The U.S. first case of the UK strain was discovered last month in Colorado.
"It's a variant that has specific mutations that was described in the UK in the middle of December," said Dr. Ramon Lorenzo Redondo, a virologist at Northwestern University Feinberg School of Medicine. "It has taken over the epidemic only in the UK."
Gov. JB Pritzker gives latest on COVID-19 in Illinois
Redondo is a part of a team of scientists at Northwestern University who have been tracking COVID-19 samples. Out of 200 recent patients, he said they found one with the UK variant.
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That finding has prompted state officials to once again warn of the need to maintain precautions, as the UK variant is believed to be more contagious. However, doctors don't believe it can cause more severe illnesses or is more deadly.
"Early studies do show that this variant spreads easier and more quickly," said Illinois' top doctor, Ngozi Ezike. "So if we do not continue to wear our masks, watch our distance and avoid gatherings, this new variant could sweep across the state as it swept across the UK."
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"We must remain vigilant if we are to maintain our progress," Governor JB Pritzker added. "Honestly, that's going to be up to each and every one of us to keep our schools open and see an economic recovery that restores jobs and businesses. We've got to wear a mask limiting interactions with people outside our household and get vaccinated when it's our turn."
Redondo said current vaccines are expected to be effective against the UK variant but he warns that now is the time to reduce the spread.
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"That's why it's so important to lower the numbers," he said. "Not because this virus is more severe but we don't want to give the virus more chances to adapt and get better at infecting."
The discovery of the variant in Chicago comes as the global death toll from COVID-19 tops 2 million, according to data compiled by Johns Hopkins University.
The milestone was reached just over a year after COVID-19 was first detected in the Chinese city of Wuhan.
It took eight months to hit 1 million dead and less than four months after that to reach the next million.