The number of hospitalised COVID-19 patients has risen nearly 50 per cent in the past week and now exceeds 100,000, according to data collected by Reuters, marking the first time that threshold has been reached in a year.

The latest surge, which forced waves of cancellations from commercial airlines flights to Broadway shows in recent weeks, was disrupting plans for public schools to welcome students back from winter vacation.

Chicago Public Schools, the nation's third-largest school district, said it would cancel classes on Wednesday after the teachers' union voted in favour of a return to remote learning.

In Los Angeles County, the presiding judge of one of the country's largest court systems ordered a two-week postponement of criminal trials due to the latest wave of infections.

The US has seen a daily average of 486,000 new cases over the past week, a rate that has doubled in seven days and far outstrips that of any other country

The 978,856 new infections documented on Monday included some cases tallied on Saturday and Sunday, when many states do not report.

The average number of US COVID-19 deaths has remained fairly steady throughout December and into early January at about 1300 a day, according to a Reuters tally, though deaths typically lag behind case numbers and hospitalisations.

Omicron was estimated to account for 95.4 per cent of cases identified in the US as of January 1, the US Centres for Disease Control and Prevention (CDC) said on Tuesday.

The World Health Organisation said evidence suggested Omicron is causing less severe illness. Nevertheless, public health officials have warned that the sheer volume of Omicron cases threatens to overwhelm hospitals, some of which are already struggling to handle COVID-19 patients, primarily among the unvaccinated.

Maryland Governor Larry Hogan declared a 30-day state of emergency on Tuesday and mobilised 1000 National Guard members to pandemic response operations as COVID-19 hospitalisations in the state hit a record high of more than 3000, up more than five-fold in seven weeks.

"The truth is that the next four to six weeks will be the most challenging time of the entire pandemic," Hogan told reporters.

Delaware, Illinois, Ohio, Virginia and Washington, DC, have also reported record numbers of hospitalised COVID patients in recent days.

'In Kentucky, where Tuesday's total of 6915 new cases was the highest daily figure since the start of the pandemic, Governor Andy Beshear urged residents to get vaccinated and wear masks.

"Omicron is causing a surge unlike anything we've seen and at this rate our hospitals will fill up," he wrote on Twitter.

The unrelenting surge has prompted more than 3200 schools to close this week, according to the website Burbio, which tracks school disruptions. Schools that have remained open are facing staff shortages and renewed concerns about virus spread.

Rank-and-file members of the Chicago Teachers Union voted 73 per cent in favour of working remotely, after the union's House of Delegates voted 88 per cent in favour of the resolution.

The union has called for more rigorous COVID-19 safety protocols, including school-based coronavirus testing and mandatory student vaccinations.

Dr Allison Arwady, Chicago's Public Health Commissioner, joined district officials and Mayor Lori Lightfoot in pressing to keep classrooms open.

Arwady cited data showing Chicago averaging just seven pediatric COVID-19 hospitalisations a day out of 550,000 children who live in the city.

Australian Associated Press