Redfield then said that widespread distribution of the vaccine would happen gradually, with those least at risk likely not receiving the vaccine until the summer of 2021. For it to be "fully available to the American public, so we begin to take advantage of vaccine to get back to our regular life," he said, as The Washington Post reported. "I think we are probably looking at late second quarter, third quarter 2021."

In his testimony, Redfield also acknowledged that a vaccine is not 100 percent effective and works best when most people in a population have it. In contrast to the vaccine, he said, a mask is a reliable and scientifically proven tool to stop the spread of the virus.

"I might even go so far as to say that this face mask is more guaranteed to protect me against COVID than when I take a COVID vaccine, because the immunogenicity may be 70 percent," he said to lawmakers, as CNN reported. "And if I don't get an immune response, the vaccine is not going to protect me. This face mask will."

He added that the American public has not yet adapted to the widespread use of masks that could stop the transmission of the coronavirus outbreak.

The remarks under oath from Redfield clashed with President Trump's insistence that a vaccine will be ready in a few weeks. Redfield's championing of mask-wearing also contrasted with Trump's reluctance to wear one in public.

In a press conference Wednesday, Trump publicly undermined Redfield's testimony, continuing to say that a vaccine will be ready shortly, and expressing skepticism about the efficacy of masks, according to The New York Times.

In response to Redfield's testimony about a timeline for a vaccine, Trump said, according to The Guardian, "I think he made a mistake with that statement. When he said it, I believe he was confused. I'm just telling you we're ready to go."

Trump then added that a vaccine would go "to the general public immediately," and "under no circumstance will it be as late as the doctor said," as The New York Times reported.

On the point of masks, Trump cast doubt on Redfield's testimony, saying that Redfield made a mistake and that he was confused by the question.

"Maybe he misunderstood it," Trump said, as CNN reported. He then added, "As far as the masks are concerned, I hope that the vaccine is going to be a lot more beneficial than the masks."

He also expressed doubt that they were effective.

"Masks have problems too ... A lot of people did not like the concept of mask initially, Dr. Fauci didn't like it initially," Trump said, as The Guardian reported. However, he did not add that Dr. Fauci later clarified his comments that he did not want to see panic that would create a run on N95 masks and leave a shortage for health care workers.

After Trump publicly rebuked the head of the world's preeminent health agency, Redfield released a statement that reiterated the importance of a vaccine and masks, but did not address the idea that he was mistaken.

"A COVID-19 vaccine is the thing that will get Americans back to normal everyday life. The best defense we currently have against this virus are the important mitigation efforts of wearing a mask, washing your hands, social distancing and being careful about crowds," he said in the statement, as CNN reported.