WASHINGTON (AP) — Once a coronavirus vaccine is approved as safe and effective, Americans should have widespread access within a reasonable time, Dr. Anthony Fauci assured lawmakers Friday.

Appearing before a House panel investigating the nation’s response to the pandemic, Fauci expressed “cautious” optimism that a vaccine would be available, particularly by next year.

“I believe, ultimately, over a period of time in 2021, that Americans will be able to get it,” Fauci said, referring to the vaccine.

There will be a priority list for who gets early vaccinations. “I don’t think we will have everybody getting it immediately,” Fauci explained.

But “ultimately, within a reasonable time, the plans allow for any American who needs the vaccine to get it,” he added.

Under direction from the White House, federal health authorities are carrying out a plan dubbed Operation Warp Speed to manufacture 300 million doses of a vaccine on a compressed timeline.

Fauci, the nation’s top infectious disease official, said a quarter-million people have expressed interest in taking part in studies of experimental vaccines for the coronavirus.

He said that 250,000 people have registered on a government website to take part in vaccine trials, which are pivotal for establishing safety and effectiveness. Not all patients who volunteer to take part in clinical trials are eligible to participate.

Fauci was joined by Centers for Disease Control and Prevention head Dr. Robert Redfield and Health and Human Services testing czar Adm. Brett Giroir.

Giroir acknowledged that currently it’s not possible for the U.S. to return all coronavirus test results to patients in two to three days. He blamed overwhelming demand across the nation.

Many health experts say that COVID-19 results are almost worthless when delivered after two or three days because by then the window for contact tracing has closed.

The latest government data shows about 75% of testing results are coming back within five days, but the remainder are taking longer, Giroir told lawmakers.

Rapid, widespread testing is critical to containing the coronavirus outbreak, but the U.S. effort has been plagued by supply shortages and backlogs since the earliest days of the outbreak.

At a time when early progress seems to have been lost and uncertainty clouds the nation’s path forward, Fauci, Giroir and Redfield are calling on Americans to go back to public health basics such as social distancing and wearing masks.

The panel, the House Select Subcommittee on the Coronavirus Crisis, is divided about how to reopen schools and businesses, mirroring divisions among Americans. Committee Chairman Rep. James Clyburn, D-S.C., said the White House must come up with a comprehensive national plan to contain the virus. Ranking Republican Steve Scalise of Louisiana said the Trump administration has plans already on vaccines, testing, nursing homes and other coronavirus-related issues.

A rebound of cases across the South and the West has dashed hopes for a quick return to normal life. Problems with the availability and timeliness of testing continue to be reported. And the race for a vaccine, though progressing rapidly, has yet to deliver a breakthrough.

Fauci’s public message in recent days has been that Americans can’t afford a devil-may-care attitude toward COVID-19 and need to double down on basic measures such as wearing masks in public, keeping their distance from others and avoiding crowds and indoor spaces such as bars. That’s echoed by Redfield and Giroir, though they are far less prominent.

Fauci’s dogged persistence has drawn the ire of some of President Donald Trump’s supporters and prompted a new round of calls for his firing.