The United States lost 15,000 people to COVID-19 last week, the deadliest seven days since April, and health officials warn that the worst is yet to come.

The Christmas holiday season could pose more risks than the Thanksgiving holiday in terms of the virus spreading, according to Dr. Anthony Fauci, the U.S. government's top infectious disease expert.

Christmas "extends that vulnerable period by two to three times," Fauci said in remarks made at the Milken Institute's Future of Health summit on Monday. Thanksgiving travel and gatherings last only for a couple of days, while Christmas events may start from late December through New Year events.

As many testing centers reopened after Thanksgiving, the number of new coronavirus cases rose 19 percent to 1.4 million in the week ending December 6, according to Reuters.

Across the United States, 10.5 percent of tests came back positive for the virus, up from 9.8 percent the prior week, according to data from The COVID Tracking Project. Out of 50 states, 34 had a positive test rate of 10 percent or higher, with record rates from Iowa and Idaho at 52 percent and South Dakota at 50 percent, respectively.

The World Health Organization considers positive test rates above five percent concerning because it suggests there are more cases in the community that have not yet been uncovered.

Government and health officials have warned that cases and deaths will rise further in the coming weeks and months due to people traveling and gathering with family – against the advice of health experts.

"I think January is going to be terrible because you're going to have the Thanksgiving surge superimposed upon the Christmas surge," Fauci told Newsweek.

He said he won't celebrate the holiday with his daughter for the first time in more than 30 years, and he and his wife will have a celebration over Zoom for his 80th birthday on Christmas Eve.

The country has added nearly 200,000 new coronavirus cases each day over the past week, and the hospitalization rate has broken records.

Fauci, who has been asked by President-elect Joe Biden to be his chief medical adviser, told CNN that his concerns for Christmas were the same as his concerns for Thanksgiving, "only this may be even more compounded because it's a longer holiday."

He said nobody wanted to modify or shut down the holiday season, but "we're at a very critical time... we've got to not walk away from the facts and the data. This is tough going for all of us."

On Sunday, former Food and Drug Administration (FDA) commissioner Scott Gottlieb warned that U.S. deaths could be near 400,000 by the end of January, adding that "as bad as things are right now, they're going to get a lot worse."

Also on Sunday, Dr. Deborah Birx, the White House coronavirus task force coordinator, criticized the Trump administration for flouting guidelines and peddling "myths" about the pandemic.

(With input from agencies)