The person was fully vaccinated with a booster dose and did not require hospitalization, the state health department said in a statement.
"Public health officials continue to perform contact tracing. Additional information about the individual is not available to protect their identity and protected health information," the statement said.
South African authorities were the first to report the omicron variant to the World Health Organization on Nov. 24.
Much remains unknown about the new omicron variant of the coronavirus, including whether it is more contagious, as some health authorities suspect, whether it can thwart vaccines and whether it makes people as sick as the original strain.
Given the realities of international travel, scientists said it was inevitable that the omicron variant would be discovered in the U.S., and they believe it may have been spreading in the country before it was detected.
Scientists monitor variants and the coronavirus’ evolution through genetic tests that are separate from the kinds of tests used to determine whether someone has COVID-19. This genetic sequencing allows scientists to monitor how the virus changes over time.
The coronavirus is continually evolving, but most mutations are inconsequential. At this point, scientists are trying to figure out whether omicron spreads more easily or causes more severe disease than the delta variant. They are also studying how well the current vaccines work against it.
“Scientists need time to learn more about the omicron COVID-19 variant, but in the meantime, we already know how to be vigilant,” said Gov. J.B. Pritzker in a statement. “So get your vaccine, get your booster, wear your mask indoors, wash your hands, and get tested for COVID-19 if you feel sick or have been exposed to someone who tested positive. I encourage all Illinois residents to make a plan for how to best protect themselves and their loved ones, especially in the holiday season.”
The first U.S. case was reported Dec. 1.
This is a developing story that will be updated.
The Associated Press contributed.
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