Despite improvements in management, S. aureus remains a major cause of mastitis. Its chronic and destructive nature often leads to premature culling and ample loss of revenue. Additional help was sought in the form of a mastitis vaccine to overcome this challenge. The study investigated the ability of a mastitis vaccine to reduce new intra-mammary infections (IMI) with Staphylococcus aureus (S. aureus), improve bacterial cure, and reduce the duration of IMI and to investigate its effect on milk somatic cell count.