Obama, in an interview with SiriusXM radio, said he would be inoculated if top US infectious disease expert Anthony Fauci signs off on a COVID-19 vaccine.

"If Anthony Fauci tells me this vaccine is safe, and can vaccinate, you know, immunize you from getting Covid, absolutely, I'm going to take it," Obama said.

"I promise you that when it's been made for people who are less at risk, I will be taking it," he said.

"I may end up taking it on TV or having it filmed, just so that people know that I trust this science, and what I don't trust is getting Covid," Obama said.

Freddy Ford, Bush's chief of staff, told CNN the former president also wanted to help promote vaccination.

"First, the vaccines need to be deemed safe and administered to the priority populations," Ford told CNN.

"Then, president Bush will get in line for his, and will gladly do so on camera."

Angel Urena, Clinton's press secretary, told CNN the former president would also be up for getting a vaccine in public on television.

Later in the day, Biden told CNN in an interview that he, too, would be willing to be vaccinated in public after government approval of vaccines, specifically saying he would rely on Fauci to say it was safe.

"It's important to communicate to the American people it's safe," the 78-year-old said. "It's safe to do this."

He also lauded the three former presidents for their commitment, saying they had "set the model as to what should be done."

Vaccines developed by Pfizer-BioNTech and Moderna-NIH are expected to be approved by US authorities shortly.

A top science official said Wednesday the United States hopes to have immunized 100 million people by the end of February.