WATCH: Trump immediately caves in debate after moderator fact checks him on having vaccine 'within weeks'
Kristen Welker and Donald Trump appear at 2020 presidential debate (OAN/screen grab)

President Donald Trump backed down on Thursday after initially promising a COVID-19 vaccine "within weeks."

Trump made the claim during his opening remarks of the final 2020 presidential debate. He was quickly pressed on the subject by moderator Kristen Welker.

"You said a vaccine will be coming within weeks?" Welker asked. "Is that a guarantee?"

"No, it's not a guarantee," Trump replied. "But it will be by the end of the year. I think it has a good chance it will be within a matter of weeks. And it will be distributed very quickly."

Welker noted that Trump administration officials have said that a vaccine would not be available for most people until 2021.

"Is your timeline realistic?" she wondered.

"I think my timeline is going to be more accurate," the president replied. "I don't know they are counting on the military the way I do."

Watch the video below from OAN.