Even as scientists race to understand more about the Omicron variant and the threat it poses, one fact is abundantly clear: It spreads quickly everywhere it lands.
In South Africa, Omicron spread twice as fast as the highly infectious Delta variant. In Britain, officials have estimated that 200,000 people are becoming infected with Omicron every day. In Denmark, Omicron cases are doubling roughly every two days.
And early data from the United States suggest that Americans will not be spared. “No part of the country will be safe from Omicron,” said Shweta Bansal, a disease ecologist at Georgetown University.
Delta remains the dominant variant nationally and was driving a surge in cases and hospitalizations even before Omicron emerged. Roughly 120,000 new Covid cases are being reported every day, a 40 percent increase from two weeks ago, although the figures remain below last winter’s peak.
But Omicron could soon overtake Delta, scientists said. Nationally, the share of cases caused by Omicron has increased to 2.9 percent from 0.4 percent in just a week, according to projections by the Centers for Disease Control and Prevention, and is far higher in some regions of the country.
Researchers at one University of Washington lab have reported that the share of viral samples seeming to contain Omicron’s telltale pattern of mutations has jumped to 20 percent from 3 percent in a matter of days.
In New York City, the test positivity rate has doubled in three days, an adviser to the Mayor said on Thursday, but officials don’t yet know how many of those infections were caused by Omicron.
Still uncertain is how serious the consequences will be, as much remains unknown about the variant, including how likely it is to cause severe disease. But the United States must redouble its fight against the virus, experts said.
“I think we need to be prepared for the possibility that this could be at least as bad as any previous wave that we’ve seen,” said Justin Lessler, an epidemiologist at the University of North Carolina at Chapel Hill. “We need to be thinking about what the plan is if things get bad.”
Omicron is not only highly transmissible but also skilled at evading the immune system’s defenses.
Breakthrough infections are common. This week, Denmark reported that three-quarters of its Omicron cases occurred in people who had received two vaccine doses. And a recent outbreak at Cornell University, where 97 percent of the population has reportedly been vaccinated, may have been driven by Omicron.
“There is no question that we’re seeing some level of immune escape and re-infection,” Dr. Lessler said. “Which means that the pool of people available to be infected with this virus is larger than it was before.”
Scientists believe that the vaccines will still provide protection against the worst outcomes; there is early evidence, for instance, that T cells, which can help prevent infections from progressing to severe disease, should still recognize the variant. And boosters are likely to provide additional protection against infection, preliminary data suggests.
Current levels of vaccination are unlikely to stop the variant, experts said. In Denmark, where Omicron is spreading rapidly, 77 percent of the population is fully vaccinated, having received a two-dose vaccine or one-dose regimen, depending on the vaccine. That suggests that the United States, where 61 percent of people have completed their primary vaccine series, should brace itself for a similar wave of cases.
“Nothing we’ve seen so far gives any reassurance that the growth won’t be similarly explosive in the U.S.,” said Joshua Salomon, an infectious disease expert at Stanford University.
The question is how much the rapid spread of Omicron will contribute to hospitalizations and deaths.
“The most challenging question is severity,” said Dr. Christopher Murray, director of the Institute for Health Metrics and Evaluation at the University of Washington.
Early evidence from South Africa has raised hopes that Omicron may be milder than Delta; in one small report, researchers there noted that patients in a hospital Covid ward were less likely to require supplemental oxygen than during previous surges, for instance.
But it is still far too early, and the data sets are too small, to make broad conclusions about Omicron’s severity, especially because hospitalizations and deaths typically lag several weeks behind infections.
“I’m not counting that as good news just yet,” Dr. Bansal said.
The population of the United States is older, and has a different pattern of vaccination and prior infection, than South Africa’s population, she noted.
And, she said, “even if infection is mild in many individuals, it’s not going to be mild in everyone.”
