The Delta variant has triggered a surge in COVID-19 cases in children across the country forcing quarantines and school closures in many districts. Experts report that 90% of the new infections in youngsters is caused by the highly contagious and virulent variant.

According to USA Today, children under the age of 12 are not eligible for COVID-19 vaccines which makes school classrooms particularly vulnerable to transmission. While parents and school officials argue about mask mandates and other safety measures, some districts have shuttered public schools after overwhelming numbers of COVID-19 cases were reported.

More than 121,000 pediatric COVID-19 cases were reported in the U.S. just last week. That’s 14 times higher than the weekly number in late June, says the American Academy of Pediatrics. According to CNN, experts believe this is the beginning of a troubling trend in weeks to come.

“This is happening before school starts. Schools are open now,” warns Dr. Peter Hotez, a noted infectious disease expert and the dean of the National School of Tropical Medicine at Baylor College of Medicine. “So, Houston Independent school district opens August 23, that’s going to be a huge accelerant. This is just the beginning, unfortunately.”

Over 20,000 students in Mississippi are currently quarantined after data from 800 schools indicated outbreaks of COVID-19. Around 600 of these schools have now mandated masking for indoor settings. Thirteen-year-old Makayla Robinson became the fifth child to die from COVID-19 in the state last week, one day after her diagnosis.

Ware County in Georgia closed all its public schools due to a sharp increase in cases, says USA Today. Many other states including Indiana and Kentucky announced they would shut down schools temporarily in districts with high rates of positivity for the virus.

“This will be a tough year, and we don’t want to have to shut down this early, but if we can determine who is positive now, we believe we can stay in school longer,” says Sarah Wasson, the superintendent in one Eastern Kentucky school district, according to USA Today.

While healthcare experts have asked for mask mandates in schools, some state governors have attempted to block this move, says CNN.

“Why tie the hands of the public health officials behind their backs?” questions Dr. Paul Offit, a top advisor and a member of the Food and Drug Administration’s vaccine advisory committee. “You have two weapons here, one is vaccines and the other is masking, and for children less than 12 that’s the only weapon they have.”