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Embargoed until 2021-12-22
Copyright: Padeniya, Seneviratne Mudiyanselage Thilini Nisansala
Embargoed until 2021-12-22
Copyright: Padeniya, Seneviratne Mudiyanselage Thilini Nisansala
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Abstract
Gonorrhoea notifications have been increasing among young Australian heterosexuals since 2009 and the reasons for this are unclear. Gonorrhoea incidence has also increased in female sex workers (FSW) since 2009. Previous studies indicate that condom-use among FSW-clients declined from 2009-2017, mainly for oral sex and a high proportion of infections in heterosexual males arise from condomless oral sex with FSW. Thus, we hypothesise that an increase in condomless sex by FSW-clients may have contributed to the rising incidence of gonorrhoea among heterosexuals in Australia. In this thesis, mathematical modelling is used to provide insights to the role of the FSW-client interaction for heterosexual gonorrhoea transmission, to explore whether the increasing notifications can be explained, even partly, by decreasing condom-use among FSW-clients, and to evaluate the potential impact of providing gonorrhoea vaccination for FSW on gonorrhoea incidence/prevalence.
A deterministic compartmental model was developed to address the stated objectives and was calibrated to reported female notifications and FSW incidence data for 2009. Using adaptations of the model that included/excluded FSW-client strata, we evaluated the role of FSW-client interactions in model dynamics and sensitivity of the reproduction number (Rt) in this population to changes in key parameters. We then estimated the annual percentage decline in condom-use between 2009 and 2017 that resulted in a model-produced notification rate that is consistent with the reported increase in heterosexual notifications using the model with FSW-client strata. Finally, the potential impact of a gonococcal vaccine for FSW on heterosexual gonorrhoea rates was assessed under different assumptions regarding the mode of vaccine conferred protection.
Our results suggest that Rt and the heterosexual notification rate are highly sensitive to changes in parameters that govern transmission in the model that accommodates FSW-client interactions and infection rates are consequently highly sensitive to changes in condom-use by FSW-clients. An annual decline of only 0.26% in condom-use by FSW-clients is predicted to lead to an increase in heterosexual notifications that is consistent with the observed increase in notifications. Vaccinating FSW with a partially efficacious vaccine has the potential to substantially reduce gonorrhoea prevalence over time in the heterosexual population in Australia.
These findings suggest that increasing condomless sex among FSW-clients can result in marked increases in heterosexual notifications. Therefore, promoting condom-use in commercial sex may help reduce the gonorrhoea burden in young heterosexuals. Additionally, targeted vaccination of FSW may be an effective means of controlling gonorrhoea in this population.