Parents against vaccinations won’t be swayed by public service announcements, according to a Dartmouth College study that is part of NPR’s larger story about a California county’s efforts to reverse sliding vaccination rates that have been accompanied by an outbreak of whooping cough.

The study included answers from 1,800 parents about the measles, mumps and rubella vaccine and researchers found that skeptical parents are tuning out PSAs and pediatrician advice.

NPR reports that a Marin County practitioner took a different route and told families that if they refused to vaccinate their children, they could not be treated at his practice. He tells NPR few families left and 150 changed their minds.