Reports of COVID-19 in emergency room visits from adolescents have nearly doubled over the past week, new Centers for Disease Control and Prevention data shows, reaching levels not seen in a year.

Measured as a share of all emergency room visits in children ages 12 to 15 years old, the figures published late Thursday by the CDC show weekly COVID-19 averages have accelerated to 2.43% through August 21. 

Rates of COVID-19 ER visits in these adolescents have increased from 1.33% the week before, and are now higher than levels seen among most other age groups except for the youngest and the oldest Americans.

By contrast, over last winter's wave, rates of ER visits from 12 to 15 year olds were among the lowest compared to other ages. ER visits have not been higher in this age group compared to others since around this time last year, amid a large wave of infections that strained some hospitals.

The increase comes as schools and businesses are now weighing a return to masks and other precautions to curb the virus, amid a weeks-long rise in new COVID-19 hospitalizations nationwide that is projected to continue. Officials are also now tracking a new, highly-mutated COVID-19 variant called BA.2.86 that experts think might fuel further spread.

Close to 10 million Americans are now in communities at "medium" COVID-19 levels that the CDC says warrants considering returning to masking and some other precautions for at-risk Americans. 

While not all emergency room visits turn into hospital admissions for COVID-19, officials say they have closely tracked this metric as an early indicator of the spread of the virus, especially after official case counts became unreliable to measure infections.

COVID-19 ER visits look worst in the Southeast, where the virus now makes up 4.46% of visits in adolescents – higher than rates seen in any other age group in the region.

This region – spanning Alabama, Florida, Georgia, Kentucky, Mississippi, North Carolina, South Carolina and Tennessee – had also reported earlier and steeper rises in COVID-19 hospital admissions for the current wave, compared to many other parts of the country.

"The increase in cases is likely due to a variety of factors such as schools and colleges starting, high temperatures sending people indoors for activities where they may be in closer proximity to each other, and new variants circulating," Dr. Kathryn Taylor, Mississippi's interim state epidemiologist, told CBS News in a statement.

Within the Southeast, Mississippi's rate of COVID-19 emergency room visits for adolescents is now averaging among the highest of any state. Taylor said that increasing COVID-19 cases mean a greater risk of being exposed to the virus.

"Mississippians should continue to be aware that COVID-19 is a concern, stay home when ill, seek care or testing when indicated, and if not already up to date on vaccination, get vaccinated," Taylor said.

Alexander Tin

CBS News reporter covering public health and the pandemic.