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The Centers for Disease Control and Prevention is helping prepare for a new Ebola vaccine trial in Sierra Leone, the country that’s now the worst hit by the Ebola epidemic.

The CDC will work with Sierra Leonean authorities to vaccinate up to 6,000 health care workers, including doctors, nurses and ambulance drivers, against a virus that’s infected more than 11,000 people in Sierra Leone alone, killing 3,400 of them.

This trial will test just one of the Ebola vaccines in development – one designed by U.S. and Canadian government researchers with a company called New Link Genetics and licensed to Merck. It uses an animal virus called vesicular stomatitis virus (VSV) to carry tiny pieces of the Ebola virus to help train the immune system to recognize it.

Ebola’s infected more than 23,000 people in West Africa and killed more than 9,300 of them, according to the World Health Organization.

“The study is designed to offer the vaccine to health care workers and other front line workers."

Several vaccine trials are under way in or planned for Liberia, Sierra Leone and Guinea and researchers are also testing drugs and blood products such as plasma to see if they help patients recover from the highly lethal virus.

Right now, the CDC is making sure the health care workers who will be vaccinated are on board with the idea.

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“We have been doing quite a bit of engagement in-country with the health care workers who will be part of the study,” said Dr. Anne Schuchat, director of CDC's National Center for Immunization and Respiratory Diseases.

“The study is designed to offer the vaccine to health care workers and other front line workers,” Schuchat told NBC News.

Other vaccine trials are testing two different vaccines head to head but Schuchat says this will be a smaller, more focused trial aimed at protecting people most at risk of catching Ebola—those caring for and in contact with patients.

“In this epidemic, that group has been hard hit by the Ebola virus. It is important for us to understand whether a vaccine can be effective in that population,” Schuchat said.

Researchers testing vaccines in Liberia say they’ll have to adjust the experiments to account for a huge drop in new cases. Liberia’s had 11 new confirmed Ebola cases in the past 21 days.

But Sierra Leone is still suffering a bad epidemic, although not as bad as late last year. WHO reports 230 new cases in Sierra Leone over the past three weeks.

“We really hope to help with their resilience and recovery.”

“We are really pleased that the number of cases are coming down,” Schuchat said. “We have adapted the design of the study based on the changing epidemiology.” The experiment may take a little longer because it takes a certain number of actual cases of disease to tell if vaccinated people are less likely to become infected than unvaccinated people.

It’s unusual for the CDC to lead a late-stage vaccine trial, but the epidemic is “so devastating” that the CDC needed to join the mix, Schuchat said.

The study will be carried out with Sierra Leonean researchers, the Ministry of Health and Sanitation, the country’s College of Medicine and Allied Health Sciences and WHO.

It’s not easy. The vaccine must be kept at extreme frozen temperatures, so trial centers must have reliable electricity and good transportation. Staff must be trained in the protocol, meticulous record keeping and high standards of care.

One benefit, the CDC hopes, will be they’ll leave behind a stronger health care system that can help prevent future such epidemics.

“CDC also interested is helping strengthen the research capacity in-country and working on partnerships,” Schuchat said. “We really hope to help with their resilience and recovery.”