A number of developed countries including Australia have experienced significant pertussis outbreaks in recent years despite consistent high levels of vaccine coverage. Evolutionary changes in Bordetella pertussis carrying variants of the gene encoding pertactin, and the emergence of pertactin-deficient strains (PRN-) in the recent epidemics suggest a possible role of vaccine-driven evolution. In this study, we considered a deterministic 2-strain compartmental model to characterize the relative fitness of PRN- strains and vaccine efficacy against PRN- infection in comparison to the wild-type pertactin-expressing (PRN+) strains. We first showed that the model's equilibrium behavior allows for replacement and co-existence, depending on key parameters related to transmission, vaccine efficacy and durations of immunity. We then fitted the model to epidemiological and pathogen PRN data from the state of New South Wales, Australia. Fitted model parameters showed that the changes in pertussis epidemiology have been governed by a vaccine-escape B. pertussis strain (PRN-) having a basic reproduction number ∼ 1/2 of the wild-type strain which was in circulation prior to April 2009, against which the vaccine was estimated to have substantially reduced efficacy. While not causal, our results suggest that selective pressure from acellular pertussis vaccination is consistent with the changing epidemiology observed in Australia over the analysis period.
Keywords: Acellular vaccine; Bacterial evolution; Bordetella pertussis; Selective pressure.
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