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-19 cases are being reported every day, a 40% 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% from 0.4% 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% from 3% in a matter of days.
In New York City, the test positivity rate has doubled in three days, an adviser to the mayor said 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.
Vaccines still the best protection
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.
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, a public health researcher 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% 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 reinfection," Lessler said. "Which means that the pool of people available to be infected with this virus is larger than it was before."
Current levels of vaccination are unlikely to stop the variant, experts said. In Denmark, where omicron is spreading rapidly, 77% 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% 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.
Milder? Or is it too early to day?
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," 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 public health researcher 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," 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."
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, 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," 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.