A pandemic upside: The flu virus became less diverse, simplifying the task of making flu shots

In the eight years leading up to the Covid-19 pandemic, one of the subtypes of influenza A viruses started acting bizarrely. Flu viruses continuously evolve, to evade the immune defenses humans develop to fend them off. But after 2012, H3N2 started to behave differently.

It was almost as if there was a falling out within a family. The viruses formed into factions — clades, in virologists’ language — drifting further and further apart with each passing year and making the process of choosing the version of H3N2 to include in flu shots an increasingly challenging task.

The greater the genetic distance between the clades, the bigger the cost of making the wrong choice. Vaccine that protects reasonably well against one might perform poorly if the other turned out to be the dominant strain in a given winter. In fact, that’s precisely what happened in the 2017-18 season, when the flu shot failed to protect three-quarters of vaccinated people in the U.S. against the H3N2 strain in circulation.

But an unexpected upside of the Covid-19 pandemic may have solved this problem for us — or at least made flu’s diversity more manageable. With Covid suppression measures like mask wearing, school closures, and travel restrictions driving flu transmission rates to historically low levels around the world, it appears that one of the H3N2 clades may have disappeared — gone extinct. The same phenomenon may also have occurred with one of the two lineages of influenza B viruses, known as B/Yamagata.

https://www.statnews.com/2021/06/02/pandemic-upside-flu-virus-became-less-diverse-simplifying-task-of-making-flu-shots/