Congo’s Ebola outbreak is all but over. Did an experimental vaccine help?

An Ebola outbreak that erupted 8 May in a remote region of the Democratic Republic of the Congo (DRC) and then threatened to explode in a highly populated city appears to have been quelled. On 12 June, the last known person infected with the deadly hemorrhagic fever had recovered, twice testing negative for the virus. That started the 42-day clock for an official declaration, expected on 24 July, that the outbreak is over.

The quick end to this outbreak—after 53 cases in Équateur province, 29 of which were fatal—is a striking contrast to the Ebola epidemic that devastated West Africa from 2014 to 2016, which sickened more than 28,000 people, killing 11,310. "I certainly haven't seen an Ebola-response time frame that looks this compressed," says epidemiologist Peter Salama, who heads the Health Emergencies Programme at the World Health Organization (WHO) in Geneva, Switzerland, and led the agency's DRC response. Much of the credit goes to unusually rapid and vigorous surveillance, contact tracing, containment, and public education efforts by the DRC, WHO, and other international partners, Salama says. "Some of the most important lessons from the West African epidemic were truly learned." But a new factor played an unknown, and perhaps important, role: an experimental vaccine, used for the first time early in an outbreak.

No one can say for sure that the vaccine—tested during the waning days of the West African epidemic—actually protected against infection. But DRC Minister of Health Oly Ilunga Kalenga calls the vaccination program a "game changer," as it clearly boosted morale and encouraged other public health efforts.

Like the DRC's eight earlier Ebola outbreaks, this one mainly hit remote villages. Each previous outbreak was controlled before growing into a full-scale epidemic—the largest had 318 cases. This time, the four confirmed cases in Mbandaka, a city of 1.2 million people on the Congo River, raised fears of an urban epidemic and wider spread. Donors quickly committed more than $50 million in aid, Salama says, and the United Nations provided badly need air transportation to hard-to-reach areas.

On 21 May, officials launched a vaccine trial with no untreated group as a control. Workers gave shots in four different locations to about 3300 people who had come in direct or secondhand contact with a confirmed case. Although analyses still are underway, Salama says none of the 53 cases occurred in a vaccinated person. Epidemiologist Emile Okitolonda, dean of the University of Kinshasa School of Public Health, says the vaccine campaign also had tangential benefits in educating the public, a cornerstone of the traditional containment response to Ebola outbreaks. "Just the fact that all the contacts we vaccinated were informed and made aware of the urgency of the situation made a difference," says Okitolonda, who advises the Ministry of Public Health.

Concentrated campaign Workers focused on vaccinating people with likely exposure to Ebola. (Orange dot indicates the number in each locale.) Democratic Republic of the Congo Republic of the Congo 1530 893 779 107 Iboko Mbandaka Ingende Bikoro Km 0 50

Jean-Jacques Muyembe-Tamfum, director of the National Institute of Biomedical Research in Kinshasa and a principal investigator of the vaccine study, says the immunizations also had a positive effect on frontline responders, who were offered the shot. "At the beginning, there was a kind of a panic among the health care workers," says Muyembe-Tamfum, who has helped respond to all of the DRC's Ebola outbreaks since the disease surfaced there in 1976. "With the arrival of the vaccine, the health care workers had more confidence that they could stay and work in the hospitals." Another lead investigator of the vaccine study, microbiologist Yap Boum of Doctors Without Borders, said the vaccine study also trained a lot of Congolese responders. "Building that capacity is really critical," Boum says. "It will ensure that the next time there's an outbreak in DRC, they'll know how to use the vaccine and they won't need as much support."

Boum, who is based in Yaoundé, helped run a controlled study of the vaccine in Guinea in 2015, at the tail end of the West African epidemic, that found no infections in the thousands of people who received it. Merck, which makes the vaccine—a harmless livestock virus engineered to carry a gene for the Ebolavirus surface protein—plans to file for regulatory approval next year; that would allow routine use of the vaccine outside of cumbersome clinical trials. The current study may not influence the licensing decision, but no downsides surfaced, says Seth Berkley, who heads Gavi, the Vaccine Alliance, a Geneva-based nonprofit that has paid Merck to build a vaccine stockpile and helped fund the current study. "We've added a real field experience and there were not any significant adverse events," he says.

More data could come from a study by epidemiologist Anne Rimoin of the University of California, Los Angeles, and Congolese researchers. To track the magnitude and duration of immune responses triggered by the shots, the team has taken blood samples from some 1000 vaccinated people and will continue to test them for at least a year. Comparing their immune responses with those of unvaccinated people could reveal signs that the vaccine provoked a protective response. "Transmission is always going to be halted using a variety of methods, and we hope to see this vaccine played a role as well," says Rimoin, who has worked in the DRC for 15 years.

If so, it is likely to play a bigger role in fighting future outbreaks. Muyembe-Tamfum thinks the DRC should consider vaccinating health workers preemptively, wherever the virus has surfaced in the past. The educational benefits of a vaccine campaign may also factor in: Early in this outbreak, a few patients fled Ebola treatment centers, some unsafe burials continued, and Boum says response teams still encountered "the idea the disease was mystical or due to witchcraft."

Rimoin wants to see the DRC's Ebola surveillance system fortified so that it's simpler to confirm cases quickly. "You have a great group of people in DRC who are ready to do this work, but they don't have the reagents needed for surveillance," she says. Now that the crisis is over, "Nobody is interested in providing the funds necessary to replenish the supplies so that they can react quickly when they're most needed."