Don’t worry. I don’t intend to treat the efficacy and safety of vaccines in principle as a thing that’s up for debate. Andrew Wakefield was wrong and quite frankly, I think he was a crook. However, a worry I’ve been noticing online lately has been a fear that getting an extra MMR vaccine could be harmful. Obviously the discussion is arising because of measles outbreaks, and the advice circulating on social media to get another shot if you were vaccinated before 1989.
So first to unpick that! The reason you may need another shot if you were vaccinated before 1989 has nothing to do with the MMR vaccine in its current formulation — except in that this version is more likely to provide protection! In the past, there have been other vaccines for measles, and the recommendation seems to stem from a reported recommendation from the CDC that people vaccinated with a “killed” vaccine. This vaccine was much less effective, so could leave people who received it still at-risk from measles.
Anyway, from searching around, I’ve found that a third dose of MMR does not seem to improve your immunity in the long-term. I’ve found no proof of the converse, that a third dose of MMR could harm you. If you aren’t sure whether you’ve had your booster, you should definitely check your own vaccination documentation — it can save the time and potential expense (depending on where you live) of getting the extra booster.
However, if you might have had the booster but you don’t think so, there should be no risk in having the booster now, from a scientific point of view. As far as I can see, there’s no mechanism by which this is likely to harm you: if you’ve already got immunity, your adaptive immune system will kick in to clear the perceived infection, and will quickly do so with no or few symptoms on your part. It should be exactly like coming into contact with measles through contacts. It’s clear that physicians don’t see a risk in exposing pre-screened people to a third MMR vaccine (the study above deliberately does so!), and I haven’t found anything contradicting my initial suspicion or that suggestive fact from the study, or in general warning against the practice.
Obviously, you should ask your clinician about this, and there may be reasons to test first for antibodies or to be more cautious, for example in cases of immunosuppression or insurance requirements. This doesn’t constitute medical advice!