The Plymouth Meeting, Pennsylvania-based company had previously said it would begin mid-stage human studies for the vaccine candidate, INO-4800, this summer.
Shares of the company were down 6% in trading after the bell.
Inovio is in active talks with the U.S. Food and Drug Administration and awaits the agency's approval to begin Phase 2/3 study in September, a company executive said during a post-earnings conference call.
The company in June reported encouraging results from an early stage human trial and received $71 million from the U.S. Department of Defense to scale up production of devices used to administer INO-4800.
Effective vaccines are seen as essential to controlling the pandemic that has claimed more than 731,700 lives worldwide, according to a Reuters tally.Many drugmakers, including Moderna Inc, Pfizer Inc , Johnson & Johnson, Sanofi and AstraZeneca Plc, are also in various stages of developing a vaccine for COVID-19.