Jim Fixx was 52 . . . HUGE difference.
Yes, Fixx was older when he died. But young runners and athletes have always had SOME risk of dying. Not a lot, but a race is very stressful and someone with a bad valve or other issue - unknown to them - can overstress their heart.
If there is proof the kid took the vaccine, then fine. Otherwise, it is just a tragedy that could be rooted in many different issues.