Liz Szabo

USA TODAY

As health officials work to contain the country's first Zika outbreak, centered in a Miami neighborhood, scientists at the National Institutes of Health announced the launch Wednesday of a clinical trial of an experimental Zika vaccine.

It's the second such study to begin this summer: Inovio Pharmaceuticals provided the first dose of its Zika vaccine July 26.

There are no effective vaccines or treatments for Zika, leaving scientists scrambling to protect people, particularly pregnant women whose unborn babies are at risk of devastating birth defects.

With 15 people infected by local mosquitoes in Miami, and more than 1,600 people in the continental U.S. infected through travel, scientists pushed themselves to begin the trial as quickly as possible, launching it a month ahead of schedule, said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

“A safe and effective vaccine to prevent Zika virus infection and the devastating birth defects it causes is a public health imperative,” Fauci said. "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," he added.

The NIH study began Tuesday, when the first volunteer was vaccinated. The vaccine will be tested in 80 healthy volunteers, ages 18 to 35, at three locations: the NIH Clinical Center in Bethesda, Md.; Emory University in Atlanta; and the University of Maryland in Baltimore, Fauci said. The vaccine is similar to an experimental West Nile Virus vaccine already developed by the NIH, but not yet approved. All 80 of the volunteers will get different doses of the active vaccine; none will get placebos.

If successful, the NIH vaccine would ideally be given to women and teens of childbearing age and their sexual partners, Fauci said. Doctors are concerned about Zika because it can cause catastrophic birth defects in fetuses, including microcephaly, in which babies are born with abnormally small skulls and, in most cases, incomplete brain development.

Vaccinating women of childbearing age would protect their fetuses when they later become pregnant and vaccinating men would protect their sexual partners, Fauci said. Although Zika is predominantly spread by mosquitoes, both men and women can transmit the virus sexually.

Both the NIH and Inovio vaccines are called DNA vaccines and work in a relatively new way.

DNA vaccines are being tested against a number of viruses, but haven’t yet been approved by the Food and Drug Administration, said Srilatha Edupuganti, an associate professor at Emory, who is leading that university’s part of the NIH vaccine study. Unlike traditional vaccines -- which are made with killed or weakened viruses, or proteins from a virus or bacteria that are designed to stimulate the immune system – DNA vaccines contain just genetic material related to key proteins.

Scientists make DNA vaccines by genetically engineering a small, circular piece of genetic material — called a plasmid — that carries instructions for making two Zika virus proteins, Edupuganti said. After the vaccine is administered, a person's cells use the instructions to make Zika virus proteins, which assemble themselves into virus-like particles. The body then mounts an immune response to these particles, protecting people from future infections, Edupuganti said.

DNA vaccines have an advantage over traditional vaccines because they can be manufactured and scaled up quickly, which helped NIH launch its trial ahead of schedule, Edupuganti said. Once DNA vaccines are proven to work, "you can turn on a dime to make them," Fauci said.

If the initial NIH study proves the vaccine is safe, a larger study in a Zika-affected country or territory could begin in January, Fauci said.

Although the first study "is a go," Fauci said he is concerned about running out of money to launch a larger study next year. President Obama asked Congress to approve $1.9 billion in emergency money for Zika in February, but lawmakers have not been able to agree on a funding package. Without more money soon, Fauci said the larger trial could be delayed.

Developing a vaccine and getting it approved by the Food and Drug Administration typically takes many years. Researchers have been rushing to create one for Zika due to the urgency of the epidemic, which is blamed for a surge in birth defects in Brazil and other countries.

Developing a vaccine for an ongoing epidemic is also challenging. Zika exploded in Latin America last year, striking a population with no natural immunity to the virus. Epidemics of new viruses often burn themselves out after a period of months, however, as more people become infected and develop lifelong immunity. By the time a vaccine is ready to be tested, there may not be enough cases to show whether it's effective.

Even if the Zika vaccine isn't ready in time to prevent many infections in Brazil, the country hardest hit by the virus, it could help prevent cases elsewhere in the world, said William Schaffner, a professor at the Vanderbilt University School of Medicine.

Although Zika first appeared in Africa, the virus has in recent years diverged into two strains: an African strain and an Asian strain, which is the one that spread rapidly in Brazil. If the Asian Zika strain spreads widely to Africa, the population could be hit hard, Schaffner said.

Zika is already causing outbreaks in Cape Verde, off the African coast. It's also spreading not just in the Americas, but in the South Pacific.