NHS North Somerset Primary Care Trust (PCT) has launched a campaign aimed at encouraging parents to immunise their children against serious diseases.
They say too few children receive the immunisations they need to protect them from "often life-threatening illnesses" due to "myths surrounding vaccination".
This follows a recent review, by the PCT, of immunisation rates for children under five years old in the county.
They said data revealed that "15% of our five-year-olds are not protected".
The campaign will feature a range of posters, leaflets, radio and cinema advertisements.
Becky Pollard, director of heath at NHS North Somerset, said around 558 children in the county were "at risk".
NHS North Somerset health improvement specialist Lianne Straus said that the controversy surrounding the MMR vaccine - in which the jab was linked by a doctor to autism - made many mothers wary of vaccinations and their possible side effects. The government and the vast majority of scientists insist that the vaccination is safe.
"Our awareness campaign aims to dispel some of the myths surrounding vaccination," she said.