Trump has repeatedly said a vaccine for COVID-19, the disease caused by the coronavirus, could be ready for distribution ahead of the Nov. 3 presidential election.

The Washington Post reported on Tuesday the US Food and Drug Administration would issue the guidance to boost transparency and public trust as health experts have become increasingly concerned the Trump administration might be interfering in the approval process to rush out a vaccine.

Trump, however, questioned why a vaccine would need to be delayed and said such a proposal by the FDA would appear to be politically-driven.

"We're looking at that and that has to be approved by the White House. We may or may not approve it," Trump told a White House news conference, when asked about the Post report.

"That sounds like a political move. Because when you have Pfizer, Johnson & Johnson, Moderna, these great companies, coming up with the vaccines, and they've done testing and everything else, I'm saying why would they have to be adding great length to the process."

Trump added he had "tremendous trust" in those companies.

Few vaccine developers were expected to have definitive trial results before the presidential election. Pfizer Inc had been the exception, although its timetable could slip with new guidance.

Moderna Inc has said it is unlikely to have data in October. AstraZeneca Plc's trial in the United States is halted while investigators try to determine whether a serious neurological problem suffered by one participant in the company's U.K. trial was caused by the vaccine.

US Food and Drug Administration head Stephen Hahn did not directly address the Washington Post report when testifying before the Senate earlier on Wednesday.

But he did say regulators would likely provide additional information on the emergency use authorization process.

At the same hearing, US Centers for Disease Control and Prevention head Robert Redfield said he expects there to be about 700 million doses of vaccines available by late March or April, enough for 350 million people.

"I think that's going to take us April, May, June, you know, possibly July, to get the entire American public completely vaccinated," Redfield told the US Senate Health, Education, Labor and Pensions Committee.