Inovio Pharmaceuticals in Plymouth Meeting said Monday that it has 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.

Inovio shares closed up 4.63 percent, or 41 cents, to $9.27, on the news.

The Centers for Disease Control and Prevention has estimated that Zika will infect more than 25 percent of the Puerto Rican population by the end of the year, Inovio said.

The study in Puerto Rico involves 80 volunteers who will receive the vaccine and 80 who will receive a placebo to evaluate the safety, tolerability, and immunogenicity of the vaccine against the mosquito-borne virus.

"The rapid progression of the Zika outbreak in Puerto Rico provides an immediate and unique opportunity to assess a preventive vaccine in a real world setting," said J. Joseph Kim, Inovio president and CEO.

"If the results are promising, we plan to meet with regulators in 2017 to map out the most efficient path forward to develop our Zika vaccine and help mitigate this widespread Zika outbreak that has expanded into the continental United States."

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

In June, Inovio began a human Zika trial at three U.S. and Canadian locations, including Philadelphia, and has enrolled 40 healthy adults. The company expects to report results later this year.

To date, there are no approved vaccines or therapies for Zika virus infection. Several companies and academic groups have announced plans to develop Zika virus vaccines. Only Inovio and a U.S. government research center have begun human clinical studies, the company said.

Inovio has tested its DNA-based vaccine - using DNA sequences of the Zika virus - in mice and monkeys with positive results. The company said its vaccine induced robust antibody and T-cell responses - the immune responses necessary to fight viral infections - in animals.

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 as from a killed or weakened virus, Kim said.

The Zika virus causes mild flulike 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.

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