Interestingly, two types of flu viruses haven't shown up on anyone's radar for a year, meaning there have been no reported cases of these viruses anywhere in the world, STAT reported.
Experts don't yet know if these types have gone extinct, but if so, officials could have an easier time picking the strains of flu viruses included in the seasonal flu shot, STAT reported.
To explain which flu viruses may have gone extinct, it helps to understand how flu viruses are classified. Two families of flu viruses cause seasonal flu: influenza A and influenza B. Influenza A viruses are divided into "subtypes" based on two proteins on their surface known as hemagglutinin (H) and neuraminidase (N), according to the Centers for Disease Control and Prevention (CDC).
Currently, H1N1 and H3N2 circulate in people, and each of these subtypes is further broken down into "clades."
Influenza B viruses, on the other hand, don't have subtypes or clades but are divided into two lineages known as B/Yamagata and B/Victoria. One clade of H3N2, known as 3c3.A, hasn't been detected since March 2020. The same is true of the lineage B/Yamagata, according to STAT.
"I think it has a decent chance that it's gone. But the world's a big place," Trevor Bedford, a computational biologist at the Fred Hutchinson Cancer Research Center in Seattle, told STAT, referring to the H3N2 clade.
Florian Krammer, a virologist at the Icahn School of Medicine Mount Sinai in New York, shared similar thoughts about the B/Yamagata lineage. "Just because nobody saw it doesn't mean it has disappeared completely, right? But it could" have disappeared, Krammer told STAT.
Less diversity among flu viruses would be a good thing.
Each year, scientists make the flu vaccine months before flu season actually starts by seeing what strains are circulating in the world and then predicting which flu strains are likely to be the most common during the upcoming season. Lower flu virus diversity means a smaller pool of circulating viruses to choose from and a greater chance that the strains in the shot will match those circulating.
H3N2 viruses are a particularly diverse group, and prior to the COVID-19 pandemic, their clades seemed to be getting more genetically diverse each year, according to STAT. So a drop in diversity for this subtype would be a "great thing," Richard Webby, director of the World Health Organization Collaborating Center for Studies on the Ecology of Influenza in Animals and Birds, based at St. Jude Children's Hospital in Memphis, told STAT.
"Currently, when we sit down to make recommendations for vaccine strains, it's always the 'headache' virus."
Webby cautioned that these virus types might still be out there even if they haven't been reported in official databases. But the dramatic drop in flu cases this year is likely to bring some changes for flu.
"Without doubt, this is definitely going to change something in terms of the diversity of flu viruses out there," Webby told STAT. "The extent to which it changes and how long it stays changed are the big question marks. But we have never seen this before."