COVID cases among children are rising “exponentially,” according to a report released by the American Academy of Pediatrics (AAP) and the Children’s Hospital Association on September 20, as the more contagious Delta variant continues to wreak havoc with students returning to school.

The AAP reported nearly 226,000 new child cases over the past week ending Sept. 16—the third-highest number of child cases in a week since the pandemic began. Children accounted for 25.7% of all cases reported last week, despite representing just 15.7% of all COVID cases since the pandemic began and making up 22.2% of the U.S. population.

Further highlighting how much the situation has worsened over the final months of summer, last week’s pediatric cases mark a 480% increase over the roughly 39,000 new cases among children reported during the week of July 15 to 22.

“After declining in early summer, child cases have increased exponentially, with over 925,000 cases in the past 4 weeks,” the AAP said in the report. In total, 5.5 million U.S. children have tested positive for COVID-19 since the beginning of the pandemic.

Currently, only children ages 12 or older are eligible to receive the COVID vaccine in the United States. However, that eligibility could soon expand to ages 5 to 11, as Pfizer and BioNTech are expected to file for emergency approval for younger children.

Health officials remain concerned over further spread among children in the classroom this fall—with mask mandate policies varying by state and school district—while the Delta variant continues to infect people at high rates. White House chief medical adviser Dr. Anthony Fauci, when speaking at a lecture hosted by the London School of Hygiene and Tropical Medicine a little over a week ago, expressed concern over the impact the virus is having on children who remain unvaccinated.

“We are almost overrun. We have a lot of children in hospitals now,” Fauci said. “So even though relatively speaking, compared to an adult they don’t get as seriously ill, we have lost more children from SARS-CoV-2 than we ever lose for influenza—and we vaccinate children against influenza.”

While child hospitalizations due to COVID are indeed occurring at a higher rate than they did early in the summer, kids have made up only between 1.6% and 4.2% of overall COVID hospitalizations among the 24 states that reported data.

Overall, between 0.1% and 2% of pediatric cases result in hospitalization, and children have accounted for no more than 0.25% of COVID deaths among the states that reported that data. Longer-term effects of the virus on children are still relatively unknown.

“At this time, it appears that severe illness due to COVID-19 is uncommon among children,” the AAP said in the report. “However, there is an urgent need to collect more data on longer-term impacts of the pandemic on children, including ways the virus may harm the long-term physical health of infected children, as well as its emotional and mental health effects.”

Timelines on vaccines for kids

Pfizer and BioNTech are pushing for quick approval from U.S. and European Union regulators of their COVID vaccine for kids age 5 and older. BioNTech CEO Ugur Sahin said in a statement that tests on that group had produced results consistent with those in older populations.

As Fortune senior writer David Meyer reported on September 21, there are several considerations that the drugmakers have had to take into account when trialing the vaccine in younger children, one of which is the amount that is to be administered. In the case of Pfizer and BioNTech’s vaccine, kids between the ages of 5 and 11 will get 10-microgram doses, which is a third of the amount that people age 12 and older get.

The companies said they are also hoping to provide results on COVID vaccine safety and effectiveness for the 2- to 5-year-old and 6-month to 2-year-old cohorts “as soon as the fourth quarter of this year.”