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December was the deadliest, most infectious month since the start of the pandemic

The U.S. also surpassed 20 million cases amid the emergence of a new virus variant and a slower than expected vaccine rollout.

The United States closed out 2020 with the deadliest and most infectious month since the start of the coronavirus pandemic, while surpassing 20 million confirmed Covid-19 cases.

More than 77,000 people lost their lives in December to Covid-19, while 6.4 million contracted the virus amid the emergence of a new virus variant and delayed efforts to vaccinate people. The second-deadliest month was April 2020, with more than 58,000 deaths.

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At least 227,651 new cases were reported on Thursday and 3,451 people died, marking the second-highest daily death toll, according to NBC News' tally.

The virus has killed more than 346,000 people in the U.S.

At least four cases of a new Covid-19 variant first detected in the United Kingdom have been reported in Colorado, California and Florida. However, Mercedes Carnethon, vice chair of preventive medicine at Northwestern University, said experts are "fairly confident this variant is now everywhere."

This variant also "appears to affect more adults under age 20," Carnethon told MSNBC. "And I think the reason why that's particularly critical is that this younger adult population is circulating more. They are performing some of the essential roles such as delivery work, factory work, and they're just generally out there in the population, and that population isn't in line to be vaccinated soon."

Only about 2.8 million people have been vaccinated since mid-December, though the Trump administration aimed to vaccinate at least 20 million people by the start of the new year. Officials in states grappling with a rise in cases have said a lack of trained staff ready to administer the vaccines has contributed to delays.

Even with vaccines becoming available, cases are almost certain to continue growing, and yet another surge is expected in the weeks after Christmas and New Year’s.

Thursday became the second consecutive day New York surpassed its record for daily new cases with 15,700. As the state prepares to potentially report its millionth case on Saturday, officials have said they hope to vaccinate at least 1 million people this month.

Virginia also hit a single-day record Thursday with 5,239 new cases.

In California, hospitalizations have climbed more than eightfold in two months and nearly tenfold in Los Angeles County. As hospitals are swamped with patients and intensive care units have no more beds for Covid-19 patients, makeshift wards are being set up in tents, arenas and classrooms to make up for the lack of space in overwhelmed hospitals.

Florida Department of Health medical workers prepare to administer a COVID-19 vaccine to seniors in the parking lot of the Gulf View Square Mall in New Port Richey near Tampa, Florida on Dec. 31, 2020.Octavio Jones / Reuters

On Thursday, the total number of California deaths surpassed 25,000, making it the third state to cross that milestone with New York and Texas.

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“Most heartbreaking is that if we had done a better job of reducing transmission of the virus, many of these deaths would not have happened,” said Barbara Ferrer, Los Angeles County’s public health director, who has pleaded with people not to get together and worsen the spread.

Nevada, North Carolina and Wyoming also set new single-day records Thursday for coronavirus deaths — reporting 59, 123 and 33, respectively.

Joe Murphy and The Associated Press contributed.