Colorado's flu season peaked in mid-May this year, an "unheard of" shift in how the disease typically operates, health officials said this week.

The shift marked the latest disruption the COVID-19 pandemic has had on the spread of standard illnesses over the past two years.

Typically, flu season begins in October or November, as cold weather shepherds people indoors, and ends in March or April, as temperatures warm. But this year, the flu in Colorado operated almost in reverse: After an initial increase in December, flu hospitalizations cratered until mid-March, when they began to increase, state data shows.

Flu would peak several weeks later, in mid-May. 

"At the peak, there were 1,148 (cases) of flu in the state of Colorado (in May), which is unheard of," said Michelle Barron, UCHealth's senior medical director of infection prevention and control. "That's a December number, that's a January number. We've never seen that much flu, flu A specifically, in May." 

That peak, while unprecedented in its timing, was still smaller than December or January peaks from years past. But it still nudged Colorado closer to pre-pandemic years, when flu-related hospitalizations would regularly top 3,000 patients per season. According to state data, 1,267 Coloradans were hospitalized with influenza since Oct. 3. 

Last year, there were just 34 flu-related hospitalizations. State officials have previously said the 2020-21 flu season was historically low because of COVID-19 related measures: school closures, mask mandates, social distancing, increased emphasis on hygiene. 

Barron said the mid-May surge this year is also explained by COVID-19. Flu hospitalizations bottomed out in January and February, as the omicron subvariant of the novel coronavirus ripped through the state and mask mandates settled into place. 

But all of those mandates were removed beginning in February, as omicron rapidly receded. 

"A lot of that had to do with timing of mask mandates going away, a lot of crowd control went away when there weren’t limits of how many people could be in a location at the same time," Barron said. "I'm sure that was partially in play as to why it suddenly surged again."

While the warm weather may help generally, she continued, the end of the few remaining COVID-19 restrictions meant people were gathering together in ways they hadn't for two years. 

She also said that waning flu immunity also likely played a role. Flu vaccinations are typically given in the fall, before the season begins in earnest. But by the spring, she said, the moment of peak protection has passed.

"Flu just sort of burns itself out at some point because everybody gets exposed," Barron said. "Nobody got exposed, and vaccine wore off, and we weren’t hanging off en masse with people (before March), so." 

While this late, strange flu season was still more mild than any recent pre-pandemic year, Barron said she's still concerned.

"What we’ve been able to predict for decades, forever — is that now going to continue to be skewed because of all the weird things that have happened with COVID?" she asked. "And also the bigger question I'm worried about is, because it’s been so mild, in the fall, are we going to get hit hard by a bad flu strain? We haven’t had it for the last years. Some of that innate immunity is gone."

Heading into the 2020-21 and 2021-22 seasons, the state emphasized flu vaccinations in an effort to avoid a "twindemic" of COVID-19 and influenza. Fortunately, COVID-19's large surges have not been exacerbated by a wave of flu. But Barron said the potential for it happening next year "keeps me up at night," particularly because many people may be burned out on hearing about vaccinations, disease mitigation and behavioral changes.

Burned out or not, those strategies remain the best way to keep Coloradans healthy and hospitals flowing. Barron urged Coloradans to get vaccinated and to seek treatment quickly if you test positive for the flu. 

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