I had an MMR soon after it was introduced (1991 ish) at school. Prior to that i had already had the single vaccines that were offered back then (not sure what they were) I had previously had a mild case of rubella. When i was tested in pregnancy, i came back as immune to rubella and had assumed i was immune to measles and mumps too, just because i had the jab.
I was recently tested by occupational health, am training to be a SALT and it turns out although i am immune to rubella i am not to either to mumps or measles.
I am not to keen to have another MMR as i dont see why it will work when it didnt before. I am not sure if i will get another immunity test to see if this one works either.
The vaccination will cancel out any natural immunity you have and unfortunately they only last a fraction of the time.
I had rubella naturally as a child but was later vaccinated. By the time I came to have children at 30 I had no immunity to rubella.
As my cousin was vaccinated with Rubella at 16 then days later following many side effects ended up paralysed from the neck down, and blind (all within weeks), I didn't take the MMR on offer after my first birth.
It's hard to advise someone but one doctor told me there's more chance of complications following vaccine as an adult than as a child. This may have been to talk me into having my kids vaccinated so im not sure if it's true.
Just to add, as I understand it the health profession seems to have decided that the MMR now only lasts for 10 years. Frankly, I don't think measles or mumps are sufficiently dangerous to warrant having the MMR but that is only my opinion (my sister had measles a couple years ago, she was ill but no worse than the flu or a bad cold really). Furthermore, I'm pretty certain her son had it too despite having been given the MMR about 6 months before. And if you don't already have children or are planning any more, Rubella is the one you have to be careful to avoid while pregnant so if you're already immune to that, that is a good thing. If it were me I wouldn't bother, but obviously you've got to decide for yourself.