If necessary, even before a pandemic, we'll start making fresh vaccine strains. We will be following it along, and hopefully be keeping up with it. If, when a pandemic strikes and it is H5, we'll pounce on that virus immediately.
Vaccines aren't wonderfully stable. You'll get 12 months out of it in the correct conditions, maybe 18 months, but it doesn't go on forever. So again, it's a bit of a gamble to stockpile a few million doses. If it's not used, it's just going to go off.
There is a huge question mark. If there is, if people had a tiny bit of immunity, the vaccine would boost it.