(CNN) -- About 99% of children in the US live in a county considered a "red" zone with high levels of Covid-19 transmission under new US Centers for Disease Control and Prevention school opening guidance, according to a CNN analysis of federal data.
The new CDC guidelines recommend virtual learning for middle and high schools and hybrid learning or reduced attendance for elementary schools in these high transmission zones.
Nearly 73 million children -- about 99% of the US population under the age of 18 -- live in such a "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.
The CDC guidance stresses 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.
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.
Fewer than 100,000 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. Most of those students live in Hawaii or Washington.
The CNN analysis used the latest federal data on new case rates and test positivity rates, published Thursday by the US Health and Human Services Department, 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.