As we enter a new flu season, many people are left wondering about what happens when the influenza virus and COVID intermingle.

In 2021, we still don’t know much about how—or how often—the flu virus and SARS-CoV-2 act in tandem within the same body. A very early study from China in January 2020 found zero cases of coinfection of these two pathogens among 99 COVID-19 patients, but a follow-up, conducted a month later at a COVID-19 hospital, concluded that about one in eight had both diseases at the same time.

Whatever the historic prevalence of coinfection, the twindemic never happened last winter. Perhaps due to mask wearing and social distancing, flu numbers in the U.S. were much, much lower than normal during the 2020–21 season.

But with pandemic restrictions relaxing, and fewer people getting flu shots, the same warnings have returned. “I actually think it’s more of a risk this year,” Griffin told me. Cases like the ones he saw almost two years ago could become far more common. In fact, new research suggests that getting coinfections—not just of COVID-19 and flu, but of many one-two punches of pathogens—may be far more common than we thought. Doctors’ understanding of what these coinfections mean for care and treatment remains preliminary, but they could well have important consequences.

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