Pennsylvania and New Jersey are entering October with coronavirus case levels at the highest they have been since the end of spring, when the region was recovering from the first devastating wave of the pandemic.

From Penn State to Rowan University, students have spurred increases since they returned to college towns in August. But data analyzed by The Inquirer indicate that the spread of the virus is also increasing in some counties that aren’t home to universities. Public health experts say community transmission is not a surprise as cold weather nears, but it is a possible sign that residents should renew efforts to prevent spread.

Since the end of August, the average number of new coronavirus cases reported each day nearly doubled in Pennsylvania and almost tripled in New Jersey. Compared with a month ago, average new cases a day has jumped by more than 400 in Pennsylvania, surpassing 1,000 per day last week, and increased by nearly 340 in New Jersey to more than 650, according to an Inquirer analysis of New York Times case data.

Philadelphia’s case numbers now show a “substantial increase” too, which is “a reason for concern,” city Health Commissioner Thomas Farley said Tuesday, though city rates are not rising as steeply as some counties.

“I don’t think there’s anything surprising about this,” said Michael LeVasseur, a Drexel University epidemiologist. “The weather is getting colder, we’re doing some back-to-school things, people are spending less time staying at home. I think the question is how bad is this going to get.”

The percentage of people tested who are positive has also increased in recent days in Pennsylvania and Philadelphia — a key indicator of community spread.

Statewide, the positivity rate was 3.7% over the last seven days, up slightly from a week earlier, according to data maintained by the state, and Philadelphia reported a rate of 3.4% on Friday, up from 2.8% a week before.

“If we go above 5% again, or trending toward 5%,” LeVasseur said, “that’s around when I’d get concerned.”

Delaware, with a similar increase in cases, had a positivity rate above 8% last week; Gov. John Carney said Tuesday he hoped to bring it down to 5% or less. Though “a good portion” of the uptick is coming from college and university towns, with a large number of new cases concentrated among people ages 18 to 34, Carney said the state is also seeing general community spread.

“It’s kind of a general malaise across the whole state, where you’re seeing a little bit of cases in every geographic area,” Carney said, “and of course, that’s not where we want to be.”

People ages 19 to 24, who usually do not get as sick as older adults, now make up a much larger portion of coronavirus patients than they did early on in the pandemic. Pennsylvania State University, which saw 2,279 cases from Sept. 4 to Oct. 1, drove the Centre County spike, the state’s biggest. An outbreak at West Chester University caused the borough of West Chester to declare a state of emergency on Friday. Temple University saw an outbreak in September that spiked city case numbers but has since eased; the university reported 50 active cases on Tuesday.

The state will likely see cases around colleges continue to increase, Pennsylvania Department of Health spokesperson Nate Wardle predicted.

“If we put each college in a bubble and they… just stayed there, then no, we would not expect to see cases increase,” Wardle said. But with “the risk of not having everybody quarantined ... that’s where there is the potential for the spread.”

Among college students, parties, house gatherings, and bar-hopping are largely blamed for transmission rates. Case increases in areas without colleges and universities have been attributed in part to shopping and eating out. About half of people with the virus who talk to state contact tracers say they had been to a business within two weeks of getting symptoms; of those, about 50% say they went to a restaurant, said Wardle.

In central Pennsylvania, Union, Montour, Snyder, and Bradford Counties all reported increases, though the raw numbers of cases in these less populous counties are comparatively small, between about 80 to 230 cases in the last month.

In Schuylkill County, where the positivity rate has shot up to 6.5%, County Commissioner George Halcovage Jr. said one of the county’s nursing homes had recently seen an increase in cases, and he also believed people may be less vigilant about COVID-19 precautions.

“I would talk to people, and they would say, ‘Maybe I’ve gotten a little bit relaxed.’ Let’s make sure if we’re talking about being relaxed, let’s just be responsible,” Halcovage said.

In Philadelphia, Farley said the spread of the virus appears to be tied to social gatherings, but officials are looking out for other causes.

“So far, we’re not seeing evidence that it’s linked to indoor dining,” he said.

Southeastern Pennsylvania, once the epicenter of the state’s outbreak, has remained fairly steady compared with elsewhere in the commonwealth. Montgomery County showed no increase in its rate of infection in a comparison between last week and the end of August. Delaware and Bucks Counties reported small increases in their daily rates over the last month.

Chester County, however, reported an average of 44 new cases a day over the last week, almost 60% more than its average during the last week of August.

“I think Southeastern Pennsylvania is the last bastion of safety here in Pennsylvania,” pediatrician David Rubin of the Children’s Hospital of Philadelphia’s PolicyLab said last week. “We need to brace ourselves, and we need to have ample testing capacity.”

In New Jersey, too, colleges have accounted for some significant case spikes. Rowan University, in Gloucester County, reported 100 new cases over the last 10 days among students, most of them living off-campus, state health officials said. In total, the university has recorded 494 cases among students and employees off and on its three campuses.

Monmouth University in Monmouth County closed its campus Thursday due to an increase in student cases as 450 students were in isolation or quarantine as of Tuesday, according to the university. Rutgers University has seen rising case numbers for the last three weeks, though its host county, Middlesex, has seen a larger increase.

And Ocean County added 3,153 new infections between Sept. 1 and Monday. The outbreak, the largest in the state, has been traced to religious gatherings, weddings, and funerals and is centered in Lakewood, which is home to a large Orthodox Jewish community.

Like in Pennsylvania, however, the rising cases are not isolated to university or religious communities.

“We are seeing a small but not insignificant increase across the state,” New Jersey Deputy Health Commissioner David J. Adinaro said. “We are still at risk — quite frankly, in danger — throughout the state.”

Cases have increased each time the state has loosened its coronavirus restrictions; the latest spike follows that pattern, Adinaro said.

“The amount of control the average New Jerseyan has or average American has to contribute to the cause cannot be underestimated,” Adinaro said. “Distance, mask, hand hygiene are as important now as they were in the height of the pandemic.”

Staff writers Ellie Silverman, Allison Steele, Laura McCrystal, Marie McCullough, and Julie Coleman contributed to this article.