First, to answer your question: that's not how vaccination works - a vaccine is not 100% effective. So if you get your vaccination, you maybe still have a 5% (just pulling that number out of the air) chance of not being immune, at which point anybody else who has the disease or is a carrier for it is dangerous to you. It's not that you get 5% sick, it's that you have a 5% chance of getting 100% sick.

Second, to address this question in the context of the article that you're commenting on, the child in question is not vaccinated.