WASHINGTON — President Biden and a top health official warned Monday that too many Americans are declaring virus victory too quickly, appealing for mask requirements and other restrictions to be maintained or restored to stave off a “fourth surge” of COVID-19. The head of the CDC said she had a feeling of “impending doom” if people keep easing off.

The double dose of warnings came even as Biden laid out hopeful new steps to expand coronavirus vaccinations, with all adults to become eligible over the next 5 weeks. Biden announced plans to expand the number of retail pharmacies that are administering vaccines, and investments to help Americans get to vaccination sites. But the optimism was tempered by stark warnings about the potential for another wave of cases.

“This is deadly serious,” Biden said, urging governors to reinstate mask mandates and other restrictions that some states have been easing.

Hours earlier, during a virtual White House health briefing, Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention, grew emotional as she reflected on her experience treating COVID-19 patients who are alone at the end of their lives.

President Biden speaks during an event on COVID-19 vaccinations and the response to the pandemic, in the South Court Auditorium on the White House campus on Monday in Washington. Evan Vucci/Associated Press

“We have so much to look forward to, so much promise and potential of where we are and so much reason for hope,” she said. “But right now, I’m scared.”

“I’m going to lose the script, and I’m going to reflect on the recurring feeling I have of impending doom.”

Cases of the virus are up about 10% over the past week from the previous week, to about 60,000 cases per day, with both hospitalizations and deaths ticking up as well, Walensky said. She warned that without immediate action the U.S. could follow European countries into another spike in cases and suffer needless deaths.

“I have to share the truth, and I have to hope and trust you will listen,” she said.

Later Monday, Biden addressed the nation from the White House, declaring, “If we let our guard down now, we can see the virus getting worse, not better. People are letting up on precautions, which is a very bad thing.”

Biden delivered a direct appeal to governors, state and local leaders to reinstate mask-wearing requirements if they have lifted them, and said he encouraged leaders to pause plans to further ease virus-related restrictions.

“Please, this is not politics, reinstate the mandate if you let it down,” he said.

Biden announced that by April 19 at least 90% of the adult U.S. population would be eligible for vaccination — and would have access to a vaccination site within 5 miles of home. Quick vaccination would still depend on supply.

Biden had previously directed that all states make all adults eligible for vaccination by May 1, but many have moved to lift eligibility requirements sooner in anticipation of supply increases.

Meanwhile, the White House is moving to double the number of pharmacies participating in the federal retail pharmacy program — which has emerged as among the most efficient avenues for administering vaccines — and increase the number of doses for them to deliver. Retail pharmacies are located relatively close to most Americans and have experience delivering vaccines like flu shots.

Biden announced that the U.S. is expecting delivery of 33 million doses of COVID-19 vaccine this week — including 11 million of the single-dose Johnson & Johnson shot.

More than one in five adults and nearly 50% of senior American are fully vaccinated, according to data from the CDC. On Thursday, the U.S. set new single-day record for shots in arms: more than 3.2 million.

“Now is not the time to let down,” Biden said. “Now’s not the time to celebrate. It is time to do what we do best as a country: our duty, our jobs, take care of one another.”

“Fight to the finish,” he added. “Don’t let up now.”

Walensky and Dr. Anthony Fauci, the nation’s top infectious disease expert, appealed to elected officials, community leaders and everyday Americans to maintain social distancing measures and mask wearing.

“We are doing things prematurely,” Fauci said, referring to moves to ease up on restrictions. Walensky appealed to Americans, “Just please hold on a little while longer.”

She added: “We are not powerless, we can change this trajectory of the pandemic.”

Walensky pointed to an uptick in travel and loosening virus restrictions for the increase in cases. “People want to be done with this. I, too, want to be done with this,” she said.

“We’ve seen surges after every single holiday,” she reiterated: “Please limit travel to essential travel for the time being.”

The White House, meanwhile is ruling out the creation of a national “vaccine passport” for Americans to verify their immunization status, saying it is leaving it to the private sector to develop a system for people show they’ve been vaccinated. Some other countries are establishing national databases to allow vaccinated people to resume normal activities.

“We do know that there is a segment of the population that is concerned that the government will play too heavy-handed of a role in monitoring their vaccinations,” said White House COVID-19 adviser Andy Slavitt. He said officials are worried that “it would discourage people” from getting vaccinated if the federal government was involved.

The administration, instead, is developing guidelines for such passports, touching on privacy, accuracy and equity, but the White House has not said when those guidelines will be ready.