Commissioner of the US Food and Drug Administration (FDA) Stephen Hahn has refused to offer a timeline for Covid-19 vaccine, which US 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 a leading media outlet on Sunday, Xinhua reported.
"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.