Modelling borderline and mild dysplasia associated with HPV 6 and 11 infection

Introduction: Low risk HPV types 6/11 are responsible for some low-grade cytological abnormalities. Most economic analyses of HPV vaccination have estimated the additional benefit of HPV 6/11 protection by the quadrivalent vaccine, over the bivalent, based on reduction of genital warts but have not included reduction in repeat smears and colposcopies due to low-grade abnormalities. We investigate the contribution of HPV types 6/11 to abnormal smears and associated costs in England. Methods: The risk of borderline or mild dysplasia due to HPV 6/11 infection was estimated from a study of type-specific HPV DNA in cervical screening specimens collected throughout England. A Markov model representing 10 million women with HPV 6/11 or with no HPV infection from 24 to 64 years was developed to estimate the number of abnormal smears, subsequent repeat smears and colposcopies due to HPV 6/11 associated with borderline or mild dysplasia. Fitting was achieved by varying the force of infection, probability of borderline or mild dysplasia if HPV-uninfected or infected with HPV 6/11 and the duration of infection. Results: The relative risks of borderline or mild dysplasia when infected with HPV 6/11 compared to not being HPV infected were 6.32 (95% credible interval 1.56-25.6) and 17.5 (1.02-300) respectively. Using best fitting parameters we find the costs incurred are between £170 and £195 per abnormal smear due to infection with HPV 6/11. Conclusions: In England, the impact of cytological abnormalities due to HPV 6/11 is relatively small, but not negligible. A vaccine that protects against HPV 6/11 infections could reduce costs associated with borderline and mild dysplasia, and associated colposcopies. These benefits should be considered when formulating immunisation policy, if possible. Smears and colposcopies in those uninfected with HPV far outnumber those in women infected with HPV 6/11.