The first country to really get hit by omicron is South Africa.
Before the new variant took off last month, coronavirus cases there were low – only several hundred per day in mid November.
But by early December, the tally of daily infections had shot up to more than 4,500 — and genomic sequencing shows that omicron is to blame.
What's more, the variant quickly swept through all regions of South Africa – and has now shown up in about 60 additional countries.
Omicron hasn't yet triggered a global wave, but many scientists who are tracking its rapid spread believe it's only a matter of time.
The reason for their concern?
Omicron is starting to gain traction in countries where the pandemic looks very different from South Africa – places where the highly transmissible delta variant is currently dominant and where vaccination levels are relatively high.
Worrisome signs from Europe
The clearest signs of trouble come from how quickly omicron is growing in Europe and the U.K., says Matt Ferrari, director of the Center for Infectious Disease Dynamics and a professor of biology at Penn State University.
"Everybody in my industry is hopeful that we're being alarmist, but more than at any other point in the course of the whole pandemic, we know how bad it could be," he says.
Early data show that one person who catches omicron is currently going on to infect two to four other people — that's at least twice as many as someone who gets the delta variant. The statistics come from countries like the U.K, Denmark and South Africa — all of which have robust surveillance systems in place for tracking coronavirus variants.
And some experts believe omicron is spreading even faster than that.
The alarming pace of new infections is on par with how quickly the first, wild-type strain of the coronavirus spread at the beginning of the pandemic – back when there were no vaccines and everyone was susceptible.
"Now we're in a world where lots of people have immunity, either from previous infection or from vaccination, and we're seeing a pretty rapid growth rate over and above delta," says Ferrari.
Indeed, the spread of omicron in the U.K., where about 70% of people are vaccinated compared to about 25% in South Africa, is a key reason scientists are worried.
Although omicron infections only make up a small portion of the overall cases in the U.K., the forecasts are sobering. The country's health agency is warning that omicron infections are doubling every few days – and could make up half of all cases there by mid-December.
"The quick steady spread is just a very clear signal that this variant can spread exponentially even in highly vaccinated populations," says Jeffrey Barrett, director of the Genomics Initiative at the Wellcome Sanger Institute, which tracks variants in the U.K.
And the U.S. should take note: What the virus does in Europe, says Barrett, often foreshadows where the pandemic is headed on the other side of the Atlantic. "By next week, we will see clear evidence of an omicron wave in European countries, and I suspect the U.S. is at most a week behind that," he predicts.
Exactly why omicron is spreading so quickly comes down to two key questions. Is this strain of the virus intrinsically more contagious — in the way that delta was compared to the variant that dominated before it arrived? And second: is omicron also just better at escaping immune defenses – antibodies resulting either from prior infection or vaccines — which means there are more people who it can infect?
It's possible the answer to both questions could be yes, although there's not enough data to say just yet.
"Looking at these early omicron cases, what we've anticipated from immune escape is turning out to be true and the [intrinsic] transmissibility seems to be at least that of delta," says Nathan Grubaugh, associate professor of epidemiology at Yale School of Public Health.
"I think we're going to see the vaccine's effectiveness against infection drop a lot," he says. This concern is not only based on how the variant is spreading in well-vaccinated places like Europe and reinfecting people who previously had COVID in South Africa, but also the many mutations on the spike protein. Scientists believe those mutations could make omicron's spike protein more adept at evading the body's antibody defenses that prevent the coronavirus from infecting cells.
This week, several preliminary lab studies seemed to confirm this fear. Researchers took blood from people who had received two shots of the Pfizer vaccine to see whether their antibodies could neutralize omicron as effectively as it could other strains of the virus. They found the antibodies had a lot more trouble disarming omicron.
It's still too early to know just how much these findings translate into lower overall protection from the vaccine. Grubaugh is one of many scientists who say it's likely the vaccines will still help ward off severe disease because the immune system relies on other defenses like T-cells.
But if omicron can more easily infect those with prior immunity, that will give it a distinct competitive advantage over delta because it will gain a foothold in populations where delta has trouble getting past the protection afforded by the vaccines. "It will probably end up replacing Delta — it's a little too early to tell how fast, but probably fairly quickly," he says.
Assessing the risk in the U.S.
Some experts caution against early predictions about what may happen in a place like the U.S. based on the situation in other countries.
John Moore, a professor of microbiology and immunology at Weill Cornell Medical School, says it's not at all certain that omicron will take over. "It's not just the transmissibility that matters, it's the ability to outcompete," he says. "Earlier variants like beta and gamma created short term angst, but they just fizzled away."
In some countries, omicron could have more trouble competing because of higher immunity levels and the increasing push to get booster shots – which early data show may help stop omicron infections, says Wafaa El-Sadr, a professor of epidemiology and medicine at Columbia University's Mailman School of Public Health.
"This in and of itself may very much influence the trajectory of omicron in a country like the United States versus in a country like South Africa," she says. "But ultimately it's the more transmissible variants that takes over and circulates more widely, so that's what can be anticipated."
So how worried should we be?
Early reports from health officials and doctors in South Africa suggest the omicron wave of infections — while breathtaking in its speed — isn't yet causing the same level of severe disease as delta did when it hit the country. For example, ICU admissions are much lower.
But many experts caution it's far too early to draw any firm conclusions. South Africa has a relatively young population, and many people had already caught the virus prior to omicron's emergence.
"The silver lining right now is we do not have any evidence that omicron is more severe," says Penn State's Matt Ferrari. Then again, he notes: "Everybody wants me to say it's milder, and we really just don't have convincing evidence that it is."
Even if omicron does end up being less severe than delta and not as many people get very ill, Barrett of the Wellcome Sanger Institute says there's plenty of reason for concern given just how contagious it appears to be: "A small proportion of a huge number all at once can still overwhelm health care systems."
This is especially true in a country like the U.S. where many hospitals are already filled with COVID-19 patients who are sick – not from omicron – but from delta.