Will repeated vaccination, if this proves necessary, begin to introduce long-term side effects that are serious?

Unfortunately, this is the case and will be borne out down the line. But again, this is something that will depend on how things go with government assessment of the data in seeing the extent of coverage and the level of protection achieved, and so on, and what their recommendations might be for widespread repeated use. If the artificially ramped up fear begins to subside, their may be less of a push for subsequent rounds if caseloads are on the wane. So as with most things, the future will depend on what the extraterrestrials do or not do with regard to keeping the pandemic going.