• Minnesota, Colorado, New York and Hawaii confirmed new cases of the omicron variant on Thursday.
• New York City Health Commissioner Dr. Dave Chokshi said the Empire State cases indicate there is community spread of the variant.
• The first U.S. case of the virus was confirmed in California on Wednesday.

At least five U.S. states have now confirmed cases of the omicron variant of the virus that causes Covid-19, as scientists investigate whether the heavily mutated strain is more infectious and virulent.

At least 9 cases were confirmed on Thursday in Minnesota, Colorado, New York, Hawaii and California. California confirmed the first U.S. case of omicron on Wednesday, bringing the total number of reported cases so far to 10.

Minnesota public health authorities confirmed Thursday morning the second U.S. case of omicron, in a resident who recently returned from New York City, the state's department of health said.

The man, who was fully vaccinated and has since recovered, traveled to New York City to attend the Anime NYC 2021 convention at the Javits Center Nov. 19-21, the department said in a statement. He developed symptoms shortly after returning and tested positive on Nov. 22.

Hours later, Colorado Gov. Jared Polis confirmed a third U.S. case in a woman who recently traveled to southern Africa and returned to the state. The patient was fully vaccinated but had not received a booster shot, he said at press conference. She is experiencing mild symptoms and is isolating at home, Polis said.

The governor said the extent of community transmission in the U.S. is unclear, but health authorities believe it is "very small" in Colorado. Polis said the state screens wastewater and has not yet detected omicron in the analysis. Colorado also runs genetic sequencing on about 15% of its Covid tests searching for variants and hasn't detected other omicron cases yet, he said.

"So if it was prevalent, we would know — it doesn't mean it's nonexistent," Polis said.

New York health officials Thursday evening confirmed 5 cases, one in Suffolk County on Long Island and four in the New York City metro area. New York City Health Commissioner Dr. Dave Chokshi said the cases indicate there is community spread of the variant.

"This is not just people who are traveling to Southern Africa or to other parts of the world where omicron has already been identified," Chokshi said.

New York Gov. Kathy Hochul told residents that the detection of multiple cases does not mean the state will revert to the sweeping shutdowns imposed during the start of the pandemic in March 2020.

"We are in a far better place, that people are informed. There's not a panic," Hochul said Thursday night. "It is still a public health crisis, but does not have to be a crisis that leads to shutdown."

It wasn't immediately clear if any of the New York cases were connected to the anime convention attended by the Minnesota resident who tested positive. Anime NYC, the convention organizer, said 53,000 people attended the three-day event.

"We have confirmed with New York's health officials that the event followed all guidance laid out by the city and state, including mask requirements and proof of vaccination for all individuals inside the Javits Center," Anime NYC spokesman Anthony Hesselius told CNBC.

Moments later, Hawaii's Department of Health confirmed one case of omicron on the island of Oahu.

Los Angeles County reported its first case of the omicron variant on Thursday evening. The individual returned to Los Angeles County after travel to South Africa via London on Nov. 22.

"The individual, who is a fully vaccinated adult and a Los Angeles County resident, is self-isolating, and their symptoms are improving without medical care," LA County health officials said. "A small number of close contacts in Los Angeles have been identified and, to date, all have tested negative and have no symptoms."

The Centers for Disease Control and Prevention, in a statement Thursday, said it is working closely with state public health authorities York City to investigate omicron. CDC Director Dr. Rochelle Walensky said the U.S. has expanded genomic sequencing over the past nine months to identify new variants.

"We have more tools to fight the variant than we had at this time last year from vaccines to boosters to the prevention strategies that we know work including masking in indoor public settings, washing your hands frequently and physical distancing," Walensky said. "These methods work to prevent the spread of COVID-19, no matter the genetic sequence."

The first omicron case in the U.S., detected in California, was in a person who recently returned to San Francisco from a trip to South Africa.

The patient in California was fully vaccinated, has mild symptoms and is improving, Gov. Gavin Newsom said Wednesday. The person, who is between 18 and 49 years old, had not received a booster dose because they were not six months out from their original vaccination course, he said.

Health officials in the U.S. and around the world are concerned that omicron is more transmissible and may evade the protection provided by currently available vaccines to some degree. The variant has some 50 mutations, more than 30 of which are on the spike protein that the virus uses to attach to human cells.

"The molecular profile of the kinds of mutations that you see [in omicron] would suggest that it might be more transmissible and that it might elude some of the protection of vaccines," White House chief medical advisor Dr. Anthony Fauci told reporters on Wednesday. "But we don't know that now."

The Biden administration Thursday laid out a plan to combat omicron and a possible winter surge of the predominant delta variant of the virus. The White House is requiring all in-bound international travelers to test for Covid within 24 hours of their departure. The administration is extending mask requirements on domestic flights and public transit through March 18. It is also expanding access to free at-home Covid tests.

President Joe Biden, in an update for the public, said his plan does not include shutdowns or any expansion of the current federal vaccine requirements.

The World Health Organization on Wednesday said omicron has been confirmed in at least 23 countries worldwide. The variant was first identified in Botswana last month and brought to the attention of the WHO by public health officials in South Africa.

Maria Van Kerkhove, the WHO's technical lead on Covid response, said Wednesday that hospitalizations are rising in South Africa but more data is needed before drawing conclusions about whether omicron causes more severe disease.

The CEOs of Pfizer and Moderna say it will take about two weeks to gather enough data to determine what impact omicron's mutations have on the effectiveness of the current vaccines. They have said it would take until early 2022 to develop a shot that specifically targets the variant. However, Moderna CEO Stephane Bancel said the company can roll out a higher-dosage booster shot much quicker.