Three patients infected with the deadly Ebola virus escaped from a hospital holding them in quarantine in the Congo city of Mbandaka, an aid group said, as medics raced to stop the disease spreading in the teeming river port.



Two of the patients got out on Monday but were found dead a day later, said Henry Gray, head of mission in the city for Doctors Without Borders (Médecins Sans Frontières).



Another left on Saturday, but was found alive the same day and is now under observation, Gray added.

"This is a hospital. It's not a prison. We can't lock everything," he said.

The report came as the World Health Organization warned that the fight to stop Congo's ninth confirmed outbreak of the hemorrhagic fever had reached a critical point.



"The next few weeks will really tell if this outbreak is going to expand to urban areas or if we're going to be able to keep it under control," WHO's emergency response chief Peter Salama told ministers and diplomats in Geneva.



"We're on the epidemiological knife edge of this response," he added at the UN body's annual assembly.

Nurses working with the World Health Organization prepare to administer vaccines in the town of Mbandaka on Monday during the launch of an Ebola vaccination campaign. (Junio D. Kannah/AFP/Getty Images)

Health officials are particularly concerned by the disease's presence in Mbandaka, a crowded trading hub upstream from Congo's capital, Kinshasa, a city of some 10 million people. The river also runs along the border with the Republic of Congo to the west.



The outbreak, which was first spotted near the town of Bikoro, about 100 km south of the city, is believed to have killed at least 27 people so far.

Detective work

Health workers have drawn up a list of 628 people who have had contact with known cases who will need to be vaccinated.



"It's really the detective work of epidemiology that will make or break the response to this outbreak," Salama said.

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WHO is accelerating its efforts with nine countries neighbouring Congo to try to prevent the current Ebola outbreak from spreading beyond the border, WHO's director for Africa said Wednesday.

The top two priority countries are Central African Republic and the Republic of Congo near the epicenter of the outbreak, Matshidiso Moeti told a World Health Assembly session. In the Republic of Congo, for example, WHO is working with government officials "to stop functioning" an active market on its side of the Congo River.

Here is the <a href="https://twitter.com/hashtag/Ebola?src=hash&ref_src=twsrc%5Etfw">#Ebola</a> response timeline so far.<br>Briefing 📄 <a href="https://t.co/UxCM72fgL6">https://t.co/UxCM72fgL6</a><br>📽️ <a href="https://t.co/7pQOoM7890">https://t.co/7pQOoM7890</a><a href="https://twitter.com/hashtag/WHA71?src=hash&ref_src=twsrc%5Etfw">#WHA71</a> <a href="https://t.co/8izMomZ5ju">pic.twitter.com/8izMomZ5ju</a> —@WHO

The other countries are Angola, Burundi, Rwanda, South Sudan, Tanzania, Zambia and, to a lesser extent, Uganda. WHO and governments are working to roll out "pre-position" supplies that might be needed if the virus spreads, Moeti said.

Communicate help for individuals

The escapes unfortunately are not surprising, said Dr. Isaac Bogoch, an infectious disease physician with Toronto's University Health Network who treats tropical diseases. Excellent communication between the clinical teams treating patients, public health teams, patients and communities around what's happening is key.

Bogoch said in outbreaks like this one, people need to know that the teams on the ground are "really trying to help and that they're trying to help prevent the spread of this infection but also help individuals."

He said over the years, communication strategy between care teams on the ground and communities have changed to try to better disseminate information.

"But it still can be quite a frightening situation and unfortunately it's not uncommon for people to run away."

It will be important to watch if the epidemic jumps international borders, spreads throughout the urban centre of Mbandaka, or spreads along the Congo River to other populations, he said.

Treatment trial

U.S. health authorities said they were preparing to send an experimental Ebola treatment to Congo to use in a

clinical trial aimed at stemming the outbreak.

Dr. Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Disease, said the trial would test the effectiveness of a treatment called mAb114. The treatment is made from the antibodies of the survivor of an Ebola outbreak in Kikwit, Congo, in 1995.

"We haven't even done the Phase 1 yet," Fauci told Reuters, but added that he was "happy to do it," as long at the trial is done in collaboration with WHO.

WHO is already in discussions over whether the government in Congo will give approval for the use of ZMapp, a

similar antibody drug developed in part in Canada and made by Mapp Biopharmaceuticals of San Diego.

Fauci said the NIH treatment has the effect of a vaccine and has a few advantages over ZMapp, which

he said is given in several doses and needs to be refrigerated.

The disease was first discovered in Congo in the 1970s. It is spread through direct contact with body fluids from an infected person, who suffers severe bouts of vomiting and diarrhea.



More than 11,300 people died in an Ebola outbreak in the West African countries of Guinea, Liberia and Sierra Leone between 2013 and 2016.