Well, in fairness and in the interest of accuracy, there's also a number in their for likelihood of contracting measles.
If you get a vaccination, there's a 100% chance you are exposed to the risks of the vaccine.
Your chances of being exposed to the measles (not contracting it, but being exposed to it) are much lower (note that we don't know how many people are actually EXPOSED to an infectious dose, because almost all vaccinated people will not....catch the measles, so we won't be able to account for them). So, there's some numerical chance of exposure. Ironically, the more successful the antivax crowd is in their evangelism....the more prevalent the disease will become, boosting this number significantly.
Then, once exposed, what are the chances of an unvaxxed person having serious complications/death?
But in the end, the math is still outsized. The ONLY way the numbers work in the anti-vaxxer's favor....is if they can successfully tag along on the herd's immunity. But their efforts to get people not to vaccinate, ironically, lead to destroying the very thing that protects them: the immunity of all the smart people who DID get vaccinated. So, the more successful antivaxxers are, the more likely their kids are to die. Like I said....bad at math, bad at thinking.