Vaccine-induced pathogen strain replacement: what are the mechanisms?

Abstract

Host immune systems impose natural selection on pathogen populations, which respond by evolving different antigenic signatures. Like many evolutionary processes, pathogen evolution reflects an interaction between different levels of selection; pathogens can win in between-strain competition by taking over individual hosts (within-host level) or by infecting more hosts (population level). Vaccination, which intensifies and modifies selection by protecting hosts against one or more pathogen strains, can drive the emergence of new dominant pathogen strains-a phenomenon called vaccine-induced pathogen strain replacement. Here, we review reports of increased incidence of subdominant variants after vaccination campaigns and extend the current model for pathogen strain replacement, which assumes that pathogen strain replacement occurs only through the differential effectiveness of vaccines against different pathogen strains. Based on a recent theoretical study, we suggest a broader range of possible mechanisms, some of which allow pathogen strain replacement even when vaccines are perfectthat is, they protect all vaccinated individuals completely against all pathogen strains. We draw an analogy with ecological and evolutionary explanations for competitive dominance and coexistence that allow for tradeoffs between different competitive and life-history traits.

A slow-fast dynamic decomposition links neutral and non-neutral coexistence in interacting multi-strain pathogens

“…Understanding the ecology underpinning this diversity is crucial to explain how these systems might respond to human interventions, such as vaccines and drugs (Lipsitch 1997;Martcheva et al 2008;Colijn and Cohen 2015), and how they might spontaneously evolve (Dercole et al 2002). We must therefore seek for comprehensive models that can provide mathematical and ecological insight into the short-and long-term dynamics of such systems.…”

Vacated niches, competitive release and the community ecology of pathogen eradication

“…Other models have rstb.royalsocietypublishing.org Phil Trans R Soc B 368: 20120150 highlighted the potential for strain replacement owing to new vaccines against rotavirus [76] and human papillomavirus [21]. The prevailing assumption is that strain replacement occurs because the vaccine is more effective against some strains than others, hence releasing non-target strains from competition, though other ecological and evolutionary mechanisms can also suffice [77]. The analogy is not perfect, but many insights from this literature may carry over to the growing theory of pathogen eradication.…”

The beneficial effects of vaccination on the evolution of seasonal influenza

“…First, by reducing the prevalence of infection, they reduce viral population size and thus the probability that antigenic escape mutants will arise. Second, although vaccination increases the growth rate of antigenically distant mutants relative to less distant mutants (which can lead to strain replacement in other pathogens [14][15][16][17][18][19][20]), it also increases the amount of immunity in the population. This increased immunity reduces the growth rate or invasion fitness of escape mutants, slowing the rate of strain replacement (SI 1.1, Eq.…”