WASHINGTON (NEXSTAR) ─ National Institutes of Health Director Dr. Francis Collins updated lawmakers Wednesday morning on Capitol Hill regarding the progress toward a safe and effective COVID-19 vaccine.

Collins said he expects one of the six vaccine candidates currently in production to be successful.

“Will it be done by a certain date? I could not possibly tell you right now … because I don’t know what’s going to happen in the coming months,” Collins said. “I do have cautious optimism that [there will be one] by the end of 2020.”

President Donald Trump said he believes a vaccine could be ready even sooner and has said one could be ready before the November election.

But some lawmakers, like Sen. Bernie Sanders, D-Vermont, don’t want the vaccine’s approval to be a political decision.

“Will you join me – in telling the President of the United States to get out of science?” Sanders asked Collins.

“I can’t say strongly enough that the decisions of how this vaccine is evaluated and assessed is going to be based on science,” Collins replied. “That will be the only measure.”

One vaccine maker, AstraZeneca, is pausing production of their vaccine after discovering an unexpected complication, but Collins said the process still has time to work.

“The reason we’re investing in six vaccines because the expectation they won’t all work,” Collins said. “To have a clinical hold is not at all unprecedented … This ought to be reassuring.”

Sen. Lamar Alexander, R-Tennessee, said the challenge of gaining the public’s trust in a vaccine will be a major hurdle.

“There’s likely to be a COVID-19 vaccine ready for the most vulnerable citizens by the end of the year,” he said.