The United States said Wednesday it has confirmed its first case of the new Omicron variant of the coronavirus, joining a growing number of countries that have detected the strain most recently deemed a concern in the ongoing pandemic.

The case involved an individual in California who returned from South Africa on Nov. 22, according to the Centers for Disease Control and Prevention. The person, who tested positive on Monday, was fully vaccinated and had mild symptoms that are improving.

All close contacts have tested negative, the CDC said.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases and the chief medical adviser to the President, gives an update on the Omicron COVID-19 variant during the daily press briefing at the White House on Dec. 1, 2021, in Washington. (Getty/Kyodo)

After the World Health Organization added Omicron to the list of coronavirus "variants of concern" last week, the United States on Monday implemented a ban on the entry of foreign travelers from eight countries in southern Africa where the strain was first reported.

The White House said President Joe Biden's medical team continues to believe that existing COVID-19 vaccines will provide "some level of protection against severe illness from Omicron, and individuals who have gotten boosters have even stronger protection."

While the Biden administration has worked to make vaccines widely available across the country free of charge, about 40 percent of the U.S. population has yet to be fully vaccinated, according to CDC data.

"We have 60 million people in this country who are not vaccinated, who are eligible to be vaccinated. Let's get them vaccinated. Let's get the people who are vaccinated boosted," Anthony Fauci, the nation's top infectious diseases expert, told a press conference.

Last month, all adults became eligible to receive a booster shot six months after their second dose of a two-shot inoculation, or two months after receiving a single-dose vaccine.

Fauci said two or three weeks will be needed to get to know more about Omicron, such as how transmissible it is, whether it will lead to severe disease for those infected, and whether its impacts will differ for vaccinated individuals who have not had a booster shot.

The individual infected with the Omicron strain was believed to have not received a booster shot, according to Fauci.

Despite the detection of Omicron, the Delta variant remains the predominant strain in the United States, according to the CDC.