Rate of change in flu virus variations in regard to yearly vaccine

For several years I've purchased the flu vaccine.
This year I may have missed it and the news is that it's a bad year to do
that.
I've read that the predominant variant, currently endangering our health is
called "Fujian". I also read that Fujian is not covered by the current
vaccine.
It's been said that  the "Panama" strain, which is covered in the current
vaccine is sufficently closely related to the "Fujian" strain as to offer
some protection from "Fujian".

Please can anyone tell me at what rate the variations in the strains
covered by the vaccine change each year? More specifically, would it be
that the vaccine I received last year contained vaccine specifically
against "Panama" and that I will still have some level of protection from
that strain due to last year's vaccine?
-- 

Arty Flinders