Even as federal health officials announced plans to speed development of an Ebola vaccine, scientists who study the virus warned that the task would be arduous and that success was hardly guaranteed.

The Ebola virus is “a survivor,” said John Dye, chief of the viral immunology branch at the United States Army Medical Research Institute of Infectious Diseases. “It does what it can to avoid the human immune system.”

The government plans to fast-track development of a vaccine shown to protect macaque monkeys, aiming to test it in humans as early as next month.

If the vaccine proves effective, it may be given to health care workers and others at high risk for infection sometime in 2015, said Dr. Anthony S. Fauci, the head of the National Institute of Allergy and Infectious Diseases.