It's a general issue with every vaccine against an illness that has a reservoir. The reservoir can be other species (birds and various other animals in case of influenza) or it can be human populations that for various reasons have not eliminated the spread of the illness yet (measles in countries that refuse or make it difficult to vaccinate, polio if not completely eradicated yet). In this reservoir the agent has opportunity to mutate, and it can never be excluded that at some point it makes inroads into populations again where it was eradicated before.

Global eradication of diseases has been achieved with smallpox and bovine rinderpest precisely because they had no natural reservoir and all vulnerable populations could be vaccinated. With Coronaviruses the situation is not so simple, but it should at least be possible to eradicate the currently relevant variant.