The pandemic coronavirus’ debut wrought universal havoc—not even seasonal flu viruses were spared. Amid travel restrictions, quarantines, closures, physical distancing, masking, enhanced hand washing, and disinfection, the 2020-2021 flu season was all but canceled. That meant not just an unprecedented global decrease in the number of people sick with the flu but also a dramatic collapse in the genetic diversity of circulating flu strains. Many subtypes of the virus all but vanished. But most notably, one entire lineage—one of only four flu groups targeted by seasonal influenza vaccines—went completely dark, seemingly extinct.
Researchers noted the absence last year as the flu was still struggling to recover from its pandemic knockout. But now, the flu has come roaring back and threatens to cause a particularly nasty season in the Northern Hemisphere. Still, the influenza B/Yamagata lineage remains missing, according to a study published this week in the journal Eurosurveillance. It has not been definitively detected since April 2020. And the question of whether it’s truly gone extinct lingers.
What B/Yamagata’s absence might mean for future flu seasons and flu shots also remains an open question. For a quick refresher: Four main types of seasonal flu have been circulating globally among humans in recent years. Two are influenza type A viruses: subtypes of H1N1 viruses and H3N2 viruses. The other two are influenza type B viruses: offshoots of the Victoria and Yamagata lineages. (For a more detailed explanation of influenza, check out our explainer here.) Current quadrivalent vaccines target season-specific versions of each of these four types of flu viruses.