“NBC News, citing unnamed officials aware of the decision,
reported it comes after new data suggests vaccinated
individuals could have higher levels of virus and infect
others amid the surge of cases driven by the delta variant
of the coronavirus.”

https://www.usatoday.com/story/news/health/2021/07/27/covid-vaccine-variant-hospitalization-children-mask-mandates/5380480001/

Note: It looks like they’ve updated the article to remove
this, but the link above will work in the archive-today
site.

Source

https://www.investmentwatchblog.com/usa-today-nbc-news-new-data-suggests-vaccinated-individuals-could-have-higher-levels-of-virus-and-infect-others/