If Colorado follows the pattern set in the Southern Hemisphere this spring, the flu season is likely to start early and be relatively severe.
Flu projections for the United States are based on what happens in Australia, where the seasons are flipped and respiratory viruses spread during what’s spring and summer in the Northern Hemisphere.
This year, the flu started spreading widely about a month earlier than it usually does in Australia, and the number of cases was higher than the average over the past five years, though the Australian national health department described its impact as “low to moderate.”
The flu typically starts circulating widely around Thanksgiving in the United States and peaks in December or January, said Dr. Michelle Barron, senior medical director for infection prevention and control at UCHealth. If the country follows the pattern set in Australia, that could mean the flu season kicks off in late October or early November, she said.
The Southern Hemisphere numbers suggest the flu season could be comparable to the 2019-2020 season, during which 3,546 people were hospitalized with influenza in Colorado, Barron said. That season was essentially cut short when flu activity dropped to almost nothing following Colorado’s COVID-19 stay-at-home order in early April 2020, and the two seasons since then have been milder.
“This year has the potential to be a bad flu year. A lot of the measures used during the COVID-19 pandemic such as masking and limiting gatherings that helped limit the spread of flu are no longer in place. We also have less immunity to flu since we haven’t had the same exposures we normally have from year to year,” Barron said.
Children younger than 5 and adults older than 65 are at the highest risk of severe illness from flu. Colorado doesn’t track adult flu deaths, which are lumped in with pneumonia from other causes in the statistics. Pediatric deaths typically fluctuate between three and seven per year.
Dr. Hector de Leon, a pediatrician at Kaiser Permanente’s Fort Collins clinic, said he’d seen a small increase in respiratory viruses as of late August, though not a large number of flu cases. Some people try to time their flu shots to have the most protection when the season peaks, but it’s better to get vaccinated early, he said.
“It’s really hard to predict when you’re going to get” the flu, he said.
Some clinics may not have flu shots in stock yet, but Walgreens and CVS pharmacies in the Denver area had appointments available as of Tuesday. People who don’t want to book multiple appointments can receive their new omicron COVID-19 boosters and seasonal flu shots at the same time without reducing their protection from either. It takes roughly two weeks after either shot to mount a full immune response.
The flu shot is less effective than the COVID-19 vaccines, generally reducing the odds of severe disease by 40% to 60%, but that’s still significant in a bad flu season, de Leon said. The flu shot’s composition changes from year to year, to try to match the strain that’s most likely to dominate.
“We don’t want to overwhelm our hospital systems, our clinics,” he said.
It’s not clear how COVID-19 and influenza will interact this fall and winter. The two viruses have appeared to seesaw, with one gaining steam as the other receded, de Leon said. But researchers aren’t sure if the virus’ characteristics were an important factor in the mild flu seasons, or if human behavior like wearing masks was the driver.
“A lot of us are worried we’re just due” for a bad season, he said.
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