“Flu and other respiratory viruses and SARS-CoV-2 just don’t get along very well together,” says virologist Richard Webby, an influenza researcher at St. Jude Children’s Research Hospital. “It’s unlikely that they will circulate widely at the same time.” “One virus tends to bully the others,” adds epidemiologist Ben Cowling at the University of Hong Kong School of Public Health. During the surge of the highly transmissible Omicron variant of SARS-CoV-2 in Hong Kong in March, Cowling found that other respiratory viruses “disappeared … and they came back again in April.” When a respiratory virus sweeps through a community, interferons can broadly raise the body’s defenses and temporarily erect a populationwide immune barrier against subsequent viruses that target the respiratory system. “Basically, every virus triggers the interferon response to some extent, and every virus is susceptible to it,” says immunologist Ellen Foxman at Yale University, who has been exploring interference between SARS-CoV-2 and other viruses in a laboratory model of the human airway. Rhinoviruses, which cause common colds, can trip up influenza A (the most prevalent flu virus). RSV can bump rhinoviruses and human metapneumoviruses. Influenza A can thwart its distant cousin influenza B. “There are a lot of major health implications from viral interference,” says Guy Boivin, a virologist at Laval University who co-authored a review (PDF) on viral interference earlier this year.
Now, viral interference researchers are closely watching the newest respiratory virus to circle the globe. “What interactions could SARS-CoV-2 have with other viruses?” Murcia asks. “To this day, there are no robust epidemiological data.” For one thing, the widespread social distancing and mask wearing in many countries meant there was little chance to see interference in action. “There was almost no circulation of other respiratory viruses during the first 3 years of the pandemic,” Boivin says. Also, SARS-CoV-2 has many defenses against interferons, including preventing their production, which might affect its interactions with other viruses. Still, Foxman has published evidence that, in her organoid model, rhinovirus can interfere with SARS-CoV-2. And Boivin’s team has reported (PDF) that influenza A and SARS-CoV-2 each can block the other in cell studies.