Nearly 72,000 children tested positive for COVID-19 last week, a “substantial” increase that troubles public health advocates amid the growing wave of infections linked to the highly transmissible delta variant of the virus.

The American Academy of Pediatrics released the figures on Tuesday, noting they were a nearly 85% increase from a week earlier. Children — defined by individual states as those aged 17 or 18 and under ― now account for 19% of the nation’s weekly COVID-19 cases.

More than 4.2 million children have tested positive for the coronavirus since the pandemic began.