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The Centers for Disease Control and Prevention (CDC) and health authorities in California confirmed the first case of COVID-19 linked to the newly discovered variant Omicron in the U.S. on Wednesday, saying an individual who had recently returned from South Africa tested positive for the strain.

The traveler was fully vaccinated and is experiencing “mild symptoms that are improving” and has been self-quarantining since testing positive, the CDC said in a statement. The person’s close contacts have been contacted by health authorities and tested negative.

The health agency said the emergence of the variant “emphasizes the importance of vaccination, boosters, and general prevention strategies needed to protect against COVID-19. Everyone 5 and older should get vaccinated boosters are recommended for everyone 18 years and older.”

The U.S. joins more than 20 countries and territories to have spotted at least one case of the strain around the world, since South African health officials sounded the alarm over the variant on November 25. The Biden administration recently classified Omicron as a “variant of concern,” echoing the World Health Organization’s designation over the holiday weekend.

“This is the first confirmed case of COVID-19 caused by the Omicron variant detected in the United States. As all of you know, of course we’ve been discussing this, we knew that it was just a matter of time before the first case of Omicron would be detected in the United States,” Dr. Anthony Fauci, the president’s chief medical adviser, told reporters at a White House briefing.

The first Omicron case

Officials said the person, who has not been identified due to privacy rules, is an adult under the age of 50 who began noticing symptoms of COVID-19 on November 25, a few days after returning from South Africa on November 22. The individual tested positive on November 29.

“The person recently traveled to South Africa and developed symptoms upon their return, and they did the right thing and got tested and reported their travel history,” Dr. Grant Colfax, San Francisco’s director of public health, told reporters on Wednesday.

The person had been fully vaccinated with Moderna’s COVID-19 vaccine less than six months ago, California’s governor said on Wednesday, but was not yet eligible to receive a booster shot. 

Scientists at the University of California, San Francisco have been sequencing the vast majority of positive tests collected in San Francisco, and had been alerted to the possibility of the possible Omicron case on Tuesday afternoon. 

“We were able to confirm the detection of Omicron within five hours and we had most of the genome within eight hours. So, 4 a.m. last night, we actually had assembled most of the genome. We were able to conclusively demonstrate that this was indeed an infection from the Omicron variant,” said Dr. Charles Chiu, professor of laboratory medicine at the university.

Preparing for more

The discovery comes as the Biden administration is mulling tougher restrictions on international travel, part of a new strategy to curb the virus that Mr. Biden plans to announce on Thursday. The CDC has also ramped up efforts to surveil for new cases of Omicron in recent days, including an unplanned expansion of a pilot program that includes San Francisco’s international airport.

On Wednesday ahead of the announcement, the CDC ordered airlines to begin handing over contact information for travelers from eight countries in Southern Africa. 

Fauci said he was not aware of any other potential Omicron cases being investigated by the CDC in the country. He said Americans should continue following CDC guidance to prevent the spread of the variant, and urged those who are fully vaccinated to “get boosted now,” saying the extra dose likely conveys some level of protection against the variant.

“There’s every reason to believe that that kind of increase that you get with the boost would be helpful at least in preventing severe disease of a variant like Omicron,” he said. “We may not need a variant-specific boost. We’re preparing for the possibility that we need a variant-specific boost.”

How exactly vaccines perform against the variant remains unclear. Though the Omicron variant shares a number of mutations with other variants of concern that could enable it to be more transmissible and evade the body’s defenses, scientists have cautioned that it could take weeks to verify in test tubes the variant’s risk. 

On “Face the Nation” on Sunday, Dr. Scott Gottlieb, a former commissioner of the Food and Drug Administration and a member of Pfizer’s board, conveyed confidence in the vaccines.

“People who have looked closely at this sequence … those individuals feel reasonably confident that three doses of vaccine is going to be protective,” Gottlieb said. “Now, that could give a really strong impetus to trying to get more people boosted.”

The Omicron variant’s emergence in the U.S. comes amid the busy holiday travel season. According to auto club AAA, over 53 million Americans were estimated to have traveled for the Thanksgiving holiday, an increase from last year.

The Food and Drug Administration says current COVID-19 tests will likely remain accurate in detecting infections by the variants, despite a quirk known as “S-Gene Target Failure” that had also been seen in earlier strains. Several labs have said they are using that to prioritize positive tests that may be caused by Omicron for further genetic sequencing, in order to verify which variant caused the infection.

Public health officials have defended the U.S. variant surveillance effort, arguing that the system of public health and commercial labs scaled up by the CDC could detect emerging variants down to 0.01% prevalence in the country.

The CDC currently estimates that the Delta variant remains virtually all circulating virus in the country.