Does it run out?
I ask as I had the MMR vaccine as a child, but when pregnant 2 years ago was checked and wasn’t immune to Rubella anymore, although was still for measles and mumps. Apparently it’s very common, and they generally only last 10-15 years ( I guess I had mine 30 years ago as a baby).
I was told to avoid people with Rubella whilst pregnant ( as if you would ever know who!), and they gave me a top up vaccination the day after Ds was born.
As polio is in baby vaccine schedule, I would assume is worse for those over 20 years who probably aren’t immune anymore