Last week, another 164,000 children in the United States tested positive for COVID-19, up by 24% from the week prior, according to a report released Monday by the American Academy of Pediatrics and the Children’s Hospital Association.

COVID-19 cases among U.S. children are "extremely high," the organizations wrote in the report.

Last week the Northeast saw its highest number of pediatric cases since the onset of the pandemic, with nearly 40,000 new cases.

So far, 21 million children ages 5 to 17 nationwide -- about 39.6% -- have received at least one dose of a COVID-19 vaccine.

Severe illness due to COVID-19 remains "uncommon" among children, the organizations wrote in the report. But they continue to warn that there is an urgent need to collect more data on the long-term consequences of the pandemic on kids, including the physical, emotional and mental health impacts.

-ABC News' Arielle Mitropoulos

A total of 50 million COVID-19 cases have now been confirmed in the U.S., according to data from Johns Hopkins University.

The U.S. has reported more cases than any other nation in the world. The U.S. currently represents nearly one-fifth of the globe's total 270.5 million cases.

-ABC News' Arielle Mitropoulos

In the weeks following Thanksgiving, the COVID-19 resurgence in the U.S. has escalated rapidly.

The U.S. is averaging more than 118,000 new cases each day -- up by about 41.8% in the last two weeks and up nearly 85% since late October, according to federal data.

Six states with some of the nation's highest vaccination rates are also among the states with the highest new case rates: New Hampshire, Vermont, Massachusetts, Maine, Rhode Island and New York. Experts say cases could be rising in the Northeast partially due to people heading inside in the cold weather.

Daily COVID-19-related hospital admissions increased by 14.4% in the last week and jumped by 48% in the last month, according to federal data.

Pediatric admissions are up by 23.8% in the last week.

-ABC News' Arielle Mitropoulos

Omicron is expected to be the dominant variant in London in the next 48 hours, the Secretary of State for Health and Social Care, Sajid Javid, warned in a statement to Parliament Monday.

Omicron represents 20% of England's cases and 44% of London's cases, Javid said.

"No variant of COVID-19 has spread this fast," Javid said.

Ten people with omicron are in U.K. hospitals.

"Hospitalisations and deaths lag infections by around two weeks," Javid said, "So we can expect those numbers to dramatically increase in the days and weeks that lie ahead."

"In preparation, the UK’s four Chief Medical Officers raised the COVID Alert level to 4, its second-highest level, this was done over the weekend. And NHS England has just announced it will return to its highest level of emergency preparedness: Level 4 National Incident," Javid said. "This means the NHS response to Omicron will be a coordinated as a national effort, rather than led by individual trusts."

Javid also urged people to get booster doses, stating that 40% of adults in the U.K. have gotten boosters so far.

-ABC News' Zoe Magee

The omicron variant appears to have a "growth advantage" over the delta variant, the World Health Organization said in a technical brief released Sunday.

"It is spreading faster than the delta variant in South Africa where delta circulation was low, but also appears to spread more quickly than the delta variant in other countries where the incidence of delta is high, such as in the United Kingdom," the WHO said in the brief, which was dated Friday. "Whether omicron's observed rapid growth rate in countries with high levels of population immunity is related to immune evasion, intrinsic increased transmissibility, or a combination of both remains uncertain. However, given the current available data, it is likely that omicron will outpace the delta variant where community transmission occurs."

Meanwhile, preliminary findings from South Africa suggest omicron may cause less severe illness than delta, and all cases of omicron reported in Europe to date have been mild or asymptomatic. But the WHO said "it remains unclear to what extent omicron may be inherently less virulent" and that "more data are needed to understand the severity profile."

The WHO also noted that "there are limited available data, and no peer-reviewed evidence, on vaccine efficacy or effectiveness to date for omicron." However, preliminary evidence, and the considerably altered antigenic profile of the variant's spike protein, suggests a reduction in vaccine efficacy against infection and transmission associated with omicron.

"There is some preliminary evidence that the incidence of reinfection has increased in South Africa, which may be associated with humoral (antibody-mediated) immune evasion," the WHO said.

The diagnostic accuracy of routinely used polymerase chain reaction (PCR) test and antigen-based rapid diagnostic test (Ag-RDT) assays does not appear to be influenced by omicron. Therapeutic interventions for the management of severe or critical COVID-19 symptoms associated with omicron are also expected to remain effective, according to the WHO.

"However, monoclonal antibodies will need to be tested individually, for their antigen binding and virus neutralization and these studies should be prioritized," the WHO added.

-ABC News' Christine Theodorou