SAN DIEGO – San Diego Unified may reinstate its indoor mask mandate in at least some of its schools as soon as Wednesday in a bid to halt the spread of COVID-19, district leaders wrote in a letter this week.

The return of a key COVID mitigation strategy comes amid “a steady increase” of infections in San Diego County and the U.S. in general, the district’s letter to staff and families reads. California’s second-largest school district intends to notify families if their school or the district reaches levels requiring indoor masking for a minimum of 14 days through the end of the school year and into its summer session.

“While this is not the rapid surge we saw in January, it is still a concern and a trend that we expect to continue,” the district said.

District leaders laid out specific criteria for returning to indoor masking in a period expected to last at least two weeks. They again will be required to wear them indoors if at least three outbreaks occur in a school and more than 5% of the school population — including students and staff — are infected.

If 10% or more of students are absent for three straight days due to illness, the requirement also will be put back into place, the letter shows. That requirement does not apply to its summer school, however.

In the district, all schools and facilities will be mandated to wear masks indoors again if San Diego County reaches the high level in the Centers for Disease Control and Prevention’s Community Level system. The county currently is charting in the CDC’s lowest level, which encourages residents to stay up to date with COVID-19 vaccines and to get tested if symptoms arise, but does not recommend indoor masking.

Masks have been optional indoors at California schools since early March when Gov. Gavin Newsom announced the state was moving away from requiring them indoors in most public spaces. Most local districts followed suit in the time after the announcement by laying out their individual roadmaps to retire indoor mask mandates.

San Diego Unified kept its mandate for another month beyond the end of the state’s, citing the county’s status in March in the CDC’s high-risk category.

When the requirement fell, district board member Richard Barrera said San Diego Unified would continue to monitor COVID-19 conditions and make changes if needed.

“(If) we start to come back into the higher-risk categories under the CDC, we will require our students to put the masks back on,” Barrera told FOX 5 in April.

The district’s last day of the school year is June 14.