Years ago, I was involved in the large class action re. MMR vaccine. Had the action happened, it may have come to light what the (2000? ) families involved were told by the lawyers. As they collated the data of vaccine damaged kids, they realised that some of the damage was done in clusters.
My own son was one of a group of about 6 kids vaccinated from the same batch, who went on to become autistic. We never met and weren't even aware of the other 5 families. And no-one knew this until the lawyers started collecting vaccine batch #s.
What this implies is - there may have been storage protocol problems, OR certain batches of the drug were indeed, somehow affected. Although maybe only kids with a certain predisposition or something else going on, went on to develop a problem. IIRC, one of the batches at least was up in the 20s.
IIRC, the dodgy vaccine was from 2 different manufacturers. Government had paid for and stockpiled this, so they weren't about to advertised that some batches might be affected. Or some clinics were doing something wrong. Interestingly - on a totally unrelated matter - the GP who ran the practice where my sons were vaccinated, was later struck off.
Older kids vaccinated from the same batch seemed less likely to have any effect. So the etiology was something to do with the neurological development (possibly?) of the younger kids.
I am not into conspiracy theories but inthis case, many people aren't aware that isolated batches of vaccine had clusters of allegedly damaged children. One of the first things Blair did as PM was cut off Legal Aid for the families involved. I was and remain a lifelong Labour supporter.