Stella Guerra performs physical therapy on an infant born with microcephaly at Altino Ventura Foundation in Recife, Brazil. (Mario Tama/Getty)

As the Zika virus continues its spread, infecting people in more than 50 countries and threatening fetal development in pregnant women, scientists have been racing to develop an effective vaccine for the disease.

Federal researchers on Wednesday announced a milestone in that effort: their first clinical trial in humans.

The trial will involve at least 80 healthy volunteers between ages 18 and 35 at three locations around the United States, including at the National Institutes of Health Clinical Center in Bethesda. The main goal of the study will be to evaluate the vaccine's safety and to see whether it generates an immune-system response in patients. If those early results are positive, researchers hope to began a larger-scale trial in Zika-affected countries in early 2017.

"A safe and effective vaccine to prevent Zika virus infection and the devastating birth defects it causes is a public health imperative," Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said in a statement. "NIAID worked expeditiously to ready a vaccine candidate, and results in animal testing have been very encouraging. We are pleased that we are now able to proceed with this initial study in people. Although it will take some time before a vaccine against Zika is commercially available, the launch of this study is an important step forward."

Fauci told The Washington Post in a previous interview that government scientists have been able to leverage research done on two similar viruses — West Nile and dengue — to quickly create vaccine candidates that target Zika, which currently has no cure or effective treatment.

Various private efforts are underway to create Zika vaccines, part of a global effort by scientists to better understand and fight the mysterious virus.

In June, Pennsylvania vaccine maker Inovio Pharmaceuticals and South Korea’s GeneOne Life Sciences announced plans to start testing a DNA vaccine, known as GLS-5700, on humans. Inovio chief executive J. Joseph Kim said the company, which is also working on vaccines for other devastating global viruses such as Ebola and Middle East respiratory syndrome (MERS), hoped to report results later this year.

The experimental vaccine that NIH researchers are testing includes a small, circular piece of DNA, known as a plasmid, which scientists engineered to contain genes that code for proteins of the Zika virus. After the vaccine is injected into a person's arm, cells read the genes and begin to produce Zika virus proteins. The goal is to jump start the body into creating an immune response that would then protect against an infection from an actual Zika virus.

Other sites participating in the Zika trial include the Center for Vaccine Development at the University of Maryland School of Medicine and Emory University in Atlanta.

Developing vaccines typically is an arduous process that can take decades and cost hundreds of millions of dollars. That's partly why, in recent years, many pharmaceuticals companies have scaled back vaccine research efforts and focused their efforts on developing newer versions of already existing vaccines. In some cases, such as during the Ebola epidemic, the government has stepped into that vacuum to fund the study of vaccines that otherwise likely would go undeveloped.

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