I've tried to find the answer to this and google is letting me down so can someone who understands the science behind this help:
Point 1 (mentioned in arguments against vaccinating babies for various things like chickenpox and rubella): getting the illness gives lifelong immunity but vaccination doesn't.
Point 2: the NHS say that one of the reasons that the varicella vaccine isn't given to children is because having the chickenpox illness circulating wild in the population "tops up" the immunity of adults who have had it therefore reduces the likelihood that they will get shingles.
Those two things seem contradictory. If you get lifelong immunity from having an illness why would that immunity need topping up? I know shingles is slightly different to chickenpox but my understanding is that the antibodies are the same?
And an immune reaction is an immune reaction, surely? The body doesn't know it's a vaccine not the real deal. So how can getting the disease ensure lifelong immunity if the vaccine doesn't?