What is known about the omicron variant that's arrived in the US

As health officials expected, the United States has now confirmed several cases of the omicron variant, which was first identified in Botswana and has since been detected in at least 35 counties.

The World Health Organization declared omicron a variant of concern on Friday. Earlier this week, the agency said the overall global risk is "very high" due to the variant's large number of mutations, including on the spike protein of the virus, though there are still many unknowns, including information on its transmissibility, severity and impact on immunity.

Here's what we know so far about the variant in the U.S.

First states to detect cases

The first three cases of the omicron variant identified in the U.S. were detected in fully vaccinated Americans with recent travel histories. All three people experienced mild symptoms, health officials said.

The first case was detected in California in an individual who had returned from South Africa on Nov. 22. The person, who was not yet eligible for a booster dose, sought COVID-19 testing a few days upon returning to San Francisco after developing symptoms, and genomic sequencing testing confirmed the case to be omicron on Wednesday, health officials said. All close contacts have been contacted and have tested negative.

Colorado confirmed a case of the omicron variant Thursday in a woman from Arapahoe County who had recently traveled to southern Africa. State epidemiologists had flagged the testing specimen for genome sequencing due to the recent travel history. The resident was eligible for a booster but had not yet received it, health officials said.

Through its regular surveillance of COVID-19 case specimens, Minnesota detected an omicron case Thursday in a Hennepin County resident who had recently traveled to New York City to attend the Anime NYC 2021 convention from Nov. 19 to 21. The man, who had received a booster dose in early November, developed mild symptoms on Nov. 22 and sought testing on Nov. 24. It seems likely that the transmission occurred at the NYC convention, but that is not certain, health officials said. His symptoms have since resolved. A close contact tested positive for COVID-19, though tests to confirm if that is also omicron have yet to be conducted, health officials said.

Thursday evening, New York Gov. Kathy Hochul announced that five cases of the omicron variant had been detected in the state. They included a 67-year-old, fully vaccinated woman in Suffolk County who recently traveled from South Africa, and four New York City residents. All cases are mild.

A second California case was also announced Thursday evening, in a fully vaccinated Los Angeles County resident who had returned from South Africa on Nov. 22, health officials said. The infection "is most likely travel-related," the Los Angeles County Department of Public Health said.

The detected cases illustrate the need to seek out testing based on travel history and symptoms, as well as the importance of sharing travel history with public health officials, health experts said. The Centers for Disease Control and Prevention currently advises that people who recently traveled internationally should get tested three to five days upon their return regardless of symptoms or vaccination status. NYC officials have also encouraged all attendees of the Anime NYC 2021 convention, estimated to be around 53,000, to get tested immediately and take precautions such as social distancing.

No cause for panic

Health experts have said the presence of the omicron variant in the U.S. is not surprising, and while concerning, it is not a reason to panic.

"At this point, I'm not terribly alarmed," Colorado Gov. Jared Polis told reporters Thursday during a briefing on the omicron case.

The governor said that if community transmission is occurring in Colorado, it's very small because no omicron variant has been discovered in wastewater analysis yet. Roughly 15% of all positive COVID-19 tests are sequenced in Colorado.

San Francisco Department of Public Health Director Dr. Grant Colfax told reporters during a briefing yesterday that the case was "not a cause for us to panic" and that the city "is prepared" for this.

Leaders in New York and Minnesota had similar messages.

"We're ready for it. This is not surprising," New York Gov. Kathy Hochul said during an on-camera briefing earlier Thursday.

"This news is concerning, but it is not a surprise," Minnesota Gov. Tim Walz said in a statement. "We know that this virus is highly infectious and moves quickly throughout the world."

Minnesota Department of Health Commissioner Jan Malcolm echoed that sentiment during a briefing Thursday, saying omicron is something to take seriously but "not a reason to panic."

"We're at a stage where there is still a lot we need to learn about omicron," she said.

Officials in the cities and states where the variant has been detected have said they don't plan to make any changes to health orders at this time.

Impact on travelers

White House chief medical adviser Dr. Anthony Fauci told reporters Wednesday he doesn't think a domestic flight policy for testing or vaccination due to omicron is necessary right now but said it's always something under consideration.

"These kinds of things we always talk about and consider. But right now, I'm not so sure we need testing for air travel in this country," he said.

There are strengthened requirements for travelers coming to the U.S. amid the spread of the omicron variant. On Thursday, President Joe Biden announced that all passengers must show proof of a negative COVID-19 test within one day of flying into the U.S., regardless of their vaccination status or nationality.

"This tighter testing timetable provides an added degree of protection as scientists continue to study the omicron variant," he said during a briefing.

Unvaccinated Americans already were required to show proof of a negative test within one day of traveling. The new rule, which goes into effect Monday, expands that one-day requirement to all vaccinated travelers coming into the U.S. from other countries. Unvaccinated nonresidents are currently barred from boarding a flight to the U.S.

Delta still dominant

Amid concerns about the omicron variant, health experts have stressed that delta is still a major issue in the U.S., where close to 100,00 new cases are diagnosed daily. Delta comprises 99.99% of new COVID-19 cases.

"I think omicron is another kind of wake-up call, and we needed another one," Malcolm, with the Minnesota Department of Health, said. "Even though we might feel like we're done with the pandemic, it is most certainly not done with us."

Health officials have urged people to get vaccinated and get boosters and to continue to follow COVID-19 guidelines such as wearing a mask indoors while in public, test if you have symptoms and stay home if you're sick.

ABC News' chief medical correspondent, Dr. Jennifer Ashton, told David Muir on Thursday that vaccinations remain the most important tool we have in combatting COVID-19 this winter.

"Some protection is better than none, and while we learn about the many mutations that omicron has ... we have to double down on what we know is going to be our best tool in the toolbox, and that is vaccination," Ashton said.

Regarding boosters, Fauci has urged Americans who are eligible to get the shot now.

"Right now, I would not be waiting," he said Wednesday. "People say, well, if we're going to have a booster-specific vaccine, should we wait? If you are eligible, namely six months with a double mRNA dose, or two months with the J&J, get boosted now."

"We may not need a variant-specific boost," he added.

ABC News' Matthew Fuhrman, Cheyenne Haslett and Arielle Mitropoulos contributed to this report.