An intriguing theory may help explain why the flu and Covid-19 never gripped the nation simultaneously — the so-called
twindemic that many public health experts had feared.
The idea is that it wasn’t just masks, social distancing or other pandemic restrictions that caused flu and other respiratory viruses to fade while the coronavirus reigned, and to resurge as it receded.
Rather, 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 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,” said Dr. Ellen Foxman, an immunologist at the Yale School of Medicine. “I don’t think we’re going to see the flu and the coronavirus peak at the same time.”
…
Only 0.2 percent of samples tested positive for influenza from September to May, compared with about 30 percent in recent seasons, and hospitalizations for flu were the lowest on record since the agency began collecting this data in 2005.
Many experts attributed the flu-free season to masks, social distancing and restricted movement, especially of young children and older adults, both of whom are at the highest risk for severe flu. Flu numbers did tick upward a year later, in the 2021-2022 season, when many states had dispensed with restrictions, but the figures were still lower than the prepandemic average.
So
far this year, the nation has recorded about five million cases, two million medical visits, and fewer than 65,000 hospitalizations and 5,800 deaths related to the flu.
….
Recent
studies have shown that
co-infections of flu and the coronavirus
are rare, and those with an active influenza infection were nearly
60 percent less likely to test positive for the coronavirus, he noted.
“Now we see a rise in flu activity in Europe and North America, and it will be interesting to see if it leads to a decrease in SARS-COV-2 circulation in the next few weeks,” he said.
Advances in technology over the past decade have made it feasible to show the biological basis of this interference. Dr. Foxman’s team used a model of human airway tissue to show that rhinovirus infection
stimulates interferons that can then fend off the coronavirus.
“The protection is transient for a certain period of time while you have that interferon response triggered by rhinovirus,” said Pablo Murcia, a virologist at the MRC Center for Virus Research at the University of Glasgow, whose team
found similar results.
But Dr. Murcia also discovered a kink in the viral interference theory: A bout with the coronavirus did not seem to prevent infection with other viruses. That may have something to do with how adept the coronavirus is at evading the immune system’s initial defenses, he said.
“Compared to influenza, it tends to activate these antiviral interferons less,” Dr. de Silva said of the coronavirus. That finding suggests that in a given population, it may matter which virus appears first.
Dr. de Silva and his colleagues have gathered additional data from Gambia — which had no pandemic-related restrictions that might have affected the viral patterns they were observing — indicating that rhinovirus, influenza and the coronavirus all peaked at different times between April 2020 and June 2021.
That data has “made me a bit more convinced that interference could play a role,” he said.