Longitudial studies in Italian turkey farms demonstrated that subtype B virus was frequently detected some period after sub type B vaccination. Sequencing showed that these later viruses were not derived form the previously applied vaccine. More detailed sequence analysis of fusion and attachment protein genes showed that these later subtype B detections formed a cluster. The attachment protein genes in this cluster were dissimilar to all those found in early B type viruses, including the established vaccines, and these themselves formed another cluster. One day old poults were vaccinated with subtype B vaccine in experimental conditions and later challenged with either early (TRT VR240) or later subtype B (205-16/04) field isolates. Protection was very poor as assessed by both clinical disease and shedding of virus after 205 challenge . The limited immunity observed may explain the dominance of the later type subtype B field viruses over a six year period in this Italian region. This may have resulted from immune pressure induced by mass subtype B vaccination.