The Omicron variant has landed in the US: California just reported its first case

• The US has identified its first case of the Omicron variant in a San Francisco, California resident.
• Public-health experts predicted last week that Omicron was already spreading in the US.

The US reported its first case of the Omicron variant on Wednesday in a San Francisco, California resident.

The University of California, San Francisco, spotted the case in an infected traveler, an adult under age 50, who had returned from South Africa on November 22 and tested positive for COVID-19 on November 29. The person is fully vaccinated and has "mild symptoms that are improving," the Centers for Disease Control and Prevention said in a Wednesday statement.

The infected person's close contacts have tested negative thus far and the CDC isn't investigating any additional Omicron cases at the moment, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said at a White House press briefing on Wednesday. He added that to his knowledge, the infected person had not received a booster dose. The CDC recommended boosters for all adults on Monday.

Omicron has spread to more than 20 countries since it was first detected in Botswana roughly three weeks ago.

Before Wednesday, public-health experts remained hopeful that the US might avoid an aggressive COVID-19 surge this winter, especially given increased availability and uptake of booster shots. The country's daily COVID-19 cases have hovered below 100,000 per day, on average, for the past month.

It's unclear whether Omicron will reverse that progress. Preliminary evidence suggests that Omicron may increase the risk of reinfection relative to other variants of concern, the World Health Organization said last week. The variant carries a concerning number of new mutations that could make it more transmissible then Delta, render vaccines less effective, or lead to more severe disease.

That means Omicron could potentially drive more "localized epidemics" in the US come February and March, Scott Gottlieb, former commissioner of the Food and Drug Administration, told SiriusXM's Doctor Radio on Wednesday.

"I don't think it's going to be a wave of infection like we've seen with Delta," he added. "I think it's going to be something that we can manage much more effectively."

Omicron may have landed in the US much earlier than Wednesday

Public-health experts had already anticipated that Omicron was spreading in the US before the California case. In an interview with Insider on Tuesday, Charity Dean, a former official at the California Department of Public Health, estimated that there were around 2,000 Omicron cases in the US, based on the variant's global spread.

The US is slow to sequence coronavirus cases relative to countries like South Africa, so it's hard to know exactly when Omicron first landed in the US. For now, public-health experts are optimistic that South African researchers detected the variant in the early stages of its spread.

"South Africa has one of the best genomic surveillance systems in the world, so we know that they're really constantly evaluating this virus," Katelyn Jetelina, an epidemiologist at UTHealth School of Public Health, told Insider.

If Omicron originated recently, it may explain South Africa's recent surge of COVID-19 cases: Average daily cases there have risen 10-fold since Omicron was first spotted on November 9. The Dutch government also detected 13 Omicron cases among 61 passengers who tested positive for COVID-19 after returning from South Africa on November 26.

"The key detail that's going to get nailed down this week, or very soon, is: When did this first make its entry into the human population?" Gottlieb told SiriusXM, adding, "There is a possibility this has been circulating for quite some time. And if that is the case, then this becomes a little less worrisome than it is right now, because it means it's probably not spreading that quickly."

Scientists are still evaluating Omicron's public health threat

Omicron contains several worrisome mutations found in other variants of concern, including Delta and Alpha. But it also carries 26 new mutations that haven't been spotted in other variants before.

That combination of mutations raises red flags among scientists, who are still waiting on lab studies to determine how well coronavirus antibodies — either from natural infection or vaccines — hold up against the new variant. On its own, a higher number of mutations doesn't make Omicron deadlier, more transmissible, or more resistant to vaccines than other variants of concern.

"We don't know yet if this new variant is outcompeting Delta," Jetelina said. "We also still don't know if it will evade our vaccines yet, either."

So far, Omicron cases in South Africa have been "extremely mild," Angelique Coetzee, chair of the South African Medical Association, told The Guardian on Friday. But Gottleib noted that many of those cases were recorded in younger people, so "it's too early to tell whether this is causing more serious illness."