Patrick J. Brennan, chief medical officer of Penn Medicine, the region’s biggest hospital system, is sounding an alarm as the surge in coronavirus cases begins to pound medical centers.

Both Pennsylvania and New Jersey have been charting record increases in confirmed cases since October, but only in recent weeks have hospitals begun to feel the impact.

To be sure, that impact is far less than in the spring. By all accounts, hospitals are managing well, without the need for the ancillary facilities and overflow tents erected in March.

The Penn system’s six hospitals, for example, had about 400 COVID-19 patients at the peak — 10 times more than in October. But Brennan sees the case load creeping up again. About 150 COVID-19 patients are now in the six hospitals, which have a combined total of about 3,100 beds.

Another troubling trend: Of those COVID-19 inpatients, 25% needed intensive care, up from 10% just a week ago. That’s because the virus is once again spreading into the most vulnerable age group; the average age of COVID-19 patients admitted to Penn hospitals is now around 60.

In an opinion piece for The Inquirer, Brennan called on civic leaders and the public to be as disciplined and unwavering about precautions as hospitals have been.

“Since the pandemic hit," he wrote, "we have learned much about the disease and the best ways to treat it, so deaths are less frequent. But the lower mortality rate has tricked many members of the public into believing that the serious hazards of the pandemic may be past. That is fool’s gold.

“We are at a crucial inflection point where civic leaders and individual members of the public need to take immediate action to reverse this course.”

His candor seems to be unusual. Most health system leaders are striving to stress the positive — they are nowhere near being overwhelmed — and downplay the renewed wave, lest non-COVID-19 patients be afraid to seek care.

Earlier this week, for example, reporters tried unsuccessfully to verify reports that hospitals in Delaware County were forced to temporarily divert emergency patients, partly because of COVID-19. On Thursday, County Council President Brian Zidek confirmed it.

Spokespeople for big hospital systems — Jefferson Health, Main Line Health, and Cooper University Health Care — offered reassuring generalizations about preparedness when asked specific questions about surges or capacity.

“Cooper University Health Care, as the Level 1 Trauma Center in the region, is coordinating the state’s response to COVID-19 for the South Jersey,” said Cooper’s emailed statement.

Why aren’t hospital leaders joining Brennan in expressing concern and being transparent about their numbers?

Brennan pointed to several factors: Overall hospital bed and ICU capacity in the region remains ample, as various pandemic tracking organizations and state officials have documented. COVID-19 admissions are not increasing as rapidly as in the spring. Hospitals have largely mastered heightened infection control procedures, as well as the best practices for COVID-19 care.

And perhaps most of all, hospitals are still suffering from the economic fallout of suspending elective and non-COVID-19 care in the spring. They do not want, or foresee, a repeat.

Hospital workers “are not perfect, but people really do adhere to the precautions,” Brennan said. “Last time, the scale back was too drastic. There was too much care that couldn’t be provided. We had too many empty beds. Now, all of us need to deliver that [regular] care while we also care for COVID patients.”

In New Jersey, state health officials disclose when a hospital is temporarily on diversion, meaning the facility has informed local ambulance and emergency medical services that certain beds are full so patients should be taken elsewhere. This week, the list included Virtua Willingboro Hospital in Burlington County, Jefferson Washington Township Hospital in Gloucester County, AtlantiCare Regional Medical Center’s Mainland Campus in Galloway, and AtlantiCare Regional Medical Center in Egg Harbor.

But hospitals have no control over the activities that public health officials and scientists say are driving the surge: casual dinners, holiday parties, sleepovers, carpools, small weddings. At such seemingly safe get-togethers, people often let their guard down and expand their circle of exposure.

In Brennan’s opinion, it may be time to reinstitute many of the unpleasant restrictions of the spring.

“As we await the results of further vaccine trials and the delivery of the products, our civic leadership must not flinch from the responsibility to take charge again,” he wrote in his op-ed. “Leaders must clearly articulate the facts about the necessity of additional measures to protect our communities... This will require more than admonitions and recommendations. It will require mandates for masking and the curtailment of non-essential services, social activities, and indoor gatherings.”

Staff writers Tom Avril and Rob Tornoe contributed to this article.