The federal government's medical research agency is starting to test a potential vaccine for Zika, which is now spreading through mosquito bites in the U.S.

Researchers are in the early stages of a clinical trial for the experimental vaccine, which they are administering to at least 80 healthy individuals at three sites around the country. Developed at the National Institutes of Health's National Institute of Allergy and Infectious Diseases earlier this year, the vaccine contains a small piece of DNA that may cause the body to mount an immune response to Zika virus proteins.

Institute Director Dr. Anthony Fauci announced the clinical trial Wednesday, calling it an "important step forward" to finding an effective vaccine for Zika that can be used widely.

"A safe and effective vaccine to prevent Zika virus infection and the devastating birth defects it causes is a public health imperative," Fauci said. "NIAID worked expeditiously to ready a vaccine candidate, and results in animal testing have been very encouraging. We are pleased that we are now able to proceed with this initial study in people."

The threat of Zika, particularly to the fetuses of pregnant women, is growing in the U.S. as mosquitos have started spreading the virus in Florida. More than 6,400 Zika cases have been reported in U.S. states and territories, with more than 1,600 of them in the continental U.S., according to the Centers for Disease Control and Prevention.

Researchers have warned that approval of a Zika vaccine could take several years, likely well after the initial Zika outbreak has subsided.

In the Zika phase one trial, volunteers will be divided into four study groups of 20 people each, receiving dosages of the vaccine at different times. Researchers hope to start a phase two trial in Zika-endemic countries early next year, if the phase one trial goes well. The approach is similar to researchers they developed a vaccine for the West Nile virus, which prompted an immune response in people who participated in a phase one clinical trial.