WASHINGTON, Dec 1 (Reuters) - The first U.S. case of the Omicron variant of COVID-19 has been detected in California, top U.S. infectious disease expert Anthony Fauci said on Wednesday.The individual in question had returned from South Africa on Nov. 22 and tested positive seven days later, he said.He said the person, who was fully vaccinated against COVID-19 but had not had a booster shot, had mild symptoms and was self-quarantining.