(CNN) About 89% of children in the US live in a county considered a red zone with high levels of Covid-19 transmission under new school opening guidelines shared by the US Centers for Disease Control and Prevention on Friday, according to a CNN analysis of federal data.
More than 65.3 million children -- about 89% of the US population under the age of 18 -- live in such a red or "high transmission" community, defined by the CDC as a county where there were at least 100 new Covid-19 cases per 100,000 people or a test positivity rate of at least 10% during the past seven days.
When the CDC guidance was released on Friday, closer to 99% of children lived in red zones, according to CNN's analysis.
The CDC guidelines stress five key mitigation strategies: requiring masks, physical distancing, handwashing, maintaining clean facilities and contract tracing. It also recommends different strategies based on how much transmission there is in the surrounding community, and has a color-coded guide with areas of high transmission colored red; substantial transmission colored orange; moderate transmission coded yellow; and low transmission as blue.
The CDC says school districts should re-assess weekly, noting that transmission levels will change over time.
If schools in "high transmission" communities cannot "strictly implement all mitigation strategies," the CDC says all extracurricular activities should be virtual. Plus middle and high schools should stick with virtual learning in these red zones, and elementary schools should maximize physical distance through hybrid learning or reduced attendance.
About 115,000 children in the US live in a county considered "low" or "moderate transmission," where the CDC recommends K-12 schools open for full in-person instruction.
The CNN analysis used the latest federal data on new case rates and test positivity rates, published Sunday by the US Department of Health and Human Services, to determine each county's risk threshold according to CDC guidelines. Population data is from the US Census Bureau's 5-Year American Community Survey 2019 estimates.