The Covid-19 pandemic killed off one strain of the flu, and that will change the next vaccines

[Ms. Toad]

18. That's a possibility.

That's the primary reason that influenza vaccines aren't more effective. The quadrivalent vaccines include two type As (drawn from two specific lines) and two Type Bs (also drawn from specific lines), based on what is circulating in the Southern Hemisphere during their winter. They guess wrong more than half of the time.

And now you know more! That's the same kind of response I have to puzzles - if something catches me off guard, I go research and find out more!