'Vaccine denial' and 'vaccine hesitancy' are not the same thing. Someone who is 'vaccine hesitant' has heard some whacked-out nonsense, and doesn't know if it's valid or not, and can have their mind changed by exposure to facts and reason. They may ask their doctor about it, and listen to what their doctor says. A 'vaccine denier' has their mind made up, and will not listen to facts or reason.

I can't explain this, though: "those parents who believe vaccination to be quite effective but might prefer their children to remain unvaccinated .... " Unless the idea is that these parents are depending on everyone else's kids getting vaccinated so their kids don't have to take those statistically minimal risks involved in vaccination ... ?