Strain replacement is the effect of substitution of a strain of higher prevalence in the population with another. Differential effectiveness of the vaccines is thought to be the mechanism responsible for the replacement effect. Recent theoretical study shows that differential effectiveness of the vaccine may not be necessary and other trade-off mechanisms can lead to it even when the vaccine is "perfect". We suggest that the mechanism of strain replacement is the reciprocal effect of vaccination on the fitness of the strains as measured by their invasion reproduction numbers. This mechanism is responsible for the substitution of one strain with another to occur both when the vaccine is perfect and when it is imperfect. We review various well-known trade-off mechanisms and investigate whether they lead to replacement effect in conjunction with "perfect" vaccination. We find that in contrast to imperfect vaccination which leads to replacement of a strain with larger intrinsic reproduction number with a strain with a lower intrinsic reproduction number, "perfect" vaccination seems to have opposite effect on the intrinsic reproduction numbers.
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