So far, 23 deaths have been recorded in the Democratic Republic of Congo's latest Ebola outbreak, the ninth since the disease made its first-known appearance near its northern Ebola river in the 1970s.
Congolese and UN officials were racing on Thursday to prevent a runaway Ebola outbreak in Congo, working out the logistics of keeping newly arrived vaccines well below freezing in a steamy region on the equator with unreliable power.
World Health Organization (WHO) spokesman Christian Lindmeier said the UN body would convene an Emergency Committee meeting on Friday to consider the international risks.
This is Democratic Republic of Congo's ninth epidemic since the disease was identified in the 1970s, but also its most alarming because of the risk of transmission via regular river transport to the capital Kinshasa, a city of 10 million.
There have already been 44 suspected, probable or confirmed cases of Ebola, and 23 people have died. Potentially most worrying is a confirmed case in Mbandaka, a city of about 1 million connected to Kinshasa by the Congo River.
"This does change the way we need to respond," Peter Salama, WHO's medical emergency programme head told Reuters TV in Geneva. "Overnight, Mbandaka has become the number one priority for preventing this outbreak from getting out of control."
The other Ebola cases were spread across sites in remote areas where the disease might not travel quickly.
An experimental but highly effective vaccine is being deployed, with health workers being vaccinated first, but it normally needs to be kept 80 degrees Celsius below freezing in a humid region where daytime temperatures hover around 30.
"For now, the cold chain is guaranteed at -80 degrees until Kinshasa," Health Minister Oly Ilunga told Reuters. "There is a fridge that will be prepared (on Thursday) ... in Mbandaka and that will be at -80."
"This vaccine is no longer experimental. The effectiveness has been proven and validated," he added. "Now that we are facing the Ebola virus we must use all the resources we have."
WHO spokesman Tarik Jasarevic told Reuters the vaccine can still be effective for up to two weeks if stored in a fridge at between 8 and 2 degrees above freezing.
No magic bullet
Mindful of criticism it received for being too slow during the huge West African Ebola outbreak, which killed at least 11,300 people in Guinea, Sierra Leone and Liberia from 2014 to 2016, the WHO is moving fast on Congo's latest outbreak.
The emergency committee will decide whether to declare a "public health emergency of international concern", which would mean getting access to more resources, Lindmeier said.
The Kinshasa government reported the outbreak on May 8, one day after two samples tested positive, and within days the WHO was sending experts, preparing a helicopter "air bridge" to the site, and planning a vaccination campaign.
The nightmare scenario is an outbreak in Kinshasa, a crowded city where millions live in unsanitary slums not connected to a sewer system.
Several public transport boats a day head from Mbandaka downstream over the river to the capital. They are so overloaded with people that they sometimes topple over, their toilets are usually filthy and water for washing absent.
"If this Ebola outbreak ever reaches Kinshasa, what we are going to see is death here," Jean Marie Mukaya, a resident of the city, told Reuters TV. "Because it is very dirty here. The government and the population must ... get rid of all the dirt."
Already the WHO has warned that there is a "moderate" regional risk because the disease could travel along the river to Central African Republic and Congo Republic. But it has said the global risk is low because of the remoteness of the area and the rapid response launched so far.
Even if the logistics of the 'fridge bridge' prove easy enough to overcome, "the vaccine is not a magic bullet," Salama told Reuters this week, especially since health workers have been infected.
"Having healthcare workers infected is usually a 'canary in the mine' for potential amplification," he said.