Type "vaccine denial" into Google and Wikipedia comes up with a more polite (and less accurate) term "Vaccine Hesitancy."

When a vaccination program successfully reduces the disease threat, it may reduce the perceived risk of disease as cultural memories of the effects of that disease fade. At this point, parents may feel they have nothing to lose by not vaccinating their children.[46] If enough people hope to become free-riders, gaining the benefits of herd immunity without vaccination, vaccination levels may drop to a level where herd immunity is ineffective.[47] According to Jennifer Reich, those parents who believe vaccination to be quite effective but might prefer their children to remain unvaccinated, are those who are the most likely to be convinced to change their mind, as long as they are approached properly.[48]

And this:

As the success of immunization programs increases and the incidence of disease decreases, public attention shifts away from the risks of disease to the risk of vaccination,[52] and it becomes challenging for health authorities to preserve public support for vaccination programs.[53]

They are trying really hard to not say that these cult-following parents are idiots, so that if one of them stumbles upon the topic they might see themselves portrayed as "reasonable" but with hesitancy instead of simply shutting down all critical thinking.