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cientists have begun a Phase 2 trial of a vaccine to protect against the Zika virus, the first time an experimental Zika vaccine has gone beyond initial safety testing.

The trial is testing a vaccine developed by scientists at the National Institute for Allergy and Infectious Diseases. If all goes well with the Phase 2 work, scientists may see clues by the end of this year as to whether the vaccine provides protection against Zika, NIAID Director Dr. Anthony Fauci said Friday.

The clinical trial involves a DNA-based vaccine, in which proteins from the Zika virus are placed into small pieces of DNA. Although there are several DNA vaccines licensed for agricultural use, there are currently no licensed DNA vaccines for use in humans.

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The Phase 2 trial began Wednesday at Baylor College of Medicine in Houston, one of 11 sites involved in the study. Most of the others are outside the continental US, in places where Zika is or has been transmitting. They include Puerto Rico, Mexico, Costa Rico, Brazil, and Peru.

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Fauci acknowledged public interest in Zika has waned. But he said it is critical to pursue the development of the vaccine. It is likely the Zika virus has become or will become entrenched in parts of the Americas, Fauci said, and thus will continue to pose a risk to pregnant women who may be exposed to it.

Infection during pregnancy — especially early in pregnancy — can lead to severe neurological birth defects.

“I’m totally intent on getting this vaccine to the point where it can be a usable vaccine,” Fauci said during a teleconference for journalists, calling the development of a Zika vaccine “a very high priority” for the NIAID.

He said his institute — part of the National Institutes of Health, which was targeted for deep cuts in President Trump’s recent budget proposal — has dedicated funding for the Phase 2 work. That will cost $100 million, he said.

If the vaccine is protective and a larger Phase 3 trial is needed, some of that cost would be borne by a commercial partner. To date, NIAID has not lined up a large vaccine manufacturer to help develop and license the experimental vaccine, but Fauci said the institute is currently in negotiations with a couple of interested companies.

The Phase 2 trial will be done in two segments. The one that began this week will involve 90 healthy volunteers — but no pregnant women —all of whom will be immunized with the vaccine. The goal is to determine the optimal dose.

A second segment of the Phase 2 trial, which will start after the dose question has been answered, will involve 2,400 healthy volunteers. Half of them will receive the vaccine and half will get a placebo. Subjects will be monitored for nearly two years.

If Zika continues to circulate in the areas where the volunteers live, comparing infection rates between the vaccine and placebo groups should show if the vaccine is warding off infection. But it’s possible, Fauci acknowledged, that the trial might have to be expanded — to 5,000 people or more — to show effectiveness, if Zika transmission drops off.

That’s a possibility, Fauci admitted. Flaviviruses — the family to which Zika belongs — tend to cause disease in waves. Transmission can taper off for a period of time after a big outbreak, but would be expected to resume at some point down the road.

A DNA vaccine cannot cause Zika; it does not include live viruses. Several other types of Zika vaccines are also in development, including others at the NIAID.

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