Can you read??? I did not say it "means it is not working", I said it is a bad sign. For people show skipped basic probability class in school: What I am saying is that conditional probability P(vaccine works | side effects) is large that unconditional probability P(vaccine works) , which, in turn, is larger than conditional probability P(vaccine works | no side effect). Please, learn basic math and the definition of the word "sign" as it is used in medicine (and any other science) to avoid making fool of yourself. Everything is "normal", but it affects the ex-post probability. so, again, having no side effect is a bad sign (reduce the probability) and having side effect is a good sign (increases the probability). Both are "normal", but one is better than another