As COVID-19 vaccination rates have jumped in Pennsylvania and New Jersey, a curious thing has happened. After a steep drop from a December peak, cases and hospitalizations have started rising again.

The virus has been especially rough on older people, who often dominated the coronavirus census at hospitals. They were among the first to get vaccines. Now, the proportion of younger patients has increased, several health officials said.

Nitin Puri, who is codirector of critical care at Cooper University Hospital, said the patients he’s seeing now are younger on average than they were before vaccination started, but most have chronic conditions that make COVID-19 more severe, such as obesity or uncontrolled diabetes. The vast majority, he said, would be eligible for vaccination because of those health problems.

“You know what the story is?” he asked. “We’re seeing people who are not getting vaccinated.”

Because they’re so sick, he hasn’t asked ICU patients if they tried to get an appointment.

The hospital has about 58 coronavirus cases now. The census doubled abruptly two weeks ago. Before vaccines were approved, 60% to 70% of Cooper’s COVID-19 patients were over 65, Puri said. Now about half are under 65, with many in their 30s and 40s. Puri said he is not seeing patients in their teens and 20s.

Public health officials have begged people to keep wearing masks and maintain social distance until more of us can be vaccinated, but they acknowledge that not everyone is staying in line, especially as the weather improves and spring break beckons. Some people may also be falling prey to more infectious forms or variants of the virus.

“The question is, after spring break, does everything fall apart?” Puri asked.

Epidemiologist Gemma Downham, director of infection prevention at AtlantiCare Regional Medical Center with campuses in Atlantic City and Pomona, has also seen a shift to younger COVID-19 patients over the last month. The COVID-19 census has crept up from a low of 40 patients in February to about 70.

”We’re seeing more younger patients and getting many fewer admissions from nursing homes, where residents are fully vaccinated,” Downham said. “Even people in their 20s and 30s are being admitted due to various exposures they’ve had in the community.” Those exposures include St. Patrick’s Day festivities in bars and other social gatherings where young adults shed their masks, she said.

The medical center has seen an 18% decrease in admissions of patients 70 years or older and increases in all younger age groups. The biggest increase was in the 40-to-59 group, which rose 10% from January to March.

At Tower Health, which includes Reading Hospital, Chestnut Hill Hospital, and several smaller hospitals, COVID-19 cases have risen to 125, from a pandemic low of 74 patients in mid-March. Cases peaked at 200 patients after Christmas. Back in December, as vaccination was just starting, about two-thirds of coronavirus patients were aged 65 or older. Now, about half are below age 65. People in their 30s, 40s, and 50s with chronic health conditions, such as obesity, diabetes, and chronic lung disease, account for a growing portion of Tower’s COVID-19 hospitalizations as older adults get vaccinated, said Debra Powell, chief of infectious diseases at Reading Hospital.

Like Puri, she is concerned about spring break and spring holidays. ”We need the public to be patient,” she said. “We’ve been in this pandemic for a year, and people want to get back to their normal activities, but we need to get more people vaccinated,” she said.

The trend toward younger hospital patients is not established everywhere. The average age of COVID-19 patients at Jefferson Health is still in the upper 60s, said John Zurlo, director of infectious disease. This suggests, he said, that “efforts to vaccinate the elderly need to be enhanced and improved upon.”

At Penn Medicine’s three Philadelphia hospitals, the average age of COVID-19 inpatients has been consistent – late 50s – in recent months and during both the spring 2020 and November 2020 peaks, a spokesman said. The average age is slightly higher for those needing intensive care.

Philadelphia Health Commissioner Thomas Farley said Tuesday that it’s too soon to say whether hospitalized patients are younger, because of lags in data. “There are still older people getting infected, and it’s occurring in all demographics and all racial groups,” he said.

In Pennsylvania, the number of hospitalized patients peaked in late December at just short of 6,100. It fell to about 1,500 on March 20 but was up to 1,644 on Monday. The number of patients on ventilators followed a similar pattern, with 741 on Dec. 28, 170 on March 19, and 185 on March 29.

While the number of hospitalized patients has declined in many Pennsylvania counties, it is up in the Philadelphia area. The state early-warning dashboard compares average daily hospitalizations over two, two-week periods: March 12-18 and March 19-25. It found that hospitalization increased by 58.6 patients in Philadelphia, 26.2 in Montgomery County, 6.3 in Chester County, 5.5 in Delaware County, and 2.2 in Bucks County.

During the most recent wave, New Jersey hospitalizations peaked at 3,802 on Dec. 23 and fell to around 1,900 in early March. They have risen over the last two weeks and were at 2,329 on March 29. There were 524 patients on ventilators on Christmas Eve. The number has been stable at around 240 for most of this month.

Neither the state data dashboard for Pennsylvania nor for New Jersey includes ages of hospitalized patients. Asked about the ages of hospitalized patients, the New Jersey Department of Health compared hospitalizations between Nov. 30 and Dec. 12, when 52% were 65 and over, to those during the last two weeks of February, when 47% were in that age group.

In New Jersey, where cases per capita are rising faster than any other state, health officials have cited the spread of highly transmissible strains of the virus as cause for increasing cases along with New Jersey’s proximity to New York City. ”The transmission is pretty rampant right now,” Health Commissioner Judith Persichilli said at a coronavirus briefing this week.

Gov. Phil Murphy and state health officials have repeatedly stressed that keeping hospitalizations down is a key factor in the state’s reopening plans, saying that the peak last spring threatened to collapse the state’s health-care system.

Nationally, vaccines are most clearly having an impact among the residents of nursing homes, a group that got top priority for vaccination. Cases fell by 96% and deaths by 91% between late December and early March, the American Health Care Association and National Center for Assisted Living (AHCA/NCAL) said Tuesday.

Downham could not immediately provide data on the growing proportion of younger patients, but she did have some COVID-19 testing data from the AtlantiCare Health system, which has facilities throughout Atlantic County. Nearly 30% of people in their 30s were testing positive. In contrast, the positivity rate was 15% for people in their 70s, and 13% for those in their 80s. Like other public health experts, Downham said that, until vaccination expands to younger groups, masking and other precautions are crucial. ”Our case levels are way too high now in our region and our nation to be relaxing standards,” she said.

More than 1.5 million New Jerseyans are now fully vaccinated, and more than 2.6 million more have received one of two shots. In Pennsylvania, 3.2 million people are partially vaccinated and 1.8 million are fully vaccinated.

Staff writers Laura McCrystal and Allison Steele contributed to this article.