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A 5-year Chlamydia vaccination programme could reverse disease-related koala population decline: Predictions from a mathematical model using field data

Craig, AP, Hanger, J, Loader, J, Ellis, WAH, Callaghan, J, Dexter, C, Jones, D, Beagley, KW, Timms, P & Wilson, DP 2014, Vaccine, vol. 32, pp. 4163-4170.

Chlamydia vaccines are being developed to mitigate the impact the disease has on koala populations. Stochastic individual-based mathematical modelling suggests targeting female koalas aged 1-2 years old with a vaccine with 75% efficacy administered to approximately 10% of koalas per year could initiate a reversal in current population declines over a 5-6 year period.

  Targeting females aged 1-2 years yielded the highest median percentage of simulations that resulted in population decline reversal in 10 years. Optimistic boosting assumptions work on the principle that immunity levels in a population can increase above initial vaccination levels. This would occur if immunity of a vaccinated koala could be boosted by mating with an infected koala. Conversely, pessimistic boosting assumptions work on the model that mating activity between an infected and a previously vaccinated koala would see no immunity increases greater than the initial efficacy level. Outcomes in population decline vary dramatically under pessimistic and optimistic boosting assumption models. When the efficacy level is at 45% in 10% of the population, the decline reversal would be eight years if optimistic boosting is assumed. On the other hand, under pessimistic boosting assumptions it would be 14 years until the decline is reversed. Predictions from this model suggest that if the vaccine program is terminated earlier than the five-year period, the prevalence of Chlamydia is likely to increase in koala populations which will again decline.

  This mathematical model is designed to increase the understanding of the effect of Chlamydia in koala population dynamics to determine the most effective vaccine delivery scenario and parameters for southeast Queensland. Targeting younger females before they reach sexual maturity is considered optimal as this should ensure immunity is conferred when breeding commences. The prediction simulation found that unless Chlamydia was totally eliminated from a population pool, without continuing vaccination, infection rates would again become prevalent.

  The outcomes of this model will help generate a long-term framework for controlling Chlamydia infection in koala populations. There are several limitations acknowledged in this study. These include but are not limited to an assumption that there would be no immigration or emigration from a given population, that the vaccination would be effective against all strains of Chlamydia, and that there was no inclusion of any mating “harem” effect. The model also excludes consideration of interactions between immunity to infection and koala retrovirus (KoRV). These factors must be considered in future research and the design of vaccination programmes.

 

Summarised by Penelope Webster

 

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