you seem to understand it correctly, the level of >protection< may be inferred by observing titre of antibodies, the strength of immune response may be inferred

you seem to understand it correctly, the level of >protection< may be inferred by observing titre of antibodies, the strength of immune response may be inferred by observing increase of titre after challenge by pathogen. the durability of immunity may be inferred by observing the increased response to challenge over points in time.