Coronavirus cases among school-age children in Pennsylvania were nearly 10 times greater last week than during the same period in 2020, state health officials said Monday, with local officials saying the rise could be due to a variety of factors, including the highly transmissible delta variant, the return of in-person school, and an increase in testing.

Between Sept. 2 and Sept. 8, nearly 5,400 Pennsylvania children between ages 5 and 18 had confirmed infections, according to the state Department of Health, compared to 574 children who were infected during the same week in 2020, when most children were in virtual school and delta was not circulating.

As widespread in-person school got underway for the first time in 18 months, many Pennsylvania counties saw upticks in cases among children earlier this month, the data show. That increase was less severe in Philadelphia’s collar counties, according to the data, though some schools in the region didn’t start until after Labor Day.

Philadelphia has seen a similar trend, said spokesperson James Garrow, with the case rate among people under age 20 about four times higher than it was in early September 2020. Current information about infections among New Jersey children was not immediately available.

Lisa O’Mahony, the medical adviser for Delaware County, called the statewide increase a “mind-blowing” and shows, among other things, just how much delta can spread among unvaccinated people, including children under 12 who aren’t yet eligible for shots.

It appears the state’s data may reflect a national trend. The American Academy of Pediatrics last week reported that cases among children were making up more than a quarter of the United States’ weekly coronavirus cases — getting sick at higher rates than they have during the pandemic overall.

In the two weeks preceding Sept. 9, nearly 500,000 new cases occurred in children — what the group called an “exponential” increase after declining case rates during the early summer. In Illinois, state data showed hospitalizations, cases, and school outbreaks were rising; last week cases in Colorado’s children were reported to have more than doubled since the school year began; and in the Chattanooga, Tenn., region, kids were making up 1 in 4 new cases.

The school year has not been the back-to-normal many had hoped for, as the rise of the delta variant and lack of approved vaccines for children under 12 have dampened the fall and debates over whether masks should be required have raged in some school districts.

Schools reopened amid a delta-driven surge that hit certain states, especially in the South, particularly hard. It also brought steadily rising case numbers among the general population in Pennsylvania and New Jersey, and hospitalizations and deaths slowly climbed in tandem.

The Pennsylvania Department of Health said Monday it would begin publishing weekly data about cases in children 4 and younger and ages 5 to 18. The commonwealth noted these new cases among children were not necessarily due to spread in the classroom: Children could have been exposed in school or child care settings, at home, or elsewhere in the community.

» READ MORE: 20,000 Philly schools employees must be vaccinated by Sept. 30. Here’s what happens if they don’t.

“We know the pediatric age group, the children, will do better in areas that are highly vaccinated,” said O’Mahony, a pediatrician. “The reality here is that we’re going to see less illness in the unvaccinated children if the community at large, that is eligible for vaccine, is immunized.”

Among children 4 and younger — none of whom can get vaccinated and some of whom can’t wear masks — 2,701 cases have been recorded since Aug. 16, with 845 of them the week of Sept. 2 to 8. State officials have said that cases in day cares increased significantly since the school year began.

It isn’t clear what exactly is causing the rise in cases among children.

The level of transmission overall could play a role: The seven-day rolling average of new cases in Pennsylvania was around 800 in early September last year, but in recent days it’s been above 3,300.

Chester County Health Director Jeanne Franklin said the county, among the state’s most vaccinated, anticipated cases to rise “as staff and students returned to in-person learning, as school athletics and extracurricular activities began, and with the implementation of testing strategies beginning to be put in place at schools.”

» READ MORE: Philly-area COVID-19 cases are leveling off, but health community is ‘bracing for an uptick’ heading into fall

Philadelphia’s Garrow also noted that the return of in-person school has also led to children being tested more than they were this time last year.

“It could be that being in school is causing our rates to go up, or that being in school is leading to more testing which is identifying more mild cases that last year wouldn’t have been seen,” he said, “or that delta may be affecting children more.”

The city’s epidemiologists believe hospitalizations may be a better metric to measure whether the virus is affecting children more severely, Garrow added. This time last year, the rate of hospitalizations among children in the city was 0 per 100,000 people; in recent days, it was .01 per 100,000 — “still very, very small overall,” he said.

O’Mahony said the best thing adults and teenagers can do to protect children is to get vaccinated if they haven’t already and to make sure their children are taking precautions.

“I think parents need to appreciate that layered mitigation strategies will keep their kids healthy and in school,” she said. “That’s masking and social distancing.”

Graphics artist Chris A. Williams contributed to this article.