The studies, one published in Nature Reviews Microbiology in September and one under review, show that the influenza B/Yamagata virus has not been detected since April 2020 anywhere in the world.
Because B/Yamagata only circulated among humans, Koutsakos says it hasn’t evolved much over the past decade, and humans have developed high levels of immunity against it, making the lineage particularly vulnerable.
“The vaccine component for the [B/Yamagata] lineage hasn’t changed in about seven or eight years,” he says. “It’s possible that a large part of the population has built up immunity to that lineage, driving it to extinction.”
Researchers have long thought that high vaccination coverage would eventually eradicate B/Yamagata, and the COVID-19 pandemic might have accelerated the process.