WASHINGTON, July 5 (Xinhua) -- Commissioner of the U.S. Food and Drug Administration (FDA) Stephen Hahn on Sunday refused to offer a timeline for COVID-19 vaccine, which U.S. President Donald Trump suggested could be available "long before the end of the year."

"I can't predict when a vaccine will be available," Hahn told ABC on Sunday. "We are seeing unprecedented speed for the development of a vaccine. But ... our solemn promise to the American people is that we will make a decision based upon the data and science on a vaccine, with respect to the safety and effectiveness of that vaccine."

During a Fourth of July address in Washington on Saturday, Trump said the country is likely to have a therapeutic and/or vaccine solution long before the end of the year.

Hahn said Thursday that he was "cautiously optimistic" about current efforts to develop a coronavirus vaccine, pointing to "year's end or early next year" as potential completion dates.

More than 2,852,000 COVID-19 cases have been reported in the United States with the fatalities surpassing 129,700 as of Sunday afternoon, according to a tally by Johns Hopkins University.