WASHINGTON — US health officials have begun enrolling volunteers for critical next-stage testing of an experimental vaccine to protect against Zika, the mosquito-borne virus that can cause devastating birth defects in pregnant women.

The first volunteer was vaccinated Wednesday at Baylor College of Medicine in Houston, as the National Institutes of Health gears up for a two-part study enrolling at least 2,400 people in Texas, Florida, Puerto Rico, and five at-risk countries: Brazil, Mexico, Panama, Costa Rica, and Peru.

Zika has caused an epidemic of birth defects — including babies with abnormally small heads and brains — in Latin America and the Caribbean, and continues to spread to other countries. For the US, the risk has largely been to travelers, although mosquitoes spread the virus last year in parts of southern Florida and Texas.

But while Zika largely disappeared from the headlines this winter, mosquito season is fast approaching — and the risk persists internationally.

‘‘It is imperative that public health research continue to work to contain the spread of the virus,’’ Dr. Anthony Fauci, director of NIH’s National Institute of Allergy and Infectious Diseases, said in announcing the $100 million study.

First-stage safety testing of a so-called DNA vaccine against Zika signaled no side effect concerns, Fauci said — allowing the NIH-created shots to progress to the next stage of testing that will help tell if they really work.

Associated Press