The outbreak, the ninth known in Congo, is the first major Ebola incident since an outbreak in three West African nations claimed the lives of at least 11,000 people in 2014 and 2015. The current eruption is the first to test a new vaccine, which appeared effective in Guinea late in 2015.

The first doses of the vaccine arrived in the three Congolese cities on May 21, and in the following two weeks about 1,200 people have received the drug, the WHO said.

Front-line health workers, those most at risk of coming into contact with a victim of the Ebola virus, are the first to receive the vaccine. The second priority is those who have come into contact with an Ebola victim, and those with whom they in turn have been in contact with — a practice known as ring vaccination, used to stop the spread of a virus.

Guinean doctors who helped deploy the vaccine in their home country have spent weeks in the hot zone administering the drug.

Still, health officials are worried about the virus spreading outside the Equateur Province. The Congolese government and WHO have identified 115 possible points of entry and exit from the province and Kinshasa, setting up special screening measures at airports in Mbandaka and Kinshasa.

Partner organizations and governments have already raced about $25 million to the region for a rapid response, aimed at containing the virus before it spreads to other cities up and down the Congo River. The U.S. Agency for International Development has contributed about $5.3 million of that total, and governments in the United Kingdom, Germany, Norway, Canada and Italy have also chipped in. The WHO has said a full-scale response could cost up to $57 million.

Teams from the Centers for Disease Control and Prevention, which maintains a satellite office in Kinshasa, have been sent to Mbandaka and Bikoro to help with the response.

More than 170 WHO experts have been sent to the three epicenters of the outbreak, and Doctors Without Borders is operating isolation facilities in Mbandaka and Bikoro. The group is building two more Ebola treatment centers in the towns of Iboko and Itipo.

The Ebola virus can incubate in a human body for up to 21 days before a person shows any symptoms. Once a virus has been identified, a country must make it two successive incubation periods — 42 days — without a new case before being declared free of the Ebola virus.