[Eric_WVGG]

IANAD, but another way of expressing that, by my understanding:

There are two major strains of the flu, and one or the other becomes the dominant strain every year; it's difficult/impossible to predict which. And due to the way vaccines are made (injecting the inert virus into unfertilized chicken eggs for replication), the annual vaccine has to be made off of one strain or the other.

This "universal booster," instead of targeting one strain or the other, would be able to immunize against most/all. That doesn't mean that it's a one-and-done vaccine like measles; it could still require annual renewal. But it would be affective against all strains, so you wouldn't find yourself with the flu despite getting the shot (i.e. what happens to me literally every year).