An understanding of the consequences of infection and vaccination on host immunity sets the stage for interpreting pertussis epidemiology. Yet, with no known serological marker of protection, such an understanding is currently not possible. This chapter interrogates longitudinal age-stratified pertussis incidence reports from Sweden and Massachusetts, United States, with the aim of quantifying the impact of infection and immunization on protective immunity. The analysis of data from Sweden during the vaccination hiatus period (1986–1996) indicates that adults contribute little to transmission. This may either be because infection-derived immunity is very long-lasting, or that individuals whose immunity has waned are subsequently less susceptible. The analysis of data from Massachusetts (1990–2005) identifies the primary mechanism of vaccine failure—for both whole-cell and acellular pertussis vaccines—to be waning. However, the average duration of immunity is identified as many decades, though the model predicts substantial individual variability in this trait. Finally, the chapter demonstrates the estimates to be consistent with those obtained from popular measures of vaccine effectiveness, though the interpretation of these findings is quite different.