CHICAGO (CBS) — Cook County and several other counties in the Chicago area are now seeing “substantial” spread of COVID-19, according to data collected by the U.S. Centers for Disease Control and Prevention, meaning fully vaccinated people are asked to resume wearing masks indoors.

Earlier this week, the CDC recommended that fully vaccinated people in parts of the country that have “substantial or high transmission” of the virus should resume wearing masks indoors, in an effort to slow the spread of the virus, driven largely by the highly contagious Delta variant.

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The vast majority of Illinois’ 102 counties are now seeing substantial or high transmission of the virus, according to CDC data.

As of Thursday, Cook, Boone, DuPage, Grundy, Kendall, LaSalle, McHenry, and Will counties are now seeing substantial transmission of the virus in the Chicago area. Lake, Kane, and Kankakee counties are still seeing moderate transmission.

Overall, all but 18 counties in Illinois are now seeing substantial or high transmission of COVID-19, according to CDC data.

The new guidance comes as a growing number of local and state health officials have already returned to mandating masks indoors, with cases of the virus surging among unvaccinated Americans.

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While neither the city of Chicago nor the state of Illinois have reinstated mask mandates or other COVID restrictions, Mayor Lori Lightfoot has said the city could consider new mitigations if cases continue to climb.

Thursday afternoon, Gov. JB Pritzker urged Illinois residents in counties with substantial or high transmission of the virus to mask up indoors in public places.

CBS 2 Chicago Staff