WASHINGTON (NYTIMES) - The proportion of coronavirus cases in the United States caused by the Omicron variant has increased sharply and may portend a significant surge in infections as soon as next month, according to new data from the Centres for Disease Control and Prevention (CDC).

During the week that ended last Saturday, Omicron accounted for 2.9 per cent of cases across the country, up from 0.4 per cent in the previous week, according to agency projections released on Tuesday (Dec 14).

In the region comprising New York, New Jersey, Puerto Rico and the US Virgin Islands, the percentage of Omicron infections had already reached 13.1 per cent.

In a briefing on Tuesday with state and local health officials and representatives of public health labs across the nation, CDC officials warned of two possible scenarios.

The first was a tidal wave of infections, both Omicron and Delta, arriving as soon as next month, just as influenza and other winter respiratory infections peak.

"The early signals say there are going to be waves coming," said Mr Scott Becker, chief executive of the Association of Public Health Laboratories, who was on the call.

"We are already expecting an uptick, just because we have seen a lot of respiratory viruses already this fall, including RSV (respiratory syncytial virus), which was extensive," he added.

Federal health officials also proposed a second scenario in which a smaller surge in Omicron cases occurs in the spring. It was unclear which forecast was more likely.

Early evidence about the variant has only begun to emerge, and it remains unclear how often infections with Omicron lead to hospitalisations or deaths.

The variant seems able to partially dodge the body's immune defences, but scientists have not yet determined to what degree vaccination and prior infection may safeguard individuals from severe disease.

To track variants, the CDC uses a national surveillance system that collects samples, as well as genetic sequences generated by commercial laboratories, academic laboratories, and state and local public health laboratories.

The US system was relatively slow to pick up cases of the variant, perhaps in part because of travel patterns or restrictive US entrance rules. But the system is also constrained by blind spots and delays.

Last week, the CDC reported that of the 43 known infections detected in the US in the first eight days of December, 34 of the patients, or 79 per cent, had been fully vaccinated when they first started showing symptoms or tested positive.

Only about one-third of the 43 people had travelled internationally in the two weeks before diagnosis, indicating some level of community spread of the variant.

The fight against Omicron may require the federal government to replenish funding for the response, Mr Xavier Becerra, secretary of the Department of Health and Human Services, suggested on Tuesday. He told reporters that about US$10 billion (S$13.7 billion) was left of the US$50 billion Congress had allocated for testing.

For public health labs, as for hospitals, staffing may be a challenge, Mr Becker said.

"It's the same staff who do molecular testing and genomic sequencing and flu surveillance," he said. "We're stretched already, so we have to begin to think about alternative plans, temporary staffing, bringing in people who helped during surge events last year.

"The lab community is tired. The health care community is tired. 'Gear up, we may have another surge', is a tough message to hear."

In Europe, health officials have warned of a spike in Omicron cases. According to estimates on Monday, cases of the variant in Denmark, which is similar to the US in terms of vaccination rates and average age, were doubling every two days.

Dr Tedros Adhanom Ghebreyesus, director-general of the World Health Organisation (WHO), said in a news briefing on Tuesday that "Omicron is spreading at a rate we have not seen with any previous variant".

Dr Tedros and senior WHO officials also cautioned against underestimating the variant.

"Even if Omicron causes less severe cases, the sheer number of cases could once again overwhelm unprepared health systems," he said.

In the US, state and local health officials urged Americans to take steps to prevent the spread of Covid-19 by getting vaccinated, going for booster shots, and wearing masks in public indoor settings.

Families and friends gathering for the holidays should get tested before celebrating together, gathering outside if possible or, if not, in well-ventilated spaces.

"As the Delta variant continues its rapid spread in the US, state and territorial health leaders are becoming increasingly concerned about emerging data from Europe and South Africa that indicate the Omicron variant may be even more transmissible," said Mr Michael Fraser, CEO of the Association of State and Territorial Health Officials.

"Hospital capacity is already at a breaking point in many states because of severe cases of Covid-19."