Why haven’t waves of COVID-19 and influenza infections spread over countries simultaneously? A piece by The New York Times recently explored potential answers to this intriguing question. They reported that the “viral interference” of significant waves of COVID-19 infection may have played a role in creating the subdued flu season.
Did COVID-19 really squash influenza?
Previously, experts promulgated the view that mask-wearing, social distancing or other pandemic restrictions caused the flu and other respiratory viruses to die down, while COVID-19 dominated.
Scientists are now exploring a new theory that suggests exposure to one respiratory virus may put the body’s immune defenses on high alert, barring other intruders from gaining entry into the airways. This biological phenomenon, called viral interference, may [place a] cap the amount of respiratory virus circulating in a region at any given time.’
“My gut feeling, and my feeling based on our recent research, is that viral interference is real,” immunologist Ellen Foxman from the Yale School of Medicine told The New York Times. “I don’t think we’re going to see the flu and the coronavirus peak at the same time.”
Foxman also stated that on an individual level, part of the population may still end up infected with two or even three viruses at the same time. But at a population level, according to this theory, one virus may tend to edge out the others.
She also cautioned, “The health care system can become overburdened well before the upper limit of circulation is reached, as the Omicron wave has shown.”
Erring on the side of caution
According to the Sydney Morning Herald, at a local level, infectious disease experts in Australia are saying it’s too early and too simplistic to say that the more than four million COVID-19 infections in Australia in 2022 will help the nation escape a bad flu season this winter.
Australia has chalked up 400 cases of influenza in March this year, a figure that’s well below pre-pandemic levels. Yet, it’s the highest monthly total since international borders shut two years ago.
Tony Cunningham, an infectious diseases physician and virologist at Sydney’s Westmead Institute for Medical Research, told the Sydney Morning Herald there was some logic to the argument that previous COVID-19 infection may prevent someone from having the flu but he “wouldn’t bet on it”.
“There are a number of different arms of the immune system,” he said, stressing that while prior infection with a virus may assist a person’s “innate” immunity against subsequent different viral infections, through those aforementioned interferons, the more important and sophisticated “adaptive” immune system develops specific responses to individual viruses.
Can immunity against COVID-19 work against the flu?
Katie Flanagan, president-elect of the Australian Society of Infectious Diseases also told the Sydney Morning Herald that it was “very well characterised” that early innate immunity from an infection or live virus vaccine may assist the body to fight off other viruses but this could not be extrapolated to recovered cases.
“If you’ve cured and cleared your [COVID-19] infection, it is not going to impact your subsequent influenza infection,” she said.
The idea that infection with one virus may build immunity against another is currently being investigated at Melbourne’s Murdoch Children’s Research Institute.
Obviously, this research area is very live, and we look forward to better understanding the links between these viruses and immunity in the future.
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