Authorities in Congo began an ambitious campaign this week to use a pioneering Ebola vaccine to help stem a growing outbreak of the deadly virus. It’s the first widespread use of the therapeutic since a devastating 2014 epidemic in West Africa and represents a major strategic shift for public health.

World Health Organization officials, criticized for their slow response four years ago, began vaccinating health workers in affected areas Monday and plan to vaccinate about 1,000 people in the next week. More than 7,500 doses have been sent to Congo, and an additional 8,000 doses will be available in the coming days, according to the WHO.

WHO epidemiologist Peter Salama, who is in charge of the United Nations agency's epidemic response, calls the vaccine a “paradigm shift” in fighting the virus, which has caused eight previous outbreaks in Congo since its discovery there in 1976. “Today marks a turning point in how we deal with Ebola. We are moving from a strategy of containment to one of offering communities protection and care,” he said.

But enormous challenges lie ahead as officials and others race against the virus to identify people in the high-risk groups being targeted for vaccination: front-line and medical workers, those who had contact with confirmed cases and then contacts of those contacts. The goal is to form a buffer of immune people to rapidly prevent the disease from spreading. The first batch of vaccines — one-dose shots — is enough to vaccinate 50 “rings” of 150 people, officials said.

“No one should underestimate how challenging this all is or expect the vaccine to be the answer to everything,” said Jeremy Farrar, director of the Wellcome Trust, a British-based global charitable foundation. “It’s not like rolling out a routine vaccination program in mothers and newborns. This is a completely different playing field in terms of complexity and difficulty.”

The vaccine, which proved highly effective in trials in Guinea and Sierra Leone toward the end of the West Africa epidemic, is considered an investigational therapeutic and not yet licensed. It is being provided at no charge by pharmaceutical giant Merck, while Gavi, the Vaccine Alliance, is providing $1 million toward the deployment of health workers, transport and other support.

As of Tuesday, according to the WHO, 58 confirmed, probable and suspected cases had been reported, with 27 deaths. Cases have been reported in three locations in the province of Equateur, further complicating outbreak control. At least one death was in Mbandaka, the provincial capital, which also has four confirmed cases. The northwestern port city of 1.2 million people sprawls along the Congo River, and health officials are most concerned about the virus reaching villages along the waterway and through tributaries to the capital of Kinshasa and its population of 10 million, as well as to the neighboring Republic of Congo and Central African Republic.

On Wednesday, there were reports that three infected patients had slipped out of an isolation ward at a hospital in Mbandaka, according to Doctors Without Borders, known by its French acronym MSF. MSF is involved in the outbreak response. The WHO's representative in Congo told local reporters that one patient was found dead, another was sent back to a hospital and died shortly afterward, and the third is alive and under observation.

The effort’s operational and cultural logistics are formidable.

The outbreak’s epicenter, the small town of Bikoro in Equateur, is in one of the most remote parts of the country. Few roads are paved; electricity and telecommunications are nonexistent. The vaccines must be transported there while being kept at a temperature of -76 to -112 degrees Fahrenheit. They have been shipped from Geneva in special containers that can retain that temperature for up to six days, but generators also are being flown by helicopter to Bikoro, according to officials working on the response in Kinshasa.

The lack of health infrastructure likely will influence how the vaccine is received by communities, noted Juliet Bedford, director of Anthrologica, a British-based research organization that specializes in applied anthropology in global health.

Across Congo, 40 percent of the communities have discontinued immunizations because of the difficulties in getting and disseminating vaccines, said Bedford, one of the anthropologists who worked with U.N. agencies during the West Africa epidemic. In Equateur, data show that only a third of children have received routine childhood immunizations.

“In the more rural and isolated areas, you are not going to see people who are used to immunizations and vaccinations,” she said.

The province is Congo’s poorest. More than a dozen languages are spoken among many ethnic groups. Ten percent to 40 percent of the people living in its remote villages and forests are pygmy groups, among the most difficult-to-reach populations, according to Pierre Rollin, a medical epidemiologist and Ebola expert at the Centers for Disease Control and Prevention. Rollin, who is on his way to Mbandaka, is among about a dozen CDC personnel deploying as part of the U.S. response. Some of the agency’s experts are part of its office in Congo.

One of the biggest lessons from the West Africa epidemic was the importance of gaining the trust of affected communities by relying on respected village elders and traditional healers. “You have to get that trust in the first five seconds,” said Dan Lucey, a Georgetown University infectious disease specialist who worked in Sierra Leone and Liberia during the 2014 epidemic.

Many of the same specialists involved in the Guinea trial with the vaccine — known as rVSV-ZEBOV — have been sent to assist in Congo. Anthropologists are also involved, in part to help with communication. All vaccine recipients are required to sign consent forms saying they have been informed of the potential side effects, including headache, fatigue and mild fever.

“Most people think about vaccines as being for children, so if we are vaccinating adults, that could raise some questions,” said Ben Dahl, an epidemiologist on the CDC team in Kinshasa. He took part in the Guinea trial, which saw “quite a bit of demand”  once the vaccine was shown to be effective.

Congolese health officials have said health care will be free as a way to encourage people to seek medical care if they are sick. Even so, there are other costs that might prohibit people from getting help — the cost of finding a motorbike ride to a clinic, for example, or of seeking help versus working.

“You lose income because you can’t collect firewood that day or go to the market, and maybe you have five children at home to feed,” Bedford explained. “Which is why active surveillance and community mobilizers who are familiar local people out on the ground are absolutely critical to containing this outbreak.”

Read more:

U.S. pledges up to $8 million for Ebola outbreak in Congo

WHO says Ebola outbreak in Congo not yet a global health emergency

Ebola vaccine shown to be 'high protective' against deadly virus in major trial 

Top White House official in charge of pandemic response exits abruptly