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 d…