(SACRAMENTO, Calif.) -- The Sacramento City Unified School District reinstated its mask mandate Monday just two months after making face coverings optional.
School officials said the decision came after the Centers for Disease Control and Prevention designated the county as having high levels of COVID-19 in the community.
When counties enter this category, the federal health agency recommends masking in indoor, public spaces and on public transportation.
"Sacramento is a community that has been devastated due to COVID," SCUSD Superintendent Jorge Aguilar told ABC News. "Our district has been really committed in the overall safety of our students and staff ... so we established the CDC threshold would be the drivers for our decision-making."
According to the CDC, the county is currently recording 283.49 new COVID-19 cases per 100,000 people every week.
Data from the district shows the case rate is currently more than seven times higher than when the mask mandate was lifted.
Dr. Peter Chin-Hong, an infectious diseases specialist at the University of California, San Francisco, told ABC News that cases in Northern California are rising for several reasons, the main being the spread of new omicron subvariants.
"The new generations of subvariants are looking so different from the original variants from Wuhan that the antibodies don't recognize them as easily," he said.
He described immunity from vaccination against the new subvariants as two levels of defense.
"I think of the antibodies as guards at the front gate that are a little bit sleepy that let the virus in," Chin-Hong said. "But you have B-cells and T-cells so, once the virus is in, they're like the guard dog that attacks. So, you get infected, but you won't go to the hospital."
SCUSD's decision comes as several school districts have reimposed mask mandates amid increasing COVID-19 cases in recent weeks.
These include Philadelphia; Brookline, Massachusetts; and Providence, Rhode Island, as well as universities such as the University of Delaware and the University of Hawaii.
"I know that April to June is not a very long time and people were just getting used to" not having a mask mandate, Aguilar said. "I do recognize that this is a very divisive topic but we've been very fortunate that the vast majority of our community has stood firm and in support of the kinds of measures that we've put in place."
Chin-Hong said SCUSD's decision to reimpose mask mandates is an example of what health officials mean when they discuss ramping up mitigation measures when cases rise and easing them when cases fall.
"This is a good example of what turning on and off the switch of protection looks like," he said. "I think we'll have to get used to it to keep hospitals intact and keep people healthy."