The first trials of an Ebola vaccine are slated to begin in West Africa in January 2015, a top official at the World Health Organization said Tuesday.

The vaccines will be available to volunteers in countries such as Mali, as well as the United States and England. But the effort will not be designed as a “mass vaccination” campaign just yet, said Dr. Marie Paule Kieny, the organization’s assistant director general for health systems and innovation.

“I’m talking about tens of thousands of doses, not millions,” Kieny told reporters in Geneva.

She cautioned that the vaccine development would be a slow – and not an entirely failsafe – process.

“There are many ‘ifs,'” Kieny said. “We don’t know if the vaccines are really safe and if they work.”

“There is no vaccine that has no side effects at all,” she added.

An Ebola vaccine has already been tested by dozens of volunteers in the United States as part of trials organized by the National Institutes of Health. 

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, has said recently that the world may not be able to entirely stop the virus outbreak without a vaccine.

Still, he and other public health officials have warned that the development of a vaccine typically takes years.