Seventeen years after the licensure of prophylactic human papillomavirus (HPV) L1 virus-like-particle vaccines, a defined antibody level that correlates with vaccine-induced protection against HPV infections and associated neoplasia is missing. In contrast, correlates of protection have been defined for many viral vaccines, including for the hepatitis B virus (HBV) vaccine. This review includes lessons learned from vaccination against HBV and the use of an established protective HBV …