In fact we have no idea how long this vaccine or any other [of the experimental covid-19 vaccines] will last for. They've got projections out to at least a year, but uncertainty becomes too great after that. It may last quite a long time, we don't really know.

for the same reasons we get annual flu vaccinations

No. We get annual flu vaccinations because flu viruses mutate significantly and quickly, so we're always getting vaccinated against new threats. Not against threats that we've forgot how to defend against.

[edited to clarify that the uncertainty is around the experimental vaccines; for old standards, we do have a very good idea]