With a caseload nearly twice that of the worst days last winter, the United States shattered its record for new daily coronavirus cases, a milestone that may not adequately illustrate the rapid spread of the delta and omicron variants because testing has slowed over the holidays.

As a second year of living with the pandemic was drawing to a close, the new daily case total topped 488,000 on Wednesday, according to a New York Times database. (The total was higher Monday, but that number should not be considered a record because it included data from the long holiday weekend.)

Wednesday’s seven-day average of new daily cases, 301,000, was also a record, compared with 267,000 the day before, according to the database. In the past week, more than 2 million cases have been reported nationally, and 15 states and territories reported more cases than in any other seven-day period.

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The rise in cases has been driven by the omicron variant, which became dominant in the United States as of last week. So far, however, those increased cases have not resulted in more severe disease, as hospitalizations have increased only 11% and deaths have decreased slightly in the past two weeks.

Because COVID-19 tests have been in short supply over the holidays, Wednesday’s numbers still may not fully illustrate the havoc caused by the two variants, which have sent caseloads soaring and have worsened a labor shortage, upending the hospitality, medical and travel industries, among others.

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Demand for tests has outstripped supply, particularly in the past month as the omicron variant has spread at an astonishing speed. And the holiday season offers its own disruptions to the U.S. case curve, with many testing sites offering limited hours, and labs and government offices not open to report test results.

Last year, the national case curve showed pronounced declines after Thanksgiving and Christmas that did not reflect real decreases in new infections. The effect of holidays may be even more noticeable this time around, as illustrated by the Labor Day holiday in September, because states are reporting data less consistently than they did a year ago.

Before Tuesday, the seven-day U.S. average had peaked Jan. 11 at 251,232. That was during a catastrophic winter when vaccinations were still relatively new. Today, more than 62% of Americans are fully vaccinated.

No matter what the true caseload is right now, the United States has confronted a new set of challenges as the delta and omicron variants have converged. The variants have disrupted holiday travel and gatherings, depleted hospital staffs and plunged the United States into another long winter.

Record caseloads have been reported in a laundry list of U.S. cities where vaccination rates are relatively high, including New York, Seattle, San Francisco, Boston, Atlanta Detroit and Washington, D.C.

Experts say there are two reasons for the high numbers in urban areas: population density and more COVID-19 testing.

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Cities are tightly packed hubs for travel and socializing, which leaves people more susceptible to the highly contagious omicron variant, said Dr. Kirsten Bibbins-Domingo, a physician and public health researcher at the University of California, San Francisco.

“Then you layer in on top of it that we do a lot of testing in these major urban centers precisely because we’re worried about big surges overwhelming hospitals,” she said Wednesday.

In fact, she said, city caseloads may be higher than reported because of the rise in at-home tests that often don’t get reported to state officials, so they don’t end up in official case totals.

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