Why do some vaccinations need to be repeated yearly?

[WeaverJackie]

Something I was pondering earlier. Why do some vaccinations last a lifetime but others fade after a year or so? Is it because of mutations?

Yes for example flu they vaccinate against different strains every year. The way they work it out for us is seeing which strains appear in the southern hemisphere , and then vaccinating against those ones come our Autumn/ winter rollout.
Would be no good if they vaccinated the same every year as the strain frequently changes so they have to keep up.