Background There have been few studies of the sexual transmission of human papillomavirus (HPV) between partners. Our objective was to estimate transmission rates among persons with documented sexual exposure to an infected partner and longitudinal follow-up.
Methods We analysed data from the HITCH Cohort Study, a study of recently-formed couples. Women aged 18–24 attending a university or junior college in Montreal, Canada and their male partners were eligible. Self-collected vaginal swabs and clinician-obtained swabs of epithelial cells from the penis and scrotum were tested for DNA of 36 HPV types. We analysed follow-up data at visit 2 from 179 couples who were discordant for one or more HPV types at enrolment. We defined the index partner as that which was infected with a type(s) not found in the other partner, and a transmission event as subsequent detection of that HPV type in the non-index partner. Transmission rates are expressed as the number of transmissions per 100 person-months (PM), with 95% CI estimated using Poisson regression.
Results Transmission was observed in 73 partnerships. There was little difference between the male-to-female (3.5 per 100PM, 95% CI 2.7 to 4.5) and the female-to-male transmission rate (4.0 per 100PM, 95% CI 3.0 to 5.5). These rates are consistent with a per-partnership transmission probability of 0.20 (95% CI 0.16 to 0.24) over 6 months. Transmission rates did not differ with the lifetime number of partners reported by the non-index partner at enrolment or with the circumcision status of the male partner. Rates were highest when the index partner was still positive for that type at follow-up; rates of male-to-female transmission quadrupled and female-to-male transmission tripled (5.2 and 6.2 per 100PM, respectively), compared to when the index partner was negative at follow-up (1.2 and 1.8 per 100PM, respectively, p<0.05).
Conclusions Transmission rates based on follow-up of discordant partners are probably underestimates of the true rate due to clearance in index partner and the depletion of susceptibles. Our results contribute to a small but growing evidence base regarding the natural history of HPV transmission and the probability of transmission. These estimates may be of utility to improve forecasting estimates from mathematical modelling efforts to project the public health impact and cost-effectiveness of HPV vaccination.
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