In one article published by the CDC, researchers found Arizona schools that opened without a school mask requirement had a 3.5 times higher likelihood of COVID-19 outbreaks compared with schools that started the year with mask requirements.
"I think they showed with excellent population and statistical data that masking works to prevent transmission of COVID-19 between children," said pediatric infectious disease specialist Dr. Santosh Nadipuram with Cedars Sinai.
The article looked at the impact of mask requirements at K-12 district schools in Maricopa and Pima counties, the state's two most populous counties.
The CDC also released two other new studies that support the effectiveness of universal masking in schools.
Another study found case rates in children and teens increased more in U.S. counties where public schools had no mask mandates than in those where schools had that requirement. These studies lacked data on other measures that could have influenced the results.
A third study counted 1,801 coronavirus-related school closures through mid-September, most of them in the South, where many schools opened earlier than those in other regions.
Still, 96% of U.S. public schools have remained open with in-person classes, the researchers say, while acknowledging that data from some districts may be missing.
The findings demonstrate masking in schools is an important strategy to prevent COVID-19 spread.
"(Masks do) prevent transmission from person to person and it does not prevent things like getting air or being able to exercise well or that kind of thing," Nadipuram said.
Meanwhile, Pfizer vaccine booster shots are already going into arms Friday, immediately following the CDC's endorsement.
CDC Director Rochelle Walensky included health-care workers and others in high exposure occupations in her official recommendation for a booster shot.
A CDC advisory panel had voted against it, saying they didn't feel comfortable including healthy, young individuals for Pfizer booster shots simply because of occupation.
For those who qualify, state and county-run websites and various pharmacies such as Walgreens and CVS aren't wasting time making appointments for third Pfizer shots available.
The Associated Press contributed to this report.