A record number of children are hospitalized with COVID-19 in Alabama, health officials said Friday as the state’s largest public school district prepared to switch to virtual classes because so many staffers are out.

Of nearly 2,100 people hospitalized statewide with COVID-19, 71 were children, the Alabama Department of Public Health said in a statement. On Thursday, 9,266 children and adults were reported to have the illness, it said.

Dr. Wes Stubblefield, a district medical officer, said immediate protective steps were critical because of the highly contagious omicron variant of the coronavirus.

“School-wide masking is at the top of the list of preventive steps that need to be implemented. Masks can still make a difference in school settings and allow students to remain in class, if properly used,” Stubblefield said.

Among children ages 5 to 11, only 10.5% have had at least one shot of vaccine, the state said, and 35.5% of young people between 12 and 17 have had a dose.

With the virus spreading quickly, Mobile County’s school system, the largest in the state, said Thursday that it will switch to virtual classes Monday because so many staffers have contracted or been exposed to COVID-19 it’s difficult to keep buildings open.

The district, with more than 53,000 students and 7,200 employees in the state’s southwest corner, joins more than 30 other systems statewide that have opted for online learning since classes resumed because of a surge in illness as the highly contagious omicron variant spread quickly.

At least 20 systems have announced moves to online teaching since Wednesday, according to a list maintained by the state.

Roughly a quarter of the state’s public school students, or more than 200,000 children, have shifted to online classes since 2022 began, yet only only a handful of the largest systems require face masks, which health experts say can help slow the spread of the coronavirus, al.com reported. The state Health Department recommends everyone wear a mask at school.

More than 16,000 new cases of COVID-19 were reported this week in Alabama schools, including more than 600 in Mobile County schools, among the highest in the state, according to data compiled by the state education and health agencies. Some districts reported even more cases, with Montgomery and Baldwin counties highest in the state with more than 800 cases each.

In Mobile County, the crush of new cases combined with school staff missing work because they’re sick or quarantining at home has meant there are too few adults to operate at times.

“Our number of cases has increased, making it difficult to staff many of our schools. Based on today’s numbers, I feel it is best for the health and safety of all students and employees for the entire district to move to virtual learning for the week of January 18–21, 2022,” said a letter sent to parents Thursday from Superintendent Chresal Threadgill.

More than 16,640 people have died of COVID-19 in Alabama, giving the state the nation’s third-highest death rate during the pandemic, according to researchers at Johns Hopkins University. Less than half of the state’s population is fully vaccinated, among the lowest in the nation, making it easier for the illness to spread.

Over the past two weeks, the rolling average of daily new cases has increased in the state by 4,907, or about 135%. One in every 82 people in Alabama tested positive in the past week, according to Johns Hopkins researchers.