The White House said 2.2 million vaccine doses have been administered in the last 24 hours -- the highest single-day total since May.

The Minnesotan man who is confirmed to have the omicron variant of COVID-19 received a booster shot in early November, health officials said.

The man attended an anime convention at the Javits Center in New York City from Nov. 19 to Nov. 21, and then, after experiencing mild symptoms on Nov. 22, he was tested on Nov. 24. He no longer has symptoms.

"It seems quite possible, perhaps, most likely, that the transmission happened at the convention in New York City, but that's not definitive," health officials said.

The Javits Center said everyone is required to show proof of vaccination and wear masks inside.

The individual had no history of international travel, officials said.

One of the man's close contacts has tested positive via a rapid test, officials said. Further tests to determine if the second individual is also positive for the omicron variant have yet to be conducted.

-ABC News' Arielle Mitropoulos

The U.K. reported 53,495 new cases on Thursday – the highest single-day rise since July 17, British health officials said.

Hospitalizations in the U.K. are down 6.5% over the last week while deaths are down 3% over the last week, officials said.

Over 80% of the 12-and-over population is fully vaccinated.

Thirty-two cases of the omicron variant have been detected in the U.K.

-ABC News' Christine Theodorou

As omicron appears to be rapidly overtaking delta as the dominant variant of the novel coronavirus in South Africa, a scientist warned Thursday that the new variant appears to be able to dodge some of the immunity from vaccines and previous infections.

Professor Anne von Gottberg, a clinical microbiologist at South Africa's National Institute for Communicable Diseases, told reporters during an online press briefing that omicron may not be more contagious or virulent than delta, but early data indicates previous infection does not provide protection against omicron as it seems to do with delta.

"We believe that omicron is widespread in the country," von Gottberg said.

South Africa and Botswana were the first countries to identify the omicron variant last month. Since then, South Africa has reported a total of 172 confirmed cases and Botswana has reported 19.

Over the past week, Southern Africa has recorded a spike in COVID-19 infections, hospitalizations and deaths, mostly driven by South Africa, where scientists believe omicron is rapidly becoming the dominant variant.

"It does look like there is a predominance of omicron throughout the country," von Gottberg said. "Almost 80% of specimens being sequenced in November are omicron. So we believe, truly, there is a link in the increase in cases with omicron variant."

Three cases of the omicron variant have been detected in Maryland, all among Baltimore area residents, Gov. Larry Hogan said.

One of these people, who is vaccinated, recently traveled to South Africa. The second case is someone in their household who is not vaccinated, Hogan said.

The third case is unrelated; this person, who is vaccinated, has no recent travel, Hogan said.

No one required hospitalization, he said.