When the Ebola virus swept through West Africa in 2014, it hit capital cities so quickly that medical professionals were left with few options to prevent its spread. Soon, health care workers and those who touched the bodies of the dead were coming down with the virus themselves, and then passing it to others. By the time the outbreak was finally contained, more than 11,000 people had died.

Now, experts hope a vaccine can help contain a new outbreak of the virus in Congo — a simple intervention that could potentially have saved thousands of lives had it been ready in 2014.

On Friday, the World Health Organization announced its plan to send the experimental vaccine to northwest Congo, where there have been about 32 suspected or confirmed cases since early April, and 18 deaths.

“We are very concerned and planning for all scenarios, including the worst case scenario,” said Peter Salama, WHO’s deputy director-general of emergency preparedness and response.

Challenges will include keeping the vaccine at low temperatures in Congo’s heat, and, with the lack of infrastructure in rural areas, getting it to those who have been exposed. This particular outbreak may be harder to contain because it has already spread across 37 miles. Some of those infected are health workers, which poses an additional risk of transmission to others. Those who help bury or clean the bodies of the infected are also at high risk.

Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, said Ebola outbreaks are dangerous in an increasingly urbanizing Africa because once infections spread to a metropolitan area, they become much more difficult to control. Already, officials in Congo fear that the virus could spread to the provincial capital Mbandaka, home to around 1 million people.

“All it would take is one or two of these infected individuals to go into a larger metropolitan area,” Osterholm said. When it comes to the danger of a large outbreak, “Kinshasa is a gas tanker waiting for a match to hit it,” he added. The Congolese capital’s population of 14 million means the disease would spread very quickly if it reaches the city, which is far from the affected area.

On Friday, Salama said he has spoken to Congo’s health minister and hoped that he would soon have approval to use the vaccine, which was developed by Merck in 2016.