A person vaccinated against COVID-19 in California becomes the first confirmed case in the U.S. omicron variant, The White House announced on Wednesday that scientists continue to study the risks posed by the new virus strain.

Dr. Anthony Fauci told reporters that the person was a traveler who returned from South Africa on November 22 and tested positive on November 29. Fauci said that the person was vaccinated but not a booster shot and showed “mild symptoms.”

The Biden administration took action at the end of last month to restrict travel from Southern Africa, where the variant was first discovered and has spread widely. Case group Was also determined to be in About two dozen other countries.

“We know that it is only a matter of time before the first omicron case was discovered in the United States,” Fauci said.

Officials said they had contacted all people who had close contact with the person, and their test results were negative.

Not a modern healthcare subscriber? Register today.

Genome sequencing was performed at the University of California, San Francisco, and the sequence was confirmed by the Centers for Disease Control and Prevention.

The U.S. Centers for Disease Control and Prevention is taking measures to tighten U.S. testing regulations for overseas travelers, including requiring all travelers to be tested within one day after boarding a flight to the U.S., regardless of vaccination status. It is also considering mandatory post-arrival testing.

Officials said these measures will only “buy time” for the country to learn more about this new variant and take appropriate precautions, but given its spreadability, it will inevitably reach the United States.

There are many unknowns about this new variant, including whether it is more contagious than the previous strain, whether it will make people sicker, and whether it will hinder vaccination. Fauci, a top American infectious disease expert, said that as scientists cultivate and test laboratory samples of the virus, they will learn more about the omicron strain in two to four weeks.

Before announcing the first case in the United States, President Joe Biden planned to outline his strategy to fight the virus in the winter. Biden tried to calm down concerns about omicron variants, saying that this was a cause of concern, but “not a cause of panic.”

Biden and public health officials are increasingly pressing for more Americans to be vaccinated – and those who have already been vaccinated to receive booster shots to maximize their protection from the virus.