Inovio Pharmaceuticals Inc said its experimental Zika vaccine induced robust immune responses in an early-stage study in human volunteers who have not been infected with the virus.

The company's shares rose 6.6 percent to $7.39 in premarket trading on Wednesday.

The Zika virus has spread to more than 60 countries and territories since the current outbreak was identified last year in Brazil, raising alarm over its ability to cause the rare birth defect microcephaly as well as other neurological disorders in infants and adults.

There are no approved vaccines or treatments for the mosquito-borne virus, although a plethora of companies are racing to mount a defensive strategy to contain the virus.

Forty volunteers - inclusive of both low- and high-dose groups - demonstrated Zika-specific immune responses after one or two vaccinations, Inovio said, adding there were no significant safety concerns after 14 weeks from the initiation of dosing.

Inovio's DNA vaccine is injected along with a brief low voltage electronic pulse that induces cell membranes to open, making them more receptive, in theory, to accepting the vaccine's genetic material.

The prospect of a blockbuster vaccine has also attracted the interest of big drugmakers, including Sanofi SA, GlaxoSmithKline Plc and Takeda Pharmaceuticals.