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2021-2022 flu season: Why are cases still on the rise in the US?

The U.S. flu season typically ends in the spring, but cases are still on the rise in much of the country, despite summer being around the corner.
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Flu season typically strikes the U.S. from the late fall to the spring, peaking from December to February, but hospitals across the country are still seeing influenza spikes even as the first day of summer approaches. The CDC’s June 10 Weekly U.S. Influenza Surveillance Report notes that “seasonal influenza viruses continue to circulate, and activity is increasing in parts of the country.”

“(We) have experienced a remarkably prolonged influenza season. It began around Thanksgiving 2021, and cases continue through now,” said Dr. Andrew Pavia, a pediatric infectious disease epidemiologist at Intermountain Primary Children’s Hospital in Salt Lake City. 

Scott Hensley, Ph.D., a professor of microbiology at the University of Pennsylvania, told TODAY that “flu is circulating at unseasonably high levels in the United States and other places in the world,” and Dr. Pritish Tosh, an infectious decision physician and researcher at Mayo Clinic, added that it’s only within the past couple of weeks that case numbers have finally started to wane. “It’s quite uncommon to see cases this far into June," he told TODAY.

What’s behind the extended 2021-2022 flu season?

There’s been a “perfect storm” of conditions that have contributed to the unexpectedly long flu season, Tosh said.

“In any given flu season, we expect about 10% of the population to become infected and develop some degree of natural immunity for the next flu season,” he explained. “But because of a near non-existent flu season the past two years (due to COVID restrictions), that natural immunity never happened, so more people have been susceptible to getting sick.”

Another factor is vacillating approaches to masking and social distancing in response to COVID waves. For example, the omicron surge in January likely contributed to the late influenza season, according to Dr. Sandra Nelson, associate clinical director of the infectious diseases division at Massachusetts General Hospital, because influenza cases rose as expected in December but then dropped off suddenly when people began masking and distancing in response to omicron. “Influenza cases then began to rise again in March, which correlated with the end of mask mandates and relaxing of other distancing measures,” Nelson explained.

What's more, this season’s flu vaccine is not a perfect match with the strain of influenza that has dominated this season, which can lead to more cases and more severe illness. And many people who were vaccinated got their shots in August or September last year, so they're now more susceptible with the virus still spreading this close to the summer, Tosh said.

All flu seasons are unpredictable 

Despite the unique challenges of the 2021 to 2022 flu season, experts stress that every flu season is unique, and there’s no cause for alarm just because a flu season is different from previous ones. “We have a saying among doctors that if you’ve seen one influenza season, you’ve seen one influenza season,” Tosh said. “Flu seasons are predictably unpredictable.” 

Nelson echoed similar sentiments: “In temperate climates, the influenza season typically begins in the late fall, peaks in February and wanes in April, but there is substantial variability from year to year.” 

One upside of the current flu season is that hospitals haven't been inundated the way some feared they would, even though high case rates have lasted longer than usual. “Before this season began, we were preparing for an overwhelming number of new cases,” Tosh explained. “Fortunately, our worst fears never happened. Though we’re seeing a prolonged number of cases, there’s been a lower amplitude overall.”

It's not clear when exactly this flu season will end, but numbers are expected to decline in the coming weeks, Tosh and Nelson agreed.

How to avoid the flu 

Many of the preventive measures that've become familiar over the pandemic also protect against influenza — including vaccination, hand hygiene, wearing masks (especially in high-risk settings) and avoiding sick people, Dr. David Brett-Major, an epidemiologist at the University of Nebraska Medical Center’s College of Public Health, advised.

Hensley had a similar take: “Flu vaccines are clearly our best protection against influenza,” he counseled. “Even in years when there are antigenic mismatches, flu vaccines prevent serious disease and death.” 

The experts also stressed the importance of staying home when you're feeling under the weather. “One human behavior the pandemic seems to have permanently shifted is that it’s no longer seen as acceptable to ‘power through’ at school or work when you’re feeling sick,” explained Tosh. “People have finally learned they should stay home when they have symptoms, and that one behavior is really cutting down on the spread of disease.”