Unlike a lot of pathogenic diseases, measles requires an extraordinarily high amount of herd immunity (upwards of 95%), so this isn't a particularly telling data point on its own.

RFK Jr has been pushing anti-vaccine bullshit for decades. He was already taking defensive stances on his activism (Oh, I'm not anti-vaccine, I'm just providing nonsense arguments and clout for anti-vaccine stances) back in 2015.

Kennedy was already pushing bullshit during the UK Measles vaccine fraud/scare, back in 2005. He was doing this along with a few other celebrities.