A lot of this becomes clear when you consider how vaccinations are selected at the start.
Throughout the course of the year, estimates are made of any number of flu strains and subtypes, called clades, of which there are thousands. These are compared to the known “immunity estimates” of people.
Say, as a group, people have been in recent years exposed to 30 different subtypes of a strain of H1N1 flu. But in the past year, seven new clades have emerged. Some of them are so closely related to a former strain that they aren’t worth consideration, because as a group, people are immune to them.
But say three of them are far enough away, say second to fifth cousins, so that our immunity doesn’t reach that far. But which of these three are spreading, actually infecting people? Say two of them are.
So these two are candidates for the vaccine, but only in the sub-finals with other candidates from other strains.
Eventually, it boils down to an educated guess, with only three types of flu vaccines to be included in the vaccine.
Most of the time, they guess right. However, this year, they missed the clade that hit. And sometimes, before the flu reaches the US, it mutates again, well out of the range of the vaccine.
Right now, in the US, there might be 50 different strains and clades of flu, with only a few being symptomatic. We live in a very septic world.
Are you talking politically or epidemiologically?