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The omicron variant of COVID-19, which was first detected in the United States at the beginning of this month, now accounts for 2.9% of all cases in the country, according to data released by the CDC on Tuesday. The delta variant is still dominant, accounting for 96.7% of all cases for the week that ended on Dec. 11, but omicron is gaining ground as it accounted for just 0.4% the previous week. Omicron was originally discovered in South Africa, where it is currently driving a surge in infections, and was named a variant of concern by the World Health Organization on Nov. 26. (Best, 12/14)

Experts have said it appears Omicron is taking over faster than Delta did as it became dominant globally earlier this year. Current data from South Africa — which had some of the first detected Omicron cases — indicate that Omicron is causing milder infections than other forms of the coronavirus, perhaps not because of a change in the virus’ inherent virulence, but because prior infections and vaccinations are keeping people from developing serious illness. But experts caution that by leading to more cases overall, better spreading viruses can cause higher numbers of severe infections, even if the rate of serious disease is lower. (Joseph, 12/14)

The World Health Organization on Tuesday warned the new Covid-19 omicron variant is spreading faster than any previous strain, and it is probably present in most countries of the world. “Omicron is spreading at a rate we have not seen with any previous variant,” WHO Director-General Tedros Adhanom Ghebreyesus said during a Covid update in Geneva. “Seventy-seven countries have now reported cases of omicron. And the reality is that omicron is probably in most countries, even if it hasn’t been detected yet.” (Kimball, 12/14)

The World Health Organization is concerned that the omicron variant is being dismissed as mild, even as it spreads at a faster rate than any previous strain of Covid-19. The recently detected variant has been reported in some 77 nations, though it’s probably in most countries already, said WHO Director-General Tedros Adhanom Ghebreyesus. “We’re concerned that people are jumping to a conclusion that this is a mild disease,” Bruce Aylward, senior adviser at the WHO, told journalists at a briefing on Tuesday. “A more transmissible virus can do just as much damage -- or more -- than one which is more severe but less transmissible.” (Gretler, 12/14)

In other news about the spread of the omicron variant —

The omicron variant of COVID-19 is moving faster than surveillance systems can track it and has so unnerved some medical experts they're starting to put the brakes on preparations for their holiday gatherings. "Personally, I'm reevaluating plans for the holidays," Bronwyn MacInnis, director of pathogen genomic surveillance at the Broad Institute of MIT and Harvard University, said on a call with reporters Tuesday. "It's the responsible thing to do and what feels right given the risk." She and a handful of other Massachusetts-based researchers on the call said they've been stunned by the pace by which omicron has been crowding out other variants and taking over the pandemic. (Weise and Weintraub, 12/14)

Enough preliminary data have been gathered to show that Omicron can undermine some of the defenses that immunized bodies have built. The variant’s spike protein—the molecular key that the virus uses to unlock cells, and the centerpiece of most of the world’s COVID-19 shots—sports more than 30 mutations compared with the original SARS-CoV-2. Last week, several teams of scientists, as well as Pfizer, released early laboratory data suggesting that these tweaks might make the variant up to 41 times better at sidestepping the neutralizing antibodies roused by vaccines. In an actual body, that could make it easier for Omicron to kick-start an infection. (Wu, 12/14)

For some Covid experts, what’s most unsettling about the Omicron variant is all the uncertainty surrounding it. That’s what keeps John Moore, a professor of microbiology and immunology at Weill Cornell Medical College, up at night. “I’ve lost more sleep worrying about answering questions about Omicron than over Omicron itself,” he said during a panel at STAT’s “A Look Ahead at Biotech 2022”event last week. (St. Fleur, 12/14)