The traveler returned to SFO International Airport from South Africa on November 22.
They began feeling symptoms on the 25th.
The individual was then tested on November 28, with a positive result coming back the next day.
The testing sample was sequenced at UCSF on Tuesday, November 30.
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"I really thought that it was only a matter of time before someone would be able to identify the first case of omicron in the United States, or the first case in California," said Dr. Charles Chiu.
Dr. Chiu is the person who discovered the traveler had been infected with the omicron variant.
He says given their recent travel history to southern Africa, his lab expedited the testing of the sample.
"Normally sequencing takes anywhere from several days to, in some cases, several weeks. We were able to sequence this virus within six hours," Chiu said.
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The passenger's case was mild. The person had the full dose of Moderna, but no booster.
Despite the variant's existence now being confirmed here in the Bay Area, local leaders say the city is prepared.
Speaking at a news conference on Wednesday, San Francisco Health Director, Dr. Grant Colfax, cited the region's high vaccination rates as well as its stringent mask policies.
"This is cause for concern, but it's also certainly not a cause for us to panic," Colfax said.
The sense of readiness was also conveyed by Governor Gavin Newsom, who was speaking at an event in the Central Valley.
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He says another lockdown is unlikely, but urged Californians to take the necessary precautions to keep us all safe.
"To get vaccinated, to get a booster and to do what is required of us," Newsom said.