U.S. researchers have begun enrolling people in the next phase of testing for a vaccine to protect against Zika, the mosquito-borne virus that can cause birth defects in pregnant women. Dr. Anthony Fauci, director of the National Institutes of Allergy and Infectious Diseases (NIAID), told reporters Friday that the Zika vaccine had cleared preliminary safety hurdles and would now be tested on human volunteers to see whether it is effective. In the study, funded by the U.S. government, researchers aim to enroll more than 2,400 healthy volunteers from areas where mosquitoes carry the Zika virus — parts of the southern United States, Puerto Rico, Brazil, Peru, Costa Rica, Panama and Mexico. Researchers at the U.S. National Institutes of Health (NIH) said the trial would begin with a small number of people testing different doses or strengths of vaccine. Once the dosage is decided, the larger part of the study could begin by June, when the volunteers will receive either the vaccine or a placebo. Participants will be monitored for two years to see whether the vaccine protects against Zika infection. The vaccine being tested is a new type, called a DNA vaccine. Traditionally, vaccines are made using killed or weakened viruses, which increase the recipients' ability to fight off an active infection. The DNA vaccine contains no actual virus, but has genes extracted from Zika viruses. Once inside the body, the genes form particles resembling Zika that cannot cause infection. If all goes well, the gene particles should induce volunteers' immune systems to produce antibodies capable of repelling the full virus. NIH researchers also are studying more traditional Zika vaccines, but those are not yet ready for human trials. The DNA vaccine trial is expected to cost $100 million, but Fauci said the government was in talks with pharmaceutical companies to share the costs of the final stage of testing, in return for rights to manufacture the vaccine in the future. Zika typically causes no symptoms or only mild ones, such as fever and body aches. If the virus infects a pregnant woman, however, it can result in birth defects in newborns, including microcephaly, which is characterized by an abnormally small head and brain, accompanied by marked developmental disorders. Zika is primarily transmitted by mosquitoes, but it can also be transmitted via sexual contact.