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Zika Virus Outbreak

New Zika Vaccine Starts Wider Testing

Federal researchers said Friday they have started a new phase of testing a vaccine to protect against the Zika virus, which has been causing severe birth defects across the Americas.

The $100 million trial will test the experimental vaccine in 2,500 volunteers in the U.S., Mexico, Puerto Rico, Brazil and other countries affected by the mosquito-borne virus, the National Institutes of Health said.

Zika threat growing; additional 'aggressive' action needed, officials say 0:17

"A safe and effective Zika vaccine is urgently needed to prevent the often-devastating birth defects that can result from Zika virus infection during pregnancy," Dr. Anthony Fauci, director of the NIH's National Institute of Allergy and Infectious Diseases (NIAID), said.

"Evidence also is accumulating that Zika can cause a variety of health problems in adults as well."

The virus has been spread by mosquitoes in more than 80 countries. It can be spread sexually, as well. Once thought to be of no importance at all, Zika turns out to cause devastating birth defects in babies born to women who become infected while pregnant.

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Will microcephaly — a small and misshapen head — is the most obvious, babies have been born with a range of other defects as well, and the virus also can cause miscarriages.

Researchers rushed a vaccine into development when it became clear the virus was so harmful and was affecting tens of thousands of people.

Several Zika vaccines are in early, Stage 1 or preclinical studies. This particular vaccine is the furthest along in development. It uses a small piece of DNA with a bit of Zika genetic material inserted.

It prompts an immune response against the virus, Fauci said.

"There are no safety concerns and the immune responses that we are getting are looking quite good so far," he told reporters.

Related: Early Trial Shows Striking Results for Zika Vaccine

How fast this stage of testing goes will depend on how bad outbreaks of Zika are in the countries taking part, and on how well the vaccine works, Fauci said. If it works very well to protect people against Zika, it's possible the U.S. Food and Drug Administration could speed its approval or allow emergency use, he said.

Although the threat is greatest to pregnant women, no pregnant women will be included in the trial, Fauci said. Eventually, if the vaccine or one of the other vaccines being tested gets approved, the plan would be to vaccinate children or teens against the virus so they are protected long before childbearing age.

The virus is likely to become endemic — a permanent resident — in parts of south and central America, Fauci said, and it's likely to cause periodic outbreaks in the U.S.

Travelers from affected regions bring the virus back when they become infected and if the Aedes aegypti mosquitoes that spread the virus live in the traveler's next stop, they can bite the infected person and themselves become infected.

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It's happened several times in south Florida and in south Texas.

And tens of thousands of Americans have been infected by travel — more than 43,000 in U.S. states and territories and probably far more never counted, Fauci said.

Of these, 5,000 are pregnant women. At least 54 babies have been born in the U.S. with Zika-linked birth defects and at least seven have miscarried or been aborted because of severe defects.

The NIH has been targeted by the Trump administration for billions of dollars in budget cuts, but Fauci said development of a vaccine was a priority for his agency and the funding was secure for this trial.

Future work is less certain.

"I really can't predict what the budget situation is going to be," he said.

Advanced development of a vaccine will involve a company, which can pay at least part of the costs, he said.