Waning immunity is known to occur for some infectious diseases after natural infection and vaccination. We present a susceptible-infectious-recovered-susceptible (SIRS)-type transmission model that includes the waning and boosting of immunity. We study how the infection prevalence changes with differences in (i) the durations of infection- and vaccine-acquired immunity and (ii) the assumed mechanism through which immune boosting acts to extend protection.
Our results show that increasing vaccine uptake always lowers the proportion of primary infections but may lead to an increase in overall transmission. Where the boosting of vaccine-acquired immunity delays a primary infection, the driver of transmission largely remains primary infections. In contrast, if immune boosting bypasses a primary infection, secondary infections become the main driver of transmission under sufficiently long-lasting vaccine-induced immunity. Our study highlights that for a particular disease and associated vaccine, a detailed understanding of how the duration of protection can influence infection prevalence is important as we seek to optimise vaccination strategies.