The US has identified its first reported case of Covid-19 caused by the Omicron variant, with health officials confirming that an individual in California had tested positive.

The Centers for Disease Control and Prevention said on Wednesday that the variant was discovered in an individual who returned from South Africa on November 22, two days before the variant was first reported to the World Health Organization by the country.

Anthony Fauci, the chief medical adviser to President Joe Biden, said the individual was fully vaccinated and displayed mild symptoms.

“We knew that it was just a matter of time before the first case of Omicron would be detected in the United States,” Fauci said. “We know what we need to do to protect people: get vaccinated if you’re not already vaccinated; get boosted if you’ve been vaccinated.”

The CDC said it “has been actively monitoring and preparing for this variant, and we will continue to work diligently with other US and global public health and industry partners to learn more”.

Biden is expected to announce on Thursday a series of measures designed to slow the spread of Covid-19 over the winter, including stricter testing requirement for international travellers. But Fauci said even with the US’s first Omicron case, the administration is not yet considering mandatory testing on arrival in the country.

“This person did what we hope other people would do,” he said. “As soon as they became symptomatic, they went and got tested, and it was positive.”

Last week, on the same day the WHO declared Omicron a “variant of concern”, Biden announced that travel to the US from several African countries would be restricted. Fauci said on Wednesday those restrictions would be temporary, though he did not say when they would end.

Asked however if the pandemic would ever end, he said: “There is no doubt that this will end. I promise you that this will end.”

Gavin Newsom, the California governor, urged residents not to panic about the Omicron variant, adding in a tweet: “We should assume that it’s in other states as well.”

In the days following the detection of Omicron in southern Africa, a growing number of governments have moved to restrict travel from those countries. Other countries have since confirmed cases of the new strain.

US stocks continued to give up earlier gains following the announcement. The S&P 500 was down 0.1 per cent in afternoon trading, having been up as much as 1.9 per cent during the morning session.