The United States has approved four coronavirus vaccine candidates for clinical trials, Food and Drug Administration (FDA) head Stephen Hahn told reporters.
"Four vaccines have been approved for moving into clinical trials... and another six are in the pipeline for us to review," Hahn said during a press briefing on Tuesday.
The US Administration launched in May Operation Warp Speed, a joint project of Health and Defense Departments, which aims to deliver 300 million doses of a vaccine for Covid-19 by January 2021.
The country's top pandemics expert Anthony Fauci warned on Tuesday, however, that there is no certainty the United States will be able to develop a vaccine against Covid-19 that works and will be safe.
Data on vaccine effectiveness, he added, may be available in the winter or early next year.