The Centers for Disease Control is urging “universal mask use” indoors for the first time as the country shatters records for coronavirus hospitalizations and deaths ahead of the holiday season.

The CDC has for months encouraged mask-wearing in public spaces with people outside the household. The new guidance, published Friday, asks people to put on masks anywhere outside their homes.

In its weekly Morbidity and Mortality report, the CDC warned Friday that the U.S. has entered "a phase of high-level transmission" as colder weather and the ongoing holiday season push Americans indoors, and said that "consistent and correct" use of face masks is critical to taming the virus.

Mask use is most crucial indoors, and in outdoor spaces where social distancing cannot be maintained, the CDC said in the report. The agency recommended mask use at home when a member of the household has been infected or potentially exposed to the virus, including those with high-risk occupations such as meatpacking or agricultural processing.

"Compelling evidence now supports the benefits of cloth face masks for both source control (to protect others)," the report said, "And to a lesser extent, protection of the wearer."

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Mitigation measures are particularly essential in light of recent research that suggests roughly 50 percent of transmission of the coronavirus is from asymptomatic people, the report said. It also recommended that communities make a plan for distributing masks to people who might struggle to access them.

Robert Redfield, head of the CDC, has called masks "the most important, powerful public health tool" in combating the coronavirus. A growing body of research shows widespread mask use can save scores of lives and stave off economic damage. One June analysis from Goldman Sachs estimated that a 15 percent increase in universal masking could prevent lockdowns and reduce associated losses of up to $1 trillion.

On Thursday, President-elect Joe Biden said that, on his first day in office, he'd ask Americans to mask up for 100 days. "Not forever. 100 days," Biden said in an interview with CNN. "And I think we'll see a significant reduction."

In addition to stepping up mask use, the CDC also recommended postponing travel plans. For those who do plan to travel, both domestically and internationally, the agency encouraged staying home, getting tested before and after traveling and quarantining for a week upon return, regardless of test results.

"Testing does not eliminate all risk and should be combined with other recommended public health strategies," including mask-wearing, social distancing and quarantines, the CDC said in the report.

Despite CDC warnings, experts are anticipating a holiday travel rush. The Sunday after Thanksgiving was the busiest day of air travel in 2020, with more than 1.1 million Americans flying, about 40 percent of the traffic on the same day last year, according to the Transportation Security Administration.

More than 277,000 Americans have died of covid-19, the disease caused by the novel coronavirus, as of Friday, and nearly 14.3 million cases have been recorded. Earlier this week, Redfield predicted that the covid-19 death toll could surpass 450,000 by February, perhaps his starkest warning yet about the dangers of the coming months.

“The reality is, December and January and February are going to be rough times,” Redfield said at a U.S. Chamber of Commerce Foundation event on Wednesday. “I actually believe they’re going to be the most difficult time in the public health history of this nation.”