For the most part, the bacterium Staphylococcus aureus is common and harmless, posing no threat to humans with whom they coexist. Occasionally, though, it can become an opportunistic pathogen, causing skin and bloodstream infections or food poisoning.
source https://www.lifetechnology.com/blogs/life-technology-science-news/familiarity-breeds-exempt-why-staph-vaccines-dont-work-in-humans