More Omicron cases are detected in the U.S., including in a Minnesota resident who had been to N.Y.C.

More known cases of the Omicron variant of the coronavirus were reported in the United States on Thursday. Minnesota health officials said that a man who lives in the state was infected with the Omicron. The man, who officials said had recently traveled to New York City, represents the second known case of the variant in the United States.

And a woman who lives in Colorado became the state’s first case of Omicron, state health officials said, after she recently returned from a trip to southern Africa for tourism.

Leaders in Minnesota said the discovery was unsurprising and credited robust disease surveillance systems for finding it.

“This news is concerning, but it is not a surprise,” Gov. Tim Walz said in a statement. “We know that this virus is highly infectious and moves quickly throughout the world. Minnesotans know what to do to keep each other safe now — get the vaccine, get tested, wear a mask indoors and get a booster.”

Much remains unknown about Omicron, including whether it is more transmissible and capable of causing more serious illness.

According to the Minnesota Department of Health, the man is a resident of Hennepin County, which includes Minneapolis. He had been vaccinated, and he got a booster shot in early November. He is no longer feeling symptoms, the department said.

The man first developed mild symptoms on Nov. 22, shortly after traveling to New York City for the Anime NYC 2021 convention at the Javits Center, the department said. Officials said the man had not been outside the United States recently. Gov. Kathy Hochul of New York said on Thursday that everyone who attended the convention should get tested for the coronavirus, while Mayor Bill de Blasio said the city had activated its contact tracing program to track down people who attended the event. He added that “we should assume there is community spread of the variant in our city.”

The Omicron variant was first identified by South African scientists, who reported their findings publicly on Nov. 25. Experts have said since then that it was only a matter of time before the variant was detected in the United States, and that once a first case was spotted, many more would probably soon be found.

The Minnesota man was tested for the coronavirus on Nov. 24. The state’s Public Health Laboratory determined late Wednesday night that he had the Omicron variant, officials said during a call on Thursday.

“Suffice to say, everybody around the country is looking for information and on alert” for the Omicron variant, said Jan Malcolm, the state’s health commissioner. “It’s very, very early hours.”

Minnesota officials said they were working with New York City and federal health officials to look into the case, and that the man was cooperating with case investigators, and following public health officials’ instructions to isolate himself.

One of his close contacts has since tested positive for the coronavirus officials said, but because a rapid test was used, scientists have not yet determined whether that person, who is also in Minnesota, had the Omicron variant as well.

In Colorado, Gov. Jared Polis said at a news conference that the resident infected with Omicron had recently returned from a trip to multiple countries in southern Africa. She had been fully vaccinated and was eligible for a booster shot but had not yet received it. She was experiencing mild symptoms and isolating at home.

“This particular case is not community transmission in Colorado,” he said. “We don’t yet know the extent of community transmission within the United States. We know if community transmission is occurring in Colorado, it is very small.”

Genetic sequencing is required to determine which variant a patient has. In recent months the United States has greatly expanded sequencing, but the process takes time — at the Centers for Disease Control, typically about 10 days — to yield results. Currently, according to the agency’s director, Dr. Rochelle P. Walensky, about 14 percent of all positive P.C.R. tests in the U.S. are being sequenced.

On Wednesday, California health officials announced that a San Francisco resident had been infected with the Omicron variant — a finding they emphasized was inevitable, as they worked to contain alarm over the variant’s discovery in the United States.

The man in Minnesota “was getting ill even before we were hearing about Omicron from South Africa,” Kris Ehresmann, the state’s infectious disease director, told reporters on Thursday.

Though the new Omicron variant is grabbing headlines, officials warned that the earlier Delta variant of the coronavirus remains prevalent and is still spreading in the United States, posing a grave risk especially to unvaccinated Americans, who are much more likely than vaccinated people to become severely ill if they are infected.

In Minnesota, Covid hospitalizations have been steadily climbing since the summer, straining health care systems and pushing health workers to the brink as intensive care units have filled with patients.

“The Minnesota surge at this point is Delta — we know that from our surveillance and sequencing work,” Ms. Malcolm said on Thursday. “We do hope this additional news underscores the urgency of paying attention and knowing Covid is very much still with us.”

Sixty-three percent of all Minnesota residents are fully vaccinated, according to C.D.C. data, and in Hennepin County, the figure is 67 percent.

Ms. Malcolm said that there were no statewide pandemic restrictions in effect for businesses in Minnesota, nor any statewide mandate that people wear masks in public indoor settings. She added that she did not believe that any “further regulatory approaches” would be necessary to manage the spread of the Omicron variant. But she urged Minnesotans to take precautions voluntarily: Mask up while indoors, and get vaccinated.

“We feel like we’ve got the tools to manage this and stay ahead of this,” she said.