A rapidly spreading virus is filling beds at Philadelphia’s hospitals — and for the first time in two years, the chief culprit is not COVID-19.

Experts hope this year’s “tridemic” of viral illnesses doesn’t represent the new normal. But they do see the numbers reflecting a new phase of the pandemic.

Current trends in viral illness are increasingly a concern for hospitals. While influenza is not as deadly as COVID-19 has been, Thomas Jefferson University’s doctors are seeing patients so sick with flu that they require intensive care.

“It’s pretty bad,” said John Zurlo, Jefferson’s director of infectious diseases.

Children’s hospitals are the hardest hit. Their emergency rooms are packed from a surge of RSV, or respiratory syncytial virus, which is especially dangerous during early childhood, in addition to flu and COVID patients. Children’s Hospital of Philadelphia and St. Christopher’s Hospital for Children have been at over 90% capacity since mid-November.

And COVID is still circulating. Yet with people no longer widely wearing masks and social distancing, respiratory viruses such as the flu and RSV are making a comeback in a population that is now more susceptible to them.

Although emergency rooms are full and hospitals are busy, physicians on the front lines are cautiously optimistic that this wave will end soon and future seasons won’t be as severe.

More cases, but not more severe ones

Flu cases have overtaken COVID-19 in Philadelphia’s ICUs in part because the overall number of severe COVID cases is now relatively low.

“There is a lot of infection — not a huge amount of hospitalization, not huge numbers of deaths,” Cheryl Bettigole, Philadelphia’s health commissioner, said of the current COVID situation.

After months of relative stability in the number of COVID cases, public health officials are bracing themselves for the full impact of holiday travel and family gatherings. Cases have risen 33% in Philadelphia in the last two weeks, although recorded numbers are likely an undercount as many people test at home or not at all.

“I’m hopeful that we won’t see the kind of huge wave we had last year,” Bettigole said.

ICUs are filling up, but not with COVID patients

During the peak of last January’s COVID wave tied to the omicron variant, about 30% of adult ICU beds in Philadelphia were used by COVID patients. That figure was down to about 2% in early December. Flu patients represent an additional 3%.

Together with RSV, these “tridemic” viruses account for a small number of the adults hospitalized in ICUs.

At Main Line Health in Philadelphia’s suburbs, for example, heart disease, sepsis, and other issues that are unrelated to respiratory viruses are now chiefly responsible for filling up ICU beds, said Ryan Reber, the director of critical care at the hospital system.

When people do get very ill from respiratory viruses, he said, they tend to be older and have underlying health issues such as advanced lung disease or weakened immune systems.

Most of the patients being hospitalized with flu are also unvaccinated.

“Those patients are at the greatest risk of progressing to severe illness requiring ICU level care,” Reber said.

RSV is adding to the strain

What makes this season unique isn’t the emergence of a new virus but that many are getting sick after not being exposed to flu and RSV in recent years. It’s a concept experts call “immunity debt.”

This has left the population “more susceptible to a bigger wave of infections,” said Ashish Jha, the White House COVID-19 response coordinator.

Prior to the pandemic, nearly all kids caught RSV before their second birthday. But after two winters of masking and social distancing to prevent COVID, the cold-like virus has been infecting a population of children who did not build up antibodies in their first years.

A surge in RSV cases has filled children’s hospitals for weeks, leading to medication shortages and frustrations for parents.

And for the first time, Robert McNamara, chair of emergency medicine at Temple University, is also seeing adults come to the hospital with RSV, adding to the crowding of hospitals already filled with patients with flu-like symptoms — and increasing wait times.

“When you get this surge of patients,” he said, “it always puts some stress on the system.”

Experts are cautiously optimistic

This year’s surge of flu and RSV infections provides the community with antibodies that could make for milder future seasons, potentially erasing the immunity debt that contributed to a harsh winter.

“I don’t think we’re going to necessarily see [a ‘tridemic’ surge] year after year,” said Zurlo, the infectious disease doctor from Jefferson.

A major unknown is the future of COVID and whether a new variant will emerge that causes wider spread or more severe outcomes.

Health officials recommend masking in crowded indoor spaces and on public transportation and testing for COVID before holiday gatherings.

Reber, from Main Line Health, also noted that people can guard against illness by getting vaccinated. The current flu shot seems to be effective at countering the virus circulating this year, according to data from the Centers for Disease Control and Prevention.

“The fight against a ‘tridemic’ begins outside of the hospital,” he said.