Is that the reason? I’ve heard that older measles vaccine was “less good” formulae than the modern MMR, and that often the second, years-later shot was forgotten, hence the current campaign for adults to get the second (now MMR) vaccination shot (and look into their vaccination passports whether they are current).
Though we still have measle outbreaks around Waldorf Schools (many parents are against vaccinations for “milder” diseases…)
Though - if that is the direction to go, I wonder: will in the future be the vaccination not a weakened Virus to Trigger T cell in the recipient, but rather the (isolated) part of the T cell itself? (Could that be done without immune activity by the recepient against the T cell?)
It’s similar to how there’s one disease where horses are used to produce antibodies which are then given to human patients (after filtering etc.)
When people were frequently exposed to measles in the “wild” a less than perfect vaccine wasn’t a problem - the initial immunity, as I said, was boosted by constant re-contact. It was only after sufficient herd immunity built up that measles became rare (at least in the West) that it became a problem, requiring a better vaccine and re-vaccination of people who had only had the prior vaccine and little or no subsequent natural exposure.