It could be a bad flu season this year -- and for a couple of years to come -- in places in the United States where COVID-19 restrictions like social distancing and masking have been lifted, researchers warn.
These sorts of measures caused flu cases to decline by more than 60% within the first 10 weeks after COVID-19 lockdowns were implemented in 2020, Columbia University researchers found.
That's because face masks, hand washing and maintaining your distance work as well at preventing influenza infections as they do to stop the spread of COVID-19, said senior researcher Sen Pei. He is an assistant professor of environmental health sciences at Columbia's Mailman School of Public Health, in New York City.
"We know COVID-19 and influenza share similar transmission routes, so measures to stop the transmission of SARS-CoV-2 will likely reduce the transmission of influenza," Pei said.
Unfortunately, many places across the United States have lifted their COVID-19 measures heading into this flu season, Pei said.
That means the flu likely will be as easily transmitted as in earlier years, but with a difference -- people now have less natural immunity against influenza because the United States essentially didn't have a flu season last year, Pei said.
"For influenza, the virus is mutating all the time," Pei said. "Every two to three or five years, people who were infected by influenza are likely to be susceptible to the virus again. Their immunity will wane over time."
For this study, Pei and his colleagues used a computer model to estimate the impact that travel restrictions, face masks, social distancing and school closures likely had on the spread of influenza in early 2020.
The new study captures a "widespread concern" among infectious disease experts heading into this year's flu season, said Dr. William Schaffner, medical director of the Bethesda, Md.-based National Foundation for Infectious Diseases.
The situation might be even more dire than depicted by this research, Schaffner said.
There were around 2,000 cases of influenza reported to the U.S. Centers for Disease Control and Prevention during the 2020-2021 flu season. The season before, the agency received reports of an estimated 35 million cases of flu.
And in the 2020-2021 season only one child died from the flu, compared to between 144 and 199 in the three previous seasons, the CDC said.
"The authors may even have underestimated the importance of all that social distancing, mask wearing, children staying home from school, because last year we had over much of the country virtually no influenza," Schaffner said. "The reduction that all those interventions had was even more profound perhaps than this model shows."
Schaffner added that there's some debate between infectious disease experts as to whether people will be more susceptible to flu this year following last year's practically non-existent season.
"It is a real point of discussion. It's so unprecedented that even the full-time fluologists are not sure about that," Schaffner said. "But it is certainly a point of discussion, and it's a reasonable concern. Could immunity have waned sufficiently on a population basis such that when influenza resurfaces and resurges, that either it will spread more readily or if you get infected you might have a greater likelihood of becoming seriously ill?"
Getting your annual flu shot is the obvious way to ward off that risk, Pei and Schaffner said.
But a National Foundation for Infectious Diseases survey last week showed that nearly half of U.S. adults are not likely to get a flu shot this season.
Large potential outbreak?
"In the next few years, flu vaccination will be very important to stop a large potential outbreak," Pei predicts. "If people stop wearing masks and stop keeping social distancing, I think it's going to be very likely there will be a big outbreak of influenza. There's more opportunity for the virus to transmit, and there are more people who can catch the disease."
But Schaffner also thinks the COVID-19 pandemic might have fundamentally changed the way that some Americans approach the flu season.
"Since now we're more or less accustomed to social distancing and mask wearing, once there's an announcement that there's influenza in your community I think many people -- particularly older people and people with underlying illnesses and (those who) are immunocompromised -- will rather quickly revert to those social distancing, mask-wearing behaviors, because they've become convinced that they are effective and will spontaneously use them," Schaffner said.
Local and state public health officials also might be more likely to call for mask wearing if they see a spike of the flu in their region, Schaffner added.
"Before COVID, you never heard public health leaders when flu picked up say wear your masks or do social distancing, rent a movie instead of going to a movie, those sorts of things," Schaffner said. "I think you'll hear more of that now."
The study was published Sept. 22 in The Journal of Infectious Diseases.
The U.S. Centers for Disease Control and Prevention has more about the 2020-21 flu season.
SOURCES: Sen Pei, PhD, assistant professor, environmental health sciences, Columbia University Mailman School of Public Health, New York City; William Schaffner, MD, medical director, National Foundation for Infectious Diseases, Bethesda, Md.; The Journal of Infectious Diseases, Sept. 22, 2021