The Democratic Republic of Congo declared an end to its ninth Ebola outbreak Tuesday, marking a victory for the new tools and experimental vaccine health agencies used to prevent a repeat of the epidemic that swept West Africa four years ago.
Congo’s Health Ministry said 42 days had passed since anyone was diagnosed with the hemorrhagic fever, carrying the central African nation past the virus’s maximum incubation period. Thirty-three people are believed to have died from the virus, which was confirmed on May 8 in the remote province of Équateur, while 21 patients recovered, the ministry said.
The virus spread to four different locations, including the city of Mbandaka, a transport hub on the Congo River and home to more than a million people. That triggered fears that patients could carry the disease to the capital Kinshasa, a city of around 12 million, and to neighboring states that, like Congo, are among the poorest in Africa.
Between 2014 and 2016, the largest outbreak of Ebola in history killed some 11,300 people, primarily in Guinea, Liberia and Sierra Leone. Isolated cases were identified in the U.S. and Europe. The World Health Organization and other public-health agencies came under fire for acting too slowly to contain the virus, which on average kills half of those infected, and for misjudging when the outbreak had ended.
Oly Ilunga, Congo’s health minister, said a fast, and more forceful, response made a difference this time for the country, where Ebola was first identified in 1976. “Although the magnitude of the crisis we faced was unprecedented, the speed and efficiency of the response implemented by the government and its partners were exceptional,” Dr. lunga said.
A first response team was dispatched from Kinshasa within hours and planes carried medical workers, equipment and supplies to the remote villages at the center of the outbreak. Health workers, including from the WHO and Doctors Without Borders, tracked and isolated people who had come into contact with patients and implemented safer burial practices.
Still, it took nearly two weeks to deploy an experimental Ebola vaccine developed by Merck & Co. Health organizations flew in fridges and generators by helicopter to keep the doses at below-freezing temperatures in a region with unreliable power supplies. Because the vaccine hasn’t been licensed yet, Congo’s government had to approve a WHO protocol that allows injections for contacts and contacts of contacts of suspected Ebola patients.
In total, 3,330 people were vaccinated, the WHO said. Health officials kept nearly 2,000 people under surveillance to ensure they didn’t develop symptoms and pass on the virus.
“None of the people who were vaccinated developed the disease,” said Jessica Ilunga, spokeswoman for Congo’s Health Ministry. “Vaccinations allowed us to break transmissions quickly and contain the virus. It was such a game changer.”
Health officials also stressed the buildup of regional know-how, including the help of several dozen nurses from Guinea, who had tested the vaccine there at the end of the West African outbreak. Activists have called for quick approval of the vaccine, as well a broad rollout targeting people outside specific outbreaks in at-risk countries such as Congo and Uganda.
Despite efforts to speed up the process after the 2014-16 outbreak, Merck’s vaccine and other experimental Ebola vaccines haven’t received marketing approval in the U.S. and Europe. Merck aimed to apply to U.S. government regulators by the end of 2017, setting the vaccine up for approval sometime this year. But the company has said that issues at a plant in Germany and clinical trials have delayed the application, and that it now expects to file for regulatory approval in 2019.
The WHO warned that there remains a risk of resurgence of Ebola from potentially undetected transmission chains and sexual transmission of the disease by male survivors, who can carry the virus in semen for up to a year. On Monday, a study published in the medical journal Lancet Infectious Diseases also for the first time documented a case in which a woman, who survived Ebola in 2014, transmitted the virus a year later.
—Betsy McKay contributed to this article.
Write to Nicholas Bariyo at nicholas.bariyo@wsj.com and Gabriele Steinhauser at gabriele.steinhauser@wsj.com