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 Centers for Disease Control and Prevention.
During the week that ended on Saturday, Omicron accounted for 2.9 percent of cases across the country, up from 0.4 percent in the previous week, according to agency projections released on Tuesday.
In the region comprising New York, New Jersey, Puerto Rico and the U.S. Virgin Islands, the percentage of Omicron infections had already reached 13.1 percent.
In a briefing on Tuesday with state and local health officials and representatives of public health labs across the nation, C.D.C. 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 Scott Becker, chief executive officer 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 R.S.V., 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 hospitalizations or deaths. The variant seems able to partly dodge the body’s immune defenses, but scientists have not yet determined to what degree vaccination and prior infection may safeguard individuals from severe disease.
To track variants, the C.D.C. 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 U.S. system was relatively slow to pick up cases of the variant, perhaps in part because of travel patterns or restrictive U.S. entrance rules. But the system is also constrained by blind spots and delays.
Last week, the C.D.C. reported that of the 43 known infections detected in the United States in the first eight days of December, 34 of the patients, or 79 percent, had been fully vaccinated when they first started showing symptoms or tested positive. Only about a third of the 43 people had traveled 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, the secretary of health and human services, Xavier Becerra, suggested on Tuesday. Mr. Becerra told reporters that about $10 billion was left of the $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,” Mr. Becker added. “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 United States in terms of vaccination rates and average age, were doubling every two days.
Dr. Tedros Adhanom Ghebreyesus, the director general of the World Health Organization, 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 W.H.O. officials 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 United States, state and local health officials urged Americans to take steps to prevent the spread of Covid by getting vaccinated, getting booster shots and wearing masks in public indoor settings. Families and friends gathering for the holidays must 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 U.S., 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 Michael Fraser, chief executive officer 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,” he added.
Nick Cumming-Bruce contributed reporting.