If Omicron spreads rapidly and infects huge swaths of the population at once, it could still sicken enough people to overwhelm health care systems, some of which are already struggling to manage a surge of Delta cases.
“Here in the U.S., especially in some parts of the U.S., we are at capacity right now in our emergency rooms,” said Maimuna Majumder, a computational epidemiologist at Boston Children’s Hospital.
Hospitals have already lost staff over the course of the pandemic, she noted; if Omicron spreads rapidly through health care workers, forcing them to isolate at home, it could further strain capacity.
“Not only are there concerns about this ripping through people who are working the floor,” Dr. Majumder said, “but also there are far fewer people who are on the floor than there were two years ago at many hospitals throughout the country.”
The Coronavirus Pandemic: Key Things to Know
A new U.S. surge. The C.D.C said that the Omicron variant’s rapid spread in the U.S. may portend a surge in infections as soon as January, but cases are already spiking, leading offices to cancel holiday parties, Broadway performances to be shuttered and college finals to be moved online.
The Omicron variant. The new Covid variant has been detected in dozens of countries. While Omicron appears to be able to partially evade antibodies, it could be less severe than other forms of the virus, and new laboratory studies indicate that vaccines, and especially boosters, may offer protection against severe disease.
The J.&J. vaccine. An expert panel to the C.D.C. voted to recommend that Covid vaccines other than Johnson & Johnson’s should be preferred, citing increasing evidence that those shots trigger a rare but potentially serious blood clotting disorder.
U.S. surpasses 800,000 deaths. Covid deaths in the United States surpassed 800,000 — the highest known number of any country. About 75 percent of the 800,000 deaths have involved people 65 or older. One in 100 older Americans has died from the virus.
There are still a lot of unanswered questions about Omicron and many possible trajectories for the coming weeks and months.
There has been some speculation that after a steep initial rise, new Omicron cases may be plateauing in the Gauteng province, the epicenter of South Africa’s latest outbreak.
“The rate of increase has certainly slowed,” said Ridhwaan Suliman, a mathematician and senior researcher at South Africa’s Council for Scientific and Industrial Research.
Still, Mr. Suliman was cautious about drawing definitive conclusions, particularly after a computer glitch and a reporting backlog last week gave the impression that cases were decreasing. A few days’ data are needed to determine whether the curve has turned, he said, and he warned that the trajectory may be different in other provinces, where vaccination rates are lower.
The good news, experts said, is that America has resources at its disposal, if it chooses to use them. Tests still work for Omicron. Vaccines are now widely available and could blunt the burden of severe disease. Antiviral pills, which are expected to be effective against Omicron, could soon be available, although supplies will be very limited at first.
“There are more tools in our toolbox now than there were during the holiday season last year,” Dr. Majumder said.
People who are not yet vaccinated should get vaccinated, experts said, and those who are eligible for boosters should get them. And masking, testing and taking precautions when gathering with others indoors — the same behaviors that experts have long been recommending — can also help slow the spread.
But beyond these individual behaviors, officials still need to do more, experts said.
The Biden administration, for instance, has focused heavily on promoting vaccination and boosters, which remain critical public health tools. But in the face of a foe like Omicron, they are not enough, experts warned.
“The way we’re seeing it unfold in other places, the way we’re starting to see it take hold here, implies that a vaccination-only strategy is going to be very fragile,” Dr. Salomon said.
Experts called on officials to improve access to testing, distribute high-quality masks, promote improved ventilation and issue clearer guidelines about what activities it is safe to engage in and in what circumstances.
Local governments, schools and employers also need to begin making plans — and making those plans public — about what they will do in the event of large outbreaks or if cases or hospitalizations rise to a certain level, they noted.
Many of these steps are long overdue, experts said, and were sorely needed even before Omicron was discovered.
“We weren’t actually prepared for even the situation that we were in before the Omicron variant emerged,” Dr. Bansal said.
And now that Omicron is here? “We’re not ready,” she said.
Lynsey Chutelcontributed reporting.