Read More: Popular Science
The Flu feels like one big thing. You get your Flu Shot to protect against The Flu during Flu Season. But part of what makes influenza so dangerous is that it’s an astonishingly diverse virus. Though we’re only aware of it for a few months each year, influenza is constantly circling the world, and as it goes it accumulates new mutations in its genome. Sometimes these mutations don’t change anything substantive, but sometimes they give the virus a survival advantage. And when that happens, the mutation can quickly spread—the older viruses don’t survive as well, so the new, mutated ones take over. That’s what’s happening in the influenza B virus right now. We spend most of our time worrying about the influenza A strains, which include H1N1 (yes, the swine flu) and H3N2, the latter of which is a particularly nasty subtype that puts more people in the hospital than any other sort of flu. Those A strains get more attention in part because they’re more dangerous, and they’re more dangerous because they mutate faster and are more diverse. That means our annual vaccine selection tends to be wrong for the A strains more often. We only get to pick one strain per subtype—one H1N1 virus and one H3N2 virus—and with so many mutations circulating, it’s hard to pin down exactly which one will cause the most trouble. The B viruses are less diverse and mutate more slowly, which makes them generally less dangerous. We also get to pick two B strains every year, since the majority of people in the U.S. get a quadrivalent vaccine (meaning it has four parts). Most years there are relatively minor changes to the B virus genome, and therefore small changes to the viruses contained in the vaccine. In the quadrivalent shot, at least, we’ve had the same B strain for the last nine years combined with a rotating set of other, less common B strains.