Individual Variation in Vaccine Immune Response Can Produce Bimodal Distributions of Protection

Prior exposure to pathogens augments host heterogeneity in susceptibility and has key epidemiological consequences

“…Another limitation is the choice of an SIR rather than an SIRS model that allows for loss of immunity over time, as is the case in the house finch-MG model system, in which protection wanes over approximately one year (40). To disentangle effects of population-level heterogeneity of susceptibility induced by prior exposure, we focus this study on a single, short timescale epidemic while ignoring demographic effects and re-infection, similar to previous work (26,62,63). Allowing for reinfection in our model would require an understanding of how susceptibility and infectivity are linked, because such correlations have an impact on the progression of epidemics (62,63).…”

“…Because the protection acquired from prior pathogen infection is often incomplete and/or wanes over time, reinfections occur even in systems where hosts have lower mean susceptibility during secondary exposures, relative to individuals exposed for the first time (21)(22)(23)(24). While there is growing appreciation for the pervasiveness of heterogeneity in acquired host protection in response to vaccination or infection across systems (23,25,26), prior work has largely focused on how host protection from vaccination or prior infection influences mean population traits, rather than variability among individuals in a given population (27). As such, it remains unknown how host protection acquired from prior pathogen exposure alters the degree of inter-individual heterogeneity in susceptibility.…”

“…Epidemiological models often select either wholly leaky protection of hosts that allow every host an identical rate of reinfection or all-or-nothing protection such that some hosts remain in the recovered class while a subset is routed back to the susceptible class, representing two extremes of how heterogeneity is incorporated into disease models (6,27). Whether and how prior exposure to pathogens alters population-level heterogeneity in susceptibility in a given system likely depends on the extent of prior pathogen exposure that hosts experience (28) and the mode of action of such protection [“leaky”, “all-or-nothing” (6,27), or some intermediate (26)]. For example, host exposure to low pathogen doses, which can occur during the natural transmission process for many pathogens (e.g.…”