Dr. Anthony Fauci, the country's divisive top infectious disease expert, returned to the White House press briefing room moments after it was announced that a case of the omicron COVID-19 variant had been found in the United States.

The individual, who was vaccinated but not boosted, tested positive in California on Nov. 29 after returning from South Africa a week earlier and was experiencing mild symptoms, according to Fauci, who last briefed at the White House in April. Those considered to have been in close contact with the San Francisco resident have tested negative, he said.

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"We knew that it was just a matter of time before the first case of omicron would be detected in the United States," Fauci told reporters Wednesday.

Fauci urged people to get vaccinated, and boosted if they received their last shot six months ago, and not to wait for variant-specific jabs. But he skirted around questions regarding whether the definition for fully vaccinated would expand to include boosters.

"I don't know. Let's see what rolls out now," he said.

President Joe Biden was last tested for COVID-19 on Monday, according to White House press secretary Jen Psaki, who joined Fauci at the podium. She also said former President Donald Trump did not advise then-candidate Biden he had tested positive for COVID-19 three days before their first debate in 2020. Biden's predecessor went on to test negative.

"We were not aware of the report that went out this morning in advance, no," Psaki said Wednesday.

While Fauci was adamant that the COVID-19 pandemic "will end," Psaki was needled on Biden's campaign pledge to "control" the virus.

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"We're working on it," she quipped.