It seems like the flu virus is the example you want right? Every year the vaccines delivered is a cocktail based on the predicted most common variants, because no one vaccine can protect against them all.
I considered that, but as I understand it (this could wrong) the variants that are more prevalent each year are pre-existing and we just don't know which ones will be 'popular' ahead of time.
Another way of looking at this is that they're not new mutations that we scramble to develop and test vaccines for, we just roll out the same tried and tested vaccines, but we have to cobble together a different blend some years.
If the above is correct, then I guess the Flu example to answer my question would be: How often is a new variant of Flu (HxNy) discovered that necessitates a new vaccine being developed?
