The first confirmed case of the Covid-19 Omicron variant in the United States has been detected in a person in California, the US Centers for Disease Control and Prevention said Wednesday.

"The individual was a traveller who returned from South Africa on November 22, 2021," the agency said. 

"The individual, who was fully vaccinated and had mild symptoms that are improving, is self-quarantining and has been since testing positive. All close contacts have been contacted and have tested negative."

The case was detected by the California and San Francisco Departments of Public Health and confirmed by the CDC.

The Omicron variant was first reported in South Africa and has been designated a variant of concern by the World Health Organization.

Scientists do not yet have hard data, but based on the variant's pattern of mutations, it is expected to at least partly evade the protective action of current vaccines, and may be more transmissible than Delta, the dominant global strain.

The US is planning tougher testing and quarantine requirements for international travelers arriving in the country, including taking a Covid-19 test one day prior to departure, the CDC said Wednesday.

Mandatory testing on arrival and even a mandatory quarantine, regardless of the test result, are also possibilities, The Washington Post reported.