Democratic nominee Joe Biden said Wednesday that he does not trust President Donald Trump when it comes to developing an effective vaccine to treat COVID-19.

“Let me be clear: I trust vaccines, I trust scientists, but I don’t trust Donald Trump,” Biden said. He called on Trump to answer three questions regarding the development of a vaccine.

Biden said his first question is: "What criteria will be used to ensure that a vaccine meets the scientific standard of safety and effectiveness?”

“Second, if the Administration greenlights a vaccine — who will validate that the decision was driven by science rather than politics?” Biden continued. "Third, how can we be sure that the distribution of the vaccine will take place safely, cost-free, and without a hint of favoritism?"

Trump promised a vaccine by the end of 2020. In an interview that aired Tuesday on Fox News, Trump said a vaccine would be ready “in a matter of weeks.”  

Democrats have expressed skepticism about Trump’s claims about a vaccine before the election. Earlier this month, Trump demanded Democrats to “immediately apologize for the reckless anti-vaccine rhetoric that they’re talking right now."

Centers for Disease Control and Prevention Director Robert Redfield told a Senate panel on Wednesday that a vaccine would not become widely available until "the second or third quarter" of 2021. For now, Redfield said facemasks are "the most important, powerful public health tool we have."

"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," the CDC director said.

At a White House press briefing Wednesday evening, Trump said he thought Redfield “made a mistake” in the vaccine comments. 

“We’re very close to that vaccine as you know and I think much closer than I think most people want to say,” Trump said. “We think we can start some time in October. So as soon as it’s announced we’ll be able to start. That will be from mid-October on. It may be a little bit later than that.”

The United States currently has the worst outbreak of COVID-19 in the world. As of Wednesday, the U.S. has 6.62 million cases of COVID-19 and over 196,000 deaths, according to Johns Hopkins University.