Inovio Pharmaceuticals in Plymouth Meeting said Thursday that it has completed enrollment and begun a clinical study of its experimental Zika vaccine in 160 healthy adult volunteers in Puerto Rico, where the virus outbreak has been declared a public health emergency.

Eighty volunteers received the vaccine, and 80 received a placebo.  Results will be known in about a year, said J. Joseph Kim, Inovio president and CEO.

In February, Inovio reported positive clinical results in a human Zika trial at three U.S. and Canadian locations, including Philadelphia. All volunteers were given three doses, several weeks apart. One dose elicited strong antibody responses in 40 percent of the volunteers. Two doses generated strong antibody responses in 37 of 39 subjects, or 95 percent, the company said, Three doses generated a robust antibody response in 100 percent of volunteers. The company said the vaccine was well tolerated and produced no serious side effects.

Inovio will  have to do another study, where people are given the vaccine and then exposed to Zika virus, to test whether the antibody response is strong enough to prevent Zika virus infection. Such a study will have to be done before the Food and Drug Administration will approve commercializing the product. "We will have to get into an area where there's active infection going on, give some people the vaccine, watch other people who didn't get the vaccine and see how many of them get infected, to be able to prove that the antibody response is strong enough to protect them," said Scott White, vice president of clinical development.

Inovio is developing its Zika vaccine with GeneOne Life Science in South Korea and academic collaborators, including the Wistar Institute in University City.

To date, there are no approved vaccines or therapies for the mosquito-borne Zika virus infection.

Inovio is the first company to report clinical data of its vaccine in humans.  The National Institutes of Health has tested a vaccine in people, but  has not publicly discussed the results, White said.

Inovio 's DNA-based vaccine does not use traditional live or killed viruses, and is quicker to develop because it creates a synthetic DNA sequence in a lab that triggers the body to create the same antigens that come from a killed or weakened virus, Kim said.

The Zika virus causes mild flu-like symptoms in most infected people, but has been linked to an increase in babies born with microcephaly, which results in abnormally small heads and impaired brain development. Health authorities also have seen an increase in Guillain-Barré syndrome, a neurological disorder, in areas affected by Zika.