NEW YORK (BLOOMBERG) - Covid-19 hospitalisations in New York state are accelerating at a rate that hasn't been seen since the early days of the pandemic.

On Tuesday (Dec 28) the state said hospitalisations rose by 647 to 6,173, marking the largest daily increase since early April 2020, according to data compiled by Bloomberg.

The total number of New Yorkers hospitalised with the virus remains far below last year's peak of almost 19,000.

The Omicron-fueled surge in Covid cases across the US has reignited concerns about hospital capacity.

The number of patients hospitalised in New York has almost doubled since the beginning of December.

While Omicron appears to be causing a lower rate of hospitalisations than earlier variants, early studies showing it to be more transmissible suggest that the sheer numbers of patients could still overload the health-care system.

New York also reported more than 40,000 new cases on Tuesday for the third time in the last five days.

Almost one in five Covid tests are positive, Governor Kathy Hochul said.

The state reported 77 new deaths, bringing the total to 48,150 since the start of the pandemic.

In the US, the seven-day average of new cases hit 206,577 on Sunday, roughly 18 per cent lower than the all-time high recorded on Jan 11, according to data from the Centres for Disease Control and Prevention (CDC).

Meanwhile, hospitalisations rose to a seven-day average of 8,964, only half their earlier peak recorded in January.

Dr Albert Ko, chair of the department of epidemiology and microbial diseases at the Yale School of Public Health, cautioned that because the new variant spreads so easily, the US will likely see continued increases in hospitalisations and deaths, though not as severe as during the delta wave that hit mid-year.

"We are seeing exponential increases in cases, and a much lower increase in hospitalisations and deaths," Dr Ko said in a phone interview.

Studies out of Britain, South Africa and Scotland show the risk of hospitalisation from Omicron is lower than from Delta.

The variant appears to have a shorter incubation period and cause less serious illness than other versions of the coronavirus, according a study of a cluster of six patients published Tuesday by the CDC.

Even when patients do end up in the hospital with omicron, they appear to spend less time there.

However, the increasing numbers of breakthrough infections among vaccinated people may skew hospitalisation data, said Professor Jeffrey Morris, director of the biostatistics division at the University of Pennsylvania's Perelman School of Medicine.

"It appears there is less risk of hospitalised disease across the board, but we have to be a little bit careful about interpreting that," he said in a phone interview.

The rate of hospitalisations and deaths may appear artificially lower because breakthrough cases tend often turn out to be mild, Morris said.