[somewhereoutth]

Region of the virus - e.g. capsid, spike etc. Even within a variant such as Delta there are sub-lineages with slight differences. Most diffences make no difference to the transmissibility etc of the virus, so can arise (and disappear) fairly randomly. If your immune system preferentially produces antibodies to proteins on the virus encoded by such a difference, then the next COVID virus it encounters may not have these proteins, and so your antibodies won't bind.