While driving, it is rational and justified to believe that the vehicle's speed is 60mph when the speedometer reads "60mph." However, if the speedometer is miscalibrated your belief is incorrect.
It cannot be said that being anti-vaccine is a justified rational belief when it has an unjustified irrational basis. It is justified and rational to believe you're going 60mph when the speedometer says so because we all have experience with the reliability of speedometers, even though it's possible to be incorrect.
To use a different analogy, imagine I believed carpet was lava and stepping on it would burn me and break my mother's back. While this would explain why I'd avoid stepping on carpet, it would hardly be accurate to describe it as a justified, rational belief.
