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Children have been presenting in large numbers and with more severe viral illnesses than typically seen, physicians at Children's Hospital of Philadelphia (CHOP) said.

The CHOP healthcare system, which includes two hospitals and more than 600 beds, is still grappling with a high volume of pediatric patients with viral infections, including respiratory syncytial virus (RSV), even after this year's "sick season" began months earlier than expected, said chief medical officer Ron Keren, MD, MPH.

"It's really important for everyone to know that volumes are extremely high right now in primary care pediatricians' offices, in urgent care centers, in our emergency departments, as well as in our inpatient units," Keren said during a press briefing on Wednesday. "It's causing a lot of strain on the system, and it's a phenomenon that's happening across the country."

Keren said that CHOP has been operating at near-full capacity every day for the last several weeks, and the demand for care is primarily coming from children, especially infants who have developed bronchiolitis caused by RSV. He attributed the surge in respiratory viruses over the past 2 months to a lack of immunity in this patient population.

"I think it gets to this idea that some folks are calling 'an immunity debt,'" Keren said. "We think that that may be because during the pandemic, there were a few cohorts of infants born who, due to social distancing and masking, probably didn't get exposed to these respiratory viruses, including RSV, and so they were not able to build up an immune defense to RSV and other respiratory viruses, leaving them vulnerable now."

Although some of the cases at CHOP have been severe, with some infants being admitted to the ICU to receive a higher level of respiratory support for breathing difficulties, Keren noted that most of these illnesses have been short-lived and that typically kids are getting better within a day or two of receiving respiratory support.

Katie Lockwood, MD, MEd, an attending physician at CHOP, said that the key to addressing this surge and the immunity debt is to get children back on track with vaccinations and to teach them healthy habits, like hand washing and masking.

"During the pandemic, there were many children who did not seek routine preventative care, especially early in 2020," Lockwood said. "So some families missed those appointments and have been slower to catch back up on those, [and] children have missed some of their routine childhood immunizations."

She noted that when a large population falls behind on the standard vaccination schedule, community vaccination rates can decline, causing a loss of herd immunity that keeps communities safe from some of these vaccine-preventable diseases.

This concern was exacerbated, Keren noted, by several years of uncertainty caused by the pandemic.

"This year, sick season is a little bit different from previous years," he said. "The timing of the appearance of common respiratory viruses has been anything but typical over the last 2 and a half years of the pandemic. What we're seeing this year is that RSV arrived early. It started in April, in the spring, which is very unusual."

Unlike this current surge, Keren explained that last fall and winter they saw very little RSV, which might have been related to the first wave of Omicron infections in December and January.

"It's a product of the infection prevention practices that were put in place -- social distancing, masking -- and so it's been a little bit topsy-turvy for us here at the hospital in terms of knowing what to expect," he said.

One positive point, Keren noted, is that COVID-19 infection rates have been mostly flat or declining over the first few months of this sick season. And while they are starting to see influenza A in the community, the numbers are still small.

"What that means [is] we all have time to get our kids vaccinated or boosted against both these viruses before their numbers increase and it becomes actually harder to avoid an infection," he added.

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