A new Ebola vaccine study starting in Sierra Leone will test the vaccine in thousands of people who are working to fight the epidemic, health officials said today.

For the study, called STRIVE, researchers will enroll about 6,000 people — all of them health care workers or others who are on the front lines, such as cleaning staff at clinics and burial workers, according to the Centers for Disease Control and Prevention. The study will be conducted in the five districts of Sierra Leone that have been most affected by the Ebola outbreak.

Participants will receive the Ebola vaccine either right away or six months later (as part of a control group). The researchers will then compare the rates of Ebola among those who got the vaccine with the rates among those waiting to get the vaccine, officials said.

"I'm hopeful that what we learn from this clinical trial will help us get closer to finding a safe and effective tool" to protect people against Ebola during the current outbreak and future ones, Dr. Anne Schuchat, director of CDC's National Center for Immunization and Respiratory Diseases, said at a news conference today (April 14). [How Do Ebola Vaccines Work?]

The vaccine, called VSV-ZEBOV, consists of a virus that mainly infects animals (including rodents, cattle, swine and horses), called the vesicular stomatitis virus (VSV). In the vaccine, one gene of VSV has been replaced with the gene for the outer protein of the Zaire strain of the Ebola virus, which is the strain causing the current outbreak.

The vaccine cannot cause Ebola, but researchers said they hope it can stimulate the immune system enough to protect against the disease. Already, the vaccine has been tested for safety in about 800 people in the United States, Canada, Africa and Europe.

The CDC is conducting the study in partnership with several health institutions in Sierra Leone.

Because it's not clear whether the vaccine provides enough protection against Ebola, people who receive the shot should still take the proper precautions to protect themselves from the disease, including wearing all the recommended personal protective equipment when treating sick people, officials said.

There has been some concern about the ability of vaccine trials to properly assess the effectiveness of Ebola vaccines, given the decline in Ebola cases in West Africa in recent months.

In the week of March 29 to April 5, there were only 30 new cases of Ebola reported in Guinea, Liberia and Sierra Leone — the lowest number of new cases in a week since May of last year, according the World Health Organization. (In Sierra Leone, there were nine new cases reported during the week of March 29 to April 5.)

"Of course, we're glad that the epidemic is improving in Sierra Leone and surrounding countries, and the case counts have come down," Schuchat said.

The researchers have adjusted the design of their trial to maximize its success, and they plan to define success in multiple ways, including looking at the level of people's immune response to the vaccine, Schuchat said. She also stressed that even though case counts have come down, people continue to be infected in the region.

Since the Ebola outbreak began in West Africa last year, there have been more than 25,500 cases and more than 10,000 deaths form the disease, according to the WHO.

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