In March 2020, Yamagata’s trail went cold.
The pathogen, one of the four main groups of flu viruses targeted by seasonal vaccines, had spent the first part of the year flitting across the Northern Hemisphere, as it typically did. As the seasons turned, scientists were preparing, as they typically did, for the virus to make its annual trek across the equator and seed new outbreaks in the globe’s southern half.
That migration never came to pass. As the new coronavirus spread, pandemic-mitigation measures started to squash flu-transmission rates to record lows. The drop-off was so sharp that several flu lineages may have gone extinct, among them Yamagata, which hasn’t been definitively detected in more than three years despite virologists’ best efforts to root it out.
Yamagata’s disappearance could still be temporary. “Right now, we’re all just kind of holding our breath,” says Adam Lauring, a virologist at the University of Michigan Medical School. The virus might be biding its time in an isolated population, escaping the notice of tests. But the search has stretched on so fruitlessly that some experts are ready to declare it officially done. “It’s been missing for this long,” says Vijaykrishna Dhanasekaran, a virologist at Hong Kong University. “At this point, I would really think it’s gone.”
If Yamagata remains AWOL indefinitely, its absence would have at least one relatively straightforward consequence: Researchers might no longer need to account for the lineage in annual vaccines. But its vanishing act could have a more head-spinning implication. Flu viruses, which have been plaguing human populations for centuries, are some of the most well-known and well-studied threats to our health. They have prompted the creation of annual shots, potent antivirals, and internationally funded surveillance programs. And yet, scientists still have some basic questions about why they behave as they do—especially about…
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