• Gov. Gavin Newsom, in a press briefing Wednesday, said the patient is age 18 to 49 and had not received a booster shot because they were not six months out from their original vaccination course.
• The person has mild symptoms, has not been hospitalized and is improving, Newsom said.
• Dr. Mark Ghaly, California's Health and Human Services secretary, said the fact that the patient is fully vaccinated and is improving underscores the importance of immunization.

The person in California who tested positive for the omicron variant of the virus that causes Covid-19 was fully vaccinated, has mild symptoms and is improving, officials said Wednesday.

The patient was otherwise healthy when they returned to the San Francisco Bay Area from traveling in South Africa on Nov. 22, developed symptoms three days later and tested positive for Covid on Nov. 29, according to public health officials in California.

Scientists at the University of California, San Francisco received the patient's sample around 3 p.m. Pacific time Tuesday and completed the sequencing in about five hours, lab director Dr. Charles Chiu told reporters at a press conference Wednesday with San Francisco Mayor London Breed.

Gov. Gavin Newsom, in a separate briefing Wednesday, said the patient is between 18 and 49 and had not received a booster shot because they were not six months out from their original vaccination course.

"This individual has not been hospitalized," Newsom said. "The individuals that this individual has come into contact with have not tested positive yet to our knowledge, and we are hopeful of full recovery and expect nothing less based on what we've learned."

The Centers for Disease Control and Prevention has recommended that all U.S. adults get a booster six months after their original Pfizer or Moderna two-dose course, and two months after their single J&J shot.

Dr. Mark Ghaly, California's Health and Human Services secretary, said the fact that the patient is improving underscores the importance of vaccination.

"We have been talking for months about the fact that vaccinations do one really, really important thing — protect against severe disease, against hospitalization and death," Ghaly told reporters at the briefing with Newsom. "The evidence that an individual with omicron identified by sequencing actually has mild symptoms, is improving, I think is a testimony to the importance of the vaccinations."

Ghaly said there's still a lot to learn about the variant but encouraged Californians to get vaccinated and get a booster shot if eligible.

White House chief medical advisor Dr. Anthony Fauci told reporters Wednesday that the profile of the omicron variant suggests its mutations could reduce the effectiveness of the vaccines currently on the market, but more data is needed.

"The molecular profile of the kinds of mutations that you see [in omicron] would suggest that it might be more transmissible and that it might elude some of the protection of vaccines," Fauci said Wednesday. "But we don't know that now."

The CEOs of Moderna and Pfizer have said it will take about two weeks to determine omicron's impact on the effectiveness of the current vaccines.

"I don't think that the result will be the vaccines don't protect," Pfizer CEO Albert Bourla told CNBC on Monday. "I think the result could be, which we don't know yet, the vaccines protect less."

Bourla said Pfizer can develop a new vaccine within 100 days. The company was able to quickly create vaccines for the beta and delta Covid variants but ended up not using them because the original vaccine remained effective against the mutations, he said.

Moderna CEO Stephane Bancel said the vaccine maker can roll out a higher-dosage booster shot quickly but that it could take until early 2022 to develop and ship a vaccine that specifically targets omicron.

"The higher dose could be done right away but it will be months before the omicron-specific variant is ready to ship in massive quantities," Bancel told CNBC's "Squawk Box" on Monday.

Maria Van Kerkhove, the World Health Organization's Covid technical lead, said Wednesday that reports from South Africa indicate some patients infected with omicron show mild symptoms but in other cases the disease has become more severe. Van Kerkhove said studies are looking at those hospitalized to see whether they have the omicron variant.

"It is certainly possible that one of the scenarios is that the virus, as it continues to evolve, may still have a fitness advantage, meaning that it can become more transmissible than delta. We'll have to see," she said. "But we don't know quite yet about the severity."