TARGETING HPV VACCINATION TO FEMALE SEX WORKERS IN THAILAND: IMPLICATIONS OF SEXUAL MIXING PATTERNS ON HERD IMMUNITY BENEFITS

   Purpose:  Vaccination against human papillomavirus (HPV) types 16 and 18 has the potential to reduce the high burden of cervical cancer in Thailand. Considering the differences in HPV prevalence and sexual activity levels among heterogeneous subgroups in the population, we assessed the benefits of targeting HPV vaccination to a high-risk group of female sex workers versus the general population of females. We explored the impact of sexual mixing patterns on levels of indirect (i.e., herd immunity) benefit in the overall population.
   

   Methods:  Using population statistics and sexual behavior data from Thailand, we contextualized a dynamic model of sexual transmission of HPV-16,-18 between males and females. The population was stratified into four sexual activity groups which governed age-specific rates of new partnerships. Model inputs, including the probabilities of HPV-16 and -18 transmission, were calibrated to fit epidemiological data from Thailand using a likelihood-based approach. Estimates of reductions in cervical cancer incidence were derived under different scenarios of sexual mixing for strategies of vaccinating all female sex workers (comprising 2% of the population) or a proportion of the general population: sexual mixing patterns were varied from fully assortative (preferential mixing with others in the same sexual activity group) to random mixing. 

    Results:  Assuming full, lifelong vaccine-induced protection, the model predicted that vaccinating nearly triple the number of females in the general population would be required to achieve equivalent reductions in cancer incidence as targeting all female sex workers. Both strategies resulted in benefits for non-vaccinated women, and the level of herd immunity benefits depended on sexual mixing within the population. When mixing was assumed to be fully assortative, reductions in cancer incidence from vaccination were greatest when all female sex workers were targeted, with herd immunity benefits exceeding 20%.When the pattern of mixing across sexual activity groups was random, the benefits of vaccinating only female sex workers decreased by over 90%, with herd immunity benefits <2%. Herd immunity benefits were always lower when broadening vaccination coverage to the general population under analogous scenarios of sexual mixing.

    Conclusions:  Countries with limited resources that are interested in introducing an HPV vaccination program should consider the additional herd immunity benefits associated with targeting highly sexually-active populations instead of broader coverage of the general population.