With sniffles as a near-constant companion this fall and winter, and previously obscure viruses becoming household names in Colorado, you’re not alone if you wondered whether this past year was normal.

Some pathogens — like respiratory syncytial virus, or RSV, and streptococcus bacteria, or strep — did damage that children’s hospitals called “unprecedented.” But the flu season was of about average severity, and a seeming cluster of unexplained hepatitis cases turned out to nothing out of the ordinary.

That doesn’t mean fall and winter weren’t miserable for some, especially after two years with minimal viral spread. A January poll from the Kaiser Family Foundation found about 38% of people reported someone in their home had been sick from flu, COVID-19 or RSV in the previous month.

In some ways, that’s a return to the way things were before the pandemic, said Dr. Sam Dominguez, a pediatric infectious disease specialist at Children’s Hospital Colorado.

An average toddler who attends daycare gets 10 to 12 respiratory viruses over the course of the six to eight months when they most commonly spread, but they had far fewer illnesses while people were masking and practicing social distancing earlier in the pandemic, he said.

“Now that those measures are gone, respiratory viruses are back,” he said.

Respiratory syncytial virus

The number of Colorado children who didn’t get common viruses in 2020 and 2021 contributed to the massive number of RSV cases in fall 2022. At the peak in November, 295 children and 31 adults were admitted to hospitals with RSV in the Denver area. (The Colorado Department of Public Health and Environment doesn’t collect statewide data on RSV.)

“It really was the worst RSV season in my memory,” Dominguez said.

Children under 1 were hit hardest, as is typical, but the average age of a hospitalized child was eight to nine months older as the virus found kids who weren’t previously infected, Dominguez said. For many respiratory diseases, including RSV, first infections tend to be the worst.

Researchers are still trying to sort out whether children who got their first infection later were more likely to become severely ill, or whether the country essentially compressed three respiratory seasons’ worth of hospitalizations into one, Dominguez said.

“That’s the million-dollar question,” he said.

It’s not clear yet if RSV will return to a more normal pattern this fall, though the sheer number of children infected over the past season suggests there won’t be a large pool that’s susceptible, Dominguez said.

The U.S. Food and Drug Administration approved an RSV vaccine for adults 60 and up on Wednesday, and is expected to approve a second vaccine for adults and a new antibody product for children, which would reduce severe cases, he said.

Unexplained hepatitis

Colorado sounded the alarm about unexplained hepatitis in children in May 2022, but new cases tapered off as the year went on. The state health department has identified 32 cases of liver inflammation in children without a clear cause since October 2021.

The rate of pediatric hepatitis cases wasn’t above what would be expected in normal times, state epidemiologist Dr. Rachel Herlihy said. Unexplained hepatitis is a rare phenomenon, and before late 2021, it didn’t get much attention.

It still isn’t entirely clear what caused the hepatitis cases, though the leading theory is that children who were infected with adeno-associated virus 2 and another virus at the same time may have been more likely to develop the condition.


The flu season has been moderate in Colorado, with most hospitalizations between early November and mid-January. Influenza A hit early and petered out relatively quickly, while influenza B has yet to cause a significant wave.

It’s still possible there could be a spring flu wave, as happened in 2022, but that’s looking less likely, Herlihy said. This year, the flu vaccine was a good match for the circulating strains and a relatively high percentage of people chose to get it, which limited the number of severe illnesses, she said.

“It certainly won’t go on record as a particularly severe season,” she said.

The Centers for Disease Control and Prevention have counted 145 flu deaths among children nationwide this season — a return to prepandemic levels. One child died during the 2020-2021 season, and 45 did during the 2021-2022 season.

Colorado has recorded one pediatric flu death so far this season — a school-aged child in the Denver area, according to the state health department.

Neither Colorado nor the CDC counts adult flu deaths as they happen.


While many of the disease waves have passed, Colorado kids are still experiencing about three to four times the usual number of cases of invasive group A streptococcus infections, which is “unprecedented,” Dominguez said.

There are fewer cases now than there were in March, but it’s too early to say the tide has turned, he said.

“I’m trying to be optimistic that we might be ending” that wave, he said.

The state health department recorded 28 cases of invasive strep A in children and two deaths in 2022, with most occurring in November and December — far higher than the usual pace of one or two cases per month. Adults also have had an above-average rate of invasive infections, Herlihy said.

Most people develop “strep throat” or mild skin infections when infected with strep A, but sometimes the bacteria gets into the bloodstream or the nervous system, or destroys tissue around a wound.

An unusually high number of kids have gotten strep throat, so it’s not clear if the increase in severe cases simply reflects the number of total infections, Dominguez said. The CDC is running genetic analysis to see if a more-severe strain is circulating, but so far it hasn’t found any sign of that, he said.

The risk of invasive strep A is higher if someone is infected with a respiratory virus at the same time. That doesn’t seem to be the only factor, though, because invasive strep A cases have remained high even as circulation of viruses has gone down, Herlihy said.

While there’s no vaccine against group A strep, keeping children up-to-date on their other shots gives the bacteria fewer opportunities to piggyback off another infection, Dominguez said. It’s also important to use hygiene practices like handwashing, he said.

Parents should also be vigilant for symptoms like prolonged fever, difficulty breathing, worsening respiratory symptoms, rapidly spreading rashes, or a child refusing to eat or move, Dominguez said.

“If you can recognize and treat group A strep sooner, the outcomes are better,” he said.

Acute flaccid myelitis

One bright spot over the last year was that an expected increase in paralysis cases in children never happened.

The condition, known as acute flaccid myelitis, followed a pattern of hitting children hard every other year from 2014 to 2018. There was no spike in 2020, though, and doctors were concerned that it would return last year.

Acute flaccid myelitis is thought to be triggered by an infection with a virus called enterovirus D68, but the number of paralysis cases remained low in 2022, despite the widespread circulation of that virus. At Children’s Hospital Colorado, there was a significant increase in patients who had EV-D68 in late summer, but only four of them developed acute flaccid myelitis, Dominguez said.

One theory is that the strain circulating in 2022 happened to be one that is far less likely to cause paralysis, Dominguez said.

“It would be great if we got lucky and the current strain continues,” he said.

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