Influenza experts fear that the low levels of flu during the pandemic have led to a smaller pool of data for predicting which flu strains will circulate next winter.

The hospitalization rate for the 2020-21 flu season was just 0.7 per 100,000 people, the lowest it’s been since the Centers for Disease Control and Prevention began collecting such data in 2005. Measures such as social distancing, wearing masks and staying indoors likely helped hold pediatric flu deaths to just one last flu season, compared to 196 in the 2019-20 season.

Public health experts are relieved that the United States avoided a “twindemic” of a strong flu season amid a spiraling Covid-19 outbreak. But the low levels of flu have left experts with a much smaller pool of data used for predicting which flu strains will predominate next winter — raising the odds that the 2021-22 flu vaccine will be less effective than normal.

Once doors start opening again and people venture out without taking a year’s worth of Covid-19 precautions, it’s possible there could be new strains of the flu circulating that scientists didn’t anticipate, said Cody Meissner, an infectious disease specialist and pediatrician at Tufts Children’s Hospital who also serves on the FDA vaccine advisory panel. Without a strong enough vaccine, the pandemic-weary country could experience a severe flu season just as it emerges from fighting the coronavirus.

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