers sometimes spend more time trying to convince locals to work with them than giving care.

Fewer than a quarter of new cases are coming from people known to have been in contact with an infected person, showing there are ongoing chains of transmission that health authorities are not aware of.

As cases mount, the likelihood grows that more infected people travel to other countries, Titanji and other experts said. France, Kenya, Nigeria and Uganda also have reported cases tied to the Congo outbreak.

Travel restrictions like those imposed by the U.S., can make it harder for the virus to leave Africa. And in many places where a traveler’s infection may surface, health officials should be able to quickly isolate an infected person and find any contacts, experts say.

“The direct risk it poses to the U.S. at this point, I’d say, is very close to zero,” Lessler said.

But Ebola could flare in poor and conflict-stricken places with weakened health systems — like South Sudan, for example, Lessler said. Viruses that cause Ebola disease spread through contact with bodily fluids such as vomit, blood and semen.

The outbreak already ranks as the second largest Dozens of Ebola outbreaks have been reported since the disease was first discovered in 1976. The largest and deadliest occurred in West Africa in 2014 to 2016, with 28,000 infections and more than 11,000 deaths.

The current outbreak already ranks second, and has had at least three times the number of cases and deaths in the time since it was declared compared with the West Africa outbreak. Health officials declared the outbreak in May, but medical evidence suggests infections were occurring months earlier One reason is that past outbreaks were almost all caused by the Ebola virus. Initial testing was geared toward that virus — not the related, but different, Bundibugyo virus that was actually causing illnesses.

A series of other challenges have hurt the response, including armed conflict in some outbreak areas, gaps in the health system, mistrust misinformation and global aid cuts, experts say.

Conditions vary in Congo, which explains why cases can be stable or falling in some places but can be trending up in others, researchers and response workers say.

“Testing, care and contact tracing only work if we work with communities, going door to door, village by village to break the chain of transmission and stop this deadly escalation,” said Katie Thomas-Canfield, the International Rescue Committee’s Ebola response director, in a statement.

Dr. Wendy Armstrong, president of the Infectious Diseases Society of America, said she thinks the situation is going to get worse. “This is very far from being controlled,” she said.

Scientists are updating projections Experts expect the outbreak will continue through the end of the year, but Titanji is hopeful about 2027. That’s because vaccines are being tested against the Bundibugyo virus, and she is optimistic an effective shot will become available in the next 12 months.

She noted no vaccines were available early in the 2014-2016 West Africa outbreak. Forecasting the outbreak is difficult, of course, but scientists have been trying.

In May, the U.S. Centers for Disease Control and Prevention published a range of scenarios generated by computer models. At the time, the Africa Centers for Disease Control and Prevention was reporting 400 confirmed cases. The U.S. CDC models estimated the growth in cases over three months could run from 10,000 to more than 20,000.

The CDC is updating its modeling work. But the agency declined to let The Associated Press interview staff scientists about how the outbreak is trending.

Alessandro Vespignani, a Northeastern University disease modeler, has been monitoring trends with U.S. and international health officials.

There’s not been the kind of “decay in the number of cases weekly that we need to see the light at the end of the tunnel,” he said.