Colorado’s flu season likely to be normal, but COVID is a “wild card”

Clues from the Southern Hemisphere suggest Colorado has a relatively normal flu season ahead, but experts are less certain about how COVID-19 and respiratory syncytial virus will behave this fall and winter.

The Centers for Disease Control and Prevention’s forecasting center projected the number of people hospitalized for COVID-19, flu and RSV combined would be similar to what it was last year, or slightly lower, director Dr. Mandy Cohen said during a press briefing on Aug. 23.

But the projection could be off if the forecasters misjudged how many people will get vaccinated, she said. The CDC assumed vaccine uptake will be similar to last year, when fewer than half of adults got a flu shot and less than one-quarter got an updated COVID-19 vaccine.

“The power is in our hands,” she said.

Typically, flu season in Colorado takes off in November and peaks in late December or January, which usually overlaps partially with a period of increased infections from RSV and COVID-19. Other viruses that cause common colds also circulate more widely in the fall and winter, but those three are responsible for most hospitalizations and deaths from respiratory infections.

The Southern Hemisphere’s flu season, which comes while the United States is experiencing summer, has followed the normal pattern, said Jenna Guthmiller, an assistant professor of immunology and microbiology at the University of Colorado School of Medicine. While the respiratory season can be different in the Northern Hemisphere, the data points to a relatively ordinary year for flu, she said.

One possible exception to the normal pattern is that influenza B may make a late-season appearance, as it has since the pandemic, Guthmiller said.

A and B are the two major flu subtypes that infect humans. Traditionally, flu B primarily affected children, but now it seems to be making a broader range of people sick after influenza A’s seasonal peak, she said.

“We don’t really understand why,” Guthmiller said.

Dr. Michelle Barron, senior medical director of infection prevention and control at UCHealth, agreed that the flu season is likely to be relatively normal, but cautioned that significant numbers of people still get sick even in an ordinary year. About 20,000 to 25,000 Americans, mostly older adults and young children, die during a typical flu season, she said.

“Typical is good, in that it’s probably not going to overrun our (health care) resources,” she said.

Experts were less sure what to expect from COVID-19. Hospitalizations started rising in June, and have leveled out since mid-August. As of Tuesday, 146 people were in Colorado hospitals for the virus, which was higher than at the same point last year, but lower than in early September 2022.

If a significant number of people had the virus recently, that could mean the population has some immunity heading into the fall, when it typically spreads more widely, Barron said. But since the state and federal government are no longer tracking COVID-19 as closely, no one really knows how many people had a recent infection, she said.

“COVID’s still my wild card,” she said.

Outpatient visits for COVID-19 went up over the summer, and Dr. Amy Duckro, an infectious disease specialist at Kaiser Permanente Colorado, said she wouldn’t be surprised if they continue increasing through the fall. RSV and flu typically come later, so she doesn’t know what to expect from them yet based on clinic visits. Colorado doesn’t start tracking flu and RSV until October.

“It’s really difficult to predict what any respiratory season will bring, so the best thing you can do is be prepared,” she said.

Guthmiller, with the CU School of Medicine, agreed that a continuation of the late-summer COVID-19 wave is more likely than a drop-off at this point. While predicting what the virus will do is never easy, it looks likely to peak around the end-of-year holidays, as it usually does, she said.

The CDC forecasted two possible scenarios: one where COVID-19 cases keep rising and peak in early fall, and another where the summer wave is peaking now, and a second wave follows in the winter. Right now, the second scenario looks more likely, but they likely would produce similar amounts of severe illness and death, said Beth Carlton, chair of environmental and occupational health at the Colorado School of Public Health.

“What we really care about now is peak hospital demand across the three viruses,” to ensure the health care system can care for everyone who needs a bed, she said. “And then it’s, how we prevent people from becoming acutely ill or dying, and that’s vaccination, vaccination, vaccination.”

None of the experts predict anywhere close to the level of hospitalizations and deaths in the first years of the pandemic, unless an evolutionary leap produced a radically different variant. Other possibilities that would cause trouble include extremely low vaccine uptake, or the avian influenza currently circulating in cows adapting to spread between people, Carlton said.

RSV also appears likely to have a more-normal year after two abnormal seasons. Infections with RSV spiked in 2022 when children who hadn’t had their first infection during the period of masking and social distancing got it, alongside younger kids who would have gotten it anyway during that time, said Dr. Kevin Messacar, an infectious disease specialist at Children’s Hospital Colorado. Infections also were higher than usual last year, but not as high as in 2022.

“In 2023, we started to see a slow return toward normal,” he said. “We’d expect this year is going to look like a more typical RSV year.”

Relatively new shots to prevent RSV could be a “game-changer” for infants, who are at the highest risk of severe disease, Messacar said. The shots debuted last year, but relatively few people got them because they arrived late in the season, he said.