The Los Angeles (LA) County Public Health Department announced that 3,186 coronavirus cases had been reported from schools from August 16-22.

The data, collected as students came back to campus en masse for the first time in more than a year, offered an early snapshot of the status of local schools in protecting against the on-campus transmission of the fatal disease, especially the Delta variant, reports Xinhua news agency.

“The early data we have on schools is somewhat sobering,” said Public Health Director Barbara Ferrer during a public briefing here on Friday.

Among the infected, 2,666 cases were reported from the Los Angeles Unified School District, the second-largest in the country with 664,774 students, 25,088 teachers, and 50,586 other employees, during the 2020-2021 school year. Ferrer said that overall the proportion of school-related cases among vaccinated individuals was in the single digits.

Data released by the authority showed that only 5 percent of reported infections were students or teachers who had been fully vaccinated.

Ferrer also noted that the school data helped the county health team better understand what drives the risk of transmission higher, saying “our findings from these outbreaks suggest that transmission risk is high with close, sustained, unmasked contact between people, especially when they are engaging in physical exertion, and in this case, most of the activities were happening indoors”/

The county’s accumulative caseload and death toll currently stood at 1,394,488 and 25,181, respectively.

And though it’s still low, the COVID-19 hospitalization rate for unvaccinated children is also much higher than “the virtually nonexistent” hospitalizations among vaccinated children, the health director said.

“We anticipate an upward trend in outbreaks as our schools have reopened, but we’re continuing to work hard to prevent investigate and manage them as they happen,” Ferrer said.

There were 3,186 new coronavirus cases reported at K-12 schools between Aug. 16 and Aug. 22, according to the county health department. Many of the cases were from the L.A. Unified School District, where children have to be tested weekly.