America’s top infectious-disease expert sounded the alarm Sunday, warning of a “surge superimposed upon” a surge of coronavirus cases over the coming weeks due to Thanksgiving travel and celebrations.

Anthony S. Fauci and other experts urged Americans to take aggressive action as the December holidays loom to mitigate the surge overwhelming hospitals across the country. As the number of coronavirus-related deaths per day rose to its highest point since April, Fauci and others highlighted the importance of complying with mask mandates and physical distancing.

“It’s going to get worse over the next several weeks, but the actions that we take in the next several days will determine how bad it is or whether or not we continue to flatten our curve,” U.S. Surgeon General Jerome Adams said on Fox News on Sunday.

Deborah Birx, coordinator of the White House coronavirus task force, warned that the number of coronavirus cases is “three, four, and 10 times” as high as it was during the second surge after Memorial Day weekend.

“It looked like things were starting to improve in our northern plain states, and now with Thanksgiving, we’re worried that all of that will be reversed,” Birx said on CBS News’s “Face the Nation,” adding that people need to “take it upon yourself to be restrictive” even in states that do not have regulations in place to curb the spread of the virus.

The number of coronavirus-related deaths are nearing record levels in the United States not seen since the spring. About 95,000 people are currently hospitalized with the disease, according to Adm. Brett Giroir, assistant secretary of health and human services. Roughly 20 percent of all hospitalized people have covid-19, he added.

Asked if the United States could reach 4,000 deaths per day in the aftermath of a record number of travelers, Giroir said that he couldn’t project how much the weekend may have exacerbated spread of the virus. “We really have to see what this weekend looks like. I can’t really project that,” Giroir said on CNN’s “State of the Union.” But he called this moment “a really dangerous time.”

“But, remember, we’re not passive bystanders,” he added. “If we do the right thing — universal mask-wearing, avoiding indoor spaces, crowded bars, restaurants, indoors, all those [sorts] of things — we can still flatten this.”

The first wave of vaccinations is expected to commence for prioritized groups in a matter of weeks, which Fauci on “Meet the Press” called a “light at the end of the tunnel” to the pandemic that has killed over 266,000 Americans this year.

The Centers for Disease Control and Prevention advisory committee on immunization practices will meet Tuesday to vote on coronavirus vaccine priority rankings. Health-care workers are likely to be the first to receive the vaccine once it has been approved, though Fauci said that states will be shipped a certain amount of the vaccine and ultimately make final distribution decisions “with strong recommendations from the CDC.”

Priority is also likely to be given to the elderly in nursing homes and nursing home staff, followed by other high risk and vulnerable populations. Giroir predicts that the United States should have enough vaccine to immunize 20 million Americans by the end of the year.

“We have to immunize for impact,” he said. “The rest of America will get it in the second quarter, third quarter of 2021, but we can maximize our impact right now.”

In the meantime, Fauci implored Americans to restrict their holiday activities to “blunt” the surge’s effect and alleviate “significant stresses” on hospital and health-care systems. Adams echoed those sentiments, begging people to “hang on just a little bit longer.”

“The science out there has never been stronger to support the wearing of masks,” said Adams, who later lamented the politicization of the virus and called wearing a mask “an instrument of freedom.”

“You shouldn’t have to have a mandate to do the right thing to protect your neighbor, to keep schools open. Make sure you’re watching your distance and make sure, again, if you’ve been in a gathering of more than 10 people without your mask on over the last several days, please get tested in the next three to five days,” he said.

Jurisdictions around the country are unevenly navigating the current surge, with some clamping down while others are resisting measures that could prevent further spread of the virus. Los Angeles County, for example, just reinstated a temporary stay-at-home order on Friday that prohibits all public and private gatherings with people who are not in their household.

“We know we are asking a lot from so many who have been sacrificing for months on end and we hope that L.A. County residents continue following Public Health safety measures that we know can slow the spread,” Barbara Ferrer, director of the Los Angeles County Department of Public Health, said in a statement.

California, along with Texas, broke the nationwide record for most new coronavirus infections reported in a single day last week, though Texas Gov. Greg Abbott (R) ruled out any further shutdowns in the state earlier this month.

New York Mayor Bill de Blasio (D) announced on Sunday a reopening of the nation’s largest public school system, starting with elementary schools, a reversal of his decision to shutter New York City public schools less than two weeks ago. The Supreme Court’s conservative majority struck down temporary pandemic-related restrictions on religious organizations last week imposed by New York Gov. Andrew M. Cuomo (D) in hot spots where the virus is raging.