Everyone now thinks that their research (hearing lurid stories from friends about their cousin's mother-in-law's window-cleaner's next-door neighbour, and reading how someone in the States dropped dead of a heart attack two days after getting a jab) trumps the research of lots of trained researchers (that is a clue) who study the way vaccinations work and the tiniest workings of the human body, collect thousands of accounts and statistics, form a possible hypothesis from what they have learnt, set up experiments to test that hypothesis, and collect thousands more bits of data from those experiments, and then think very hard about what they have found out, and draw a conclusion. They usually still call that conclusion a "theory" because with proper science more is being discovered about everything whenever someone does all those careful measurements again and collects masses more data.
If you haven't done all the work, you have no right to reach the conclusion.