People wait in line for COVID-19 tests at a mobile testing site on Times Square in New York, the United States, Dec. 20, 2021. (Xinhua/Wang Ying)

The 7-day average daily increase of COVID-19 cases reached more than 206,000 on Sunday, according to latest data of the U.S. CDC. This is also the highest figure since Jan. 18.

The lightning spread of the new Omicron variant in the United States has driven daily COVID-19 new cases to a record high of over 510,000 on Monday.

It is the highest single-day increase of cases since the onset of the pandemic in the country, according to latest data of Johns Hopkins University.

A total of 512,553 new cases and 1,762 new deaths were reported across the nation on Monday. Over the past week, nearly 1,660,000 new cases and over 10,000 new deaths were added to the tally, according to Johns Hopkins University.

The 7-day average daily increase of cases reached more than 206,000 on Sunday, according to latest data of the U.S. Centers for Disease Control and Prevention (CDC). This is also the highest figure since Jan. 18.

The Omicron variant, first identified late last month in southern Africa, has sent infections soaring to levels not seen since last winter, bringing the total number of U.S. coronavirus cases to more than 52.9 million, with more than 819,000 deaths.

The first known case of the Omicron variant in the United States was identified on Dec. 1. Since then, the variant has been detected in most U.S. states and territories, and is rapidly increasing the proportion of COVID-19 cases it is causing.

The Omicron variant was estimated to account for 58.6 percent of coronavirus variants circulating in the United States for the week ending Dec. 25, according to new data released Tuesday by the CDC.

According to the estimate, the prevalence of the Omicron variant had eclipsed that of the Delta variant, which accounted for 41.1 percent of variants.

Omicron's surge has prompted more cancellations in travel, sports and other events across the country.

Some states have seen hospitalizations peak amid Omicron surge. The number of children hospitalized with COVID-19 is also rising.

COVID-19 cases among U.S. children are "extremely high and increasing," according to a latest report of the American Academy of Pediatrics (AAP) and the Children's Hospital Association.

Over 7.5 million children have tested positive for COVID-19 since the onset of the pandemic in the United States, representing 1 in 10 American children.

For the week ending Dec. 23, almost 199,000 child COVID-19 cases were reported, a 50 percent increase over the weekly new cases in the beginning of December. This marked the 20th consecutive week child COVID-19 cases are above 100,000.

Since the first week of September, there have been over 2.5 million additional child cases, according to the AAP.

COVID-19 testing sites across the United States have seen unprecedented demand due to the surge in new infections.