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.