An experimental Zika vaccine has moved successfully into broader testing, with the first volunteer receiving a test dose in Houston earlier this week. Testing will also begin in Miami and San Juan, Puerto Rico, and by June, researchers hope to enroll more than 2,000 volunteers in those cities and other regions in the Americas to determine whether the vaccine is effective in preventing infection, a top U.S. researcher said Friday.

The experimental vaccine, developed by scientists at the National Institute of Allergy and Infectious Diseases, is one of the first to progress to expanded testing of at least a half-dozen candidates in the development pipeline in the United States. Initial results could be available by the end of this year. If this next phase shows good results, and another outbreak of Zika flares in South America or elsewhere, it’s possible that the Food and Drug Administration could make the vaccine available on an emergency basis, said NIAID Director Anthony Fauci in a conference call with reporters.

“If there is a good vaccine efficacy signal and there is an outbreak in South America, the FDA could make that vaccine available by different mechanisms,” he said. “But it depends on the emergent need of the vaccine and the quality of the data.”

Public health experts and scientists have warned that urgent measures are needed to prevent infections because of the devastating birth defects that can result from Zika virus infection during pregnancy. Severe microcephaly, characterized by abnormally small head size and often an underdeveloped brain, is only one of the many defects. There is a range of problems that affect vision, hearing and joints, and often don’t become apparent until many months after birth.

Although Zika has faded from the headlines, officials are warning people to take preventive measures with the return of warm weather and mosquito season in the coming months. The virus can also be transmitted through sex. Florida has already reported two cases of Zika this year that were acquired locally. In the continental United States, of 1,228 Zika-infected pregnancies, 54 babies have been born with birth defects and seven resulted in miscarriages, stillbirths or abortions, according to federal data.

“Evidence also is accumulating that Zika can cause a variety of health problems in adults as well, including Guillain-Barré syndrome and heart-related issues,” Fauci said, underscoring the urgent need for effective medical countermeasures.

The costs of the trial, $100 million, have been funded and will not be affected by the Trump administration's proposed budget cuts to the National Institutes of Health, Fauci said.

“That money has been allocated to this trial, and it's not going to be unallocated,” he said.

Researchers began testing the safety of the experimental vaccine in the summer. Results have been encouraging, Fauci said, and scientists have moved to the next phase of testing, which will take place in two stages.

First, researchers want to further evaluate the vaccine’s safety and its ability to stimulate an immune response in communities where Zika has been confirmed. They also want to determine the optimal dose. They are seeking to enroll 90 healthy men and nonpregnant women at three sites in Houston, Miami and San Juan. The Houston site at Baylor College of Medicine began testing the vaccine on its first volunteer Wednesday, he said. All the participants will either receive a standard dose or a high dose of the investigational vaccine. The testing will take place over the course of 12 weeks, although participants will be followed for much longer.

Starting in June, researchers hope to enroll at least 2,400 men and nonpregnant women, ages 15 to 35, in many more locations to determine whether the vaccine can protect them in places where they are naturally exposed to the virus. In addition to Houston, Miami and San Juan, researchers plan to enroll volunteers in Brazil, where the Zika outbreak began, Costa Rica, Mexico, Panama and Peru. Participants will be randomly assigned to receive either the experimental vaccine or a placebo. They will receive three doses, each four weeks apart. The volunteers will be followed for nearly two years and be assessed for any adverse symptoms or signs of Zika infection.

The experimental vaccine does not have any infectious material, so researchers say it can’t cause a person to become infected with Zika. NIAID researchers engineered a small circular piece of DNA called a plasmid so it contains genes coded for proteins of the Zika virus. When injected, a person’s cells read the genes and make those Zika proteins, which in turn trick the body into mounting a defense with antibodies and T cells.

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