Now your creating a straw man argument. Of course probability is not certainty and I never claimed it was. But, understanding probability is crucial to determining risk vs reward. And if one has no clue about probability, best to just follow the advice of the experts, which are all saying "Get vaccinated", well, unless we want to start going down the quack watch list and pretend that they are experts, in which case you can find just about any opinion under the sun.
It certainly can be irrational. If the fear is not based on science, but based on emotion, that would fit the definition of irrational. As I shared, my daughter had a bad reaction to the MMR vaccine, which led to ITP. If the family then avoided vaccines based our emotions, rather than science, this would be irrational- which by definition means not logical or reasonable. But, as I noted, perhaps understandable if a family in such circumstances did let their emotions get the better of them.
People are choosing not to get the vaccine, due to fears which are either imaginary, or emotionally amplified, and in doing so, they are putting themselves at significant risk. But to see this it does require either trusting the experts or having about an 8th grade level of math comprehension, so that one can read the studies and understand the statistics.