Strain replacement occurs when after a vaccination campaign one (or more) strains decline in prevalence while another strain (or strains) rise in prevalence. Differential effectiveness of the vaccine is the widely accepted and the most important mechanism which leads to this replacement effect. Recent theoretical studies have suggested that strain replacement may occur even if the vaccine is perfect, that is, the vaccine is completely effective with respect to all strains present. It has already been shown that perfect vaccination, along with a trade-off mechanism, such as co-infection or super-infection, lead to strain replacement. In this paper, we examine the hypothesis that strain replacement with perfect vaccination occurs only with trade-off mechanisms which allow a strain with a lower reproduction number to eliminate a strain with a higher reproduction number in the absence of vaccination. We test this hypothesis on a two-strain model with vertical transmission. We first show that vertical transmission as a trade-off mechanism can lead to dominance of a strain with suboptimal reproduction number. Based on the hypothesis we expect, and we show, that strain replacement occurs with vertical transmission.
