Cases of RSV, flu, COVID, and norovirus are on the rise as winter ‘sick season’ arrives

It’s sick season in Philly: the cold-weather months when cases of respiratory illnesses and the “stomach bug” norovirus rise. Some public health experts have called the combination of viruses circulating “the quad-demic” — referring to a wave of illness from the simultaneous spike in COVID, flu, respiratory syncytial virus or RSV, and norovirus.

The “quad-demic” of seasonal illness follows another sick surge. In the late fall, children’s hospitals in Philadelphia dealt with rising cases of another respiratory illness: walking pneumonia, or mycoplasma pneumonia.

Now cases of walking pneumonia are waning, and it’s hard to gauge the extent of the misery locally due to limited reporting.

The CDC reports that wastewater testing shows “very high” levels of COVID and “moderate levels” of the flu in Pennsylvania. Wastewater testing is one of the more accurate ways to understand the spread of COVID in a region because most at-home COVID tests are not reported to public health authorities.

Emergency-room visits for COVID in the state are moderate, the CDC reported, and visits for flu and RSV are high.

And cases of norovirus, a gastrointestinal illness that causes vomiting and diarrhea and is spread through oral contact with infected fecal matter, are also on the rise.

Because neither Pennsylvania nor New Jersey reports all cases of norovirus to the CDC, it’s difficult to track the extent of cases. But several area hospitals voluntarily report positive norovirus tests to the agency, which reports that the percentage of positive tests in the Northeastern U.S. are the highest they’ve been since summer 2018.

The Philadelphia health department said that since early December, the city has seen a “steady increase” in people seeking care for vomiting or diarrhea at local hospitals, although that number also likely includes other gastrointestinal illnesses as well.

Donald Schaffner, a food microbiology professor at Rutgers University, said that though norovirus and respiratory illnesses spread through different means, they both spike in the winter for similar reasons. Some evidence shows viruses survive better in colder, drier weather. And more people are inside, interacting with each other in close quarters in the winter.

“It’s two separate issues, but it may be driven by the same underlying behaviors. The fundamental thing that’s driving all of this is person-to-person contact,” he said.

The isolation of COVID-19 pandemic brought about shifts in typical seasonal illnesses — norovirus cases, for example, decreased, and the virus’s typical seasonal peak came later in the year, he said. But now cases are adhering more closely to their pre-pandemic patterns, Schaffner said.

At the height of the rise of walking pneumonia cases last fall, Doug Thompson, the chief medical officer at St. Christopher’s Hospital for Children, saw more children hospitalized with the illness than at any other time in his career. Typically, the disease affects teenagers and adults, but last year, his patients were younger and sicker.

It’s a mild pneumonia that typically causes a severe cough but less fatigue than more serious forms of the disease.

Nationally, mycoplasma pneumonia cases were high in 2024, especially among children, the CDC reported. Because the CDC does not track the disease nationally, the agency said, it’s hard to know the full extent of cases.

“It typically peaks in the late spring, and can remain high in late summer and early fall, but we saw higher than usual rates in the early fall,” said Katie Lockwood, a Children’s Hospital of Philadelphia pediatrician.

These days, she said, many patients with a cough and fatigue visit her office assuming they have walking pneumonia, after hearing reports that cases are on the rise.

“But when I check them out, they have flu and RSV,” she said.

“Over the holiday break, things seem to have shifted, and now RSV and flu have seemed to rise, as well as stomach viruses like norovirus,” she said.

It’s unclear why mycoplasma pneumonia surged last year, but isolation during COVID was likely a factor as now is playing out with the norovirus.

Many children build some immunity to respiratory illnesses through everyday exposures. (Those with underlying medical conditions, newborns, and infants are particularly at risk during typical seasonal patterns of illness.)

“When we isolated during COVID, we saw dramatic decreases in our typical respiratory viruses like flu and RSV. We had a whole year where we didn’t see a single case of flu in the hospital — that’s an incredible track record,” St. Christopher’s Thompson said.

“We disrupted the normal exposure to these viruses, the natural immunity you develop. Then your body is less used to seeing [a virus] and it isn’t as prepared, so all of a sudden your immune system has to respond.”

There isn’t a vaccine for mycoplasma pneumonia, which is caused by a bacteria.

COVID isolations could have kept some children from getting vaccinated on time for other respiratory diseases, like whooping cough, which also saw a surge in cases in Pennsylvania last year. There’s also growing vaccine skepticism among some parents, which could also contribute to fewer kids getting vaccinated.

Vaccination rates for adult and child immunizations dropped in the months after stay-at-home orders in Philadelphia, the city health department reported, but returned to normal rates the next year. Still, in the 2023 and 2024 school year, the city’s kindergartners were slightly less likely to have received recommended vaccines than their 12th-grade counterparts.

Cases of walking pneumonia have leveled off, but other respiratory illnesses are on the rise.

“We’re starting to see the typical viruses climb up again,” CHOP’s Lockwood said.

“It’s not too late to get your flu shot, and folks should make sure they’re up to date with their COVID vaccination status as well,” she said.

Mycoplasma pneumonia in particular has a two-week window between exposure and symptoms, so people with significant coughs should be careful not to expose others.

Rutger’s Schaffner added that people should also wear masks to protect themselves from respiratory illnesses, and should wash their hands thoroughly and avoid others if they’re sick to prevent the spread of both respiratory disease and norovirus.

“The bottom line is to just stay away from people who are sick as best you can, and wash your hands,” Schaffner said. “With norovirus, it doesn’t take much to make you sick.”