We took this into account by longitudinal modelling, see more age adjustment, matching, and age restriction. We also included analyses of the number of partners before age 18, which ensured that all respondents had the same time interval available

to gain partners since all survey participants were at least 18 years old. Still, the possibility of residual confounding by age cannot be entirely excluded for the analyses that included women of a wide age range. It is thus reassuring that the main finding of this study is supported by all analyses, even the narrowly age restricted and the age matched analyses. As yet this is the largest study to address potential differences in sexual behaviour between HPV vaccinees and non-vaccinees. Another strength of this study is the representativeness of the study sample. To our knowledge, this is the first study of HPV vaccination and sexual behaviour surveying large random samples drawn from complete population registries. Moreover, by use of reported ages, we addressed the sequence of vaccination and sexual behaviour in the relevant order, thus limiting the analyses to events that may be temporally attributable to HPV vaccination. Women vaccinated against HPV did not engage more in sexual risk taking behaviour than unvaccinated women. This held true for analyses of opportunistic as well as organized catch-up GDC-0199 in vitro vaccination. Hence, concerns that HPV vaccination may lead to increased

sexual risk-taking seem unwarranted, at least in the vaccination settings investigated here. Since HPV vaccines have high efficacy and favourable safety profiles, the success of HPV vaccination as a public health intervention largely seems to be a matter of vaccine uptake. Information from this study could be useful to parents and others involved in decisions regarding HPV vaccination, and may thus help to increase vaccine uptake. B.T.H., also S.K.K., L.A.D.,

K.L.L., K.E.J., C.M. and M.N. designed the questionnaire and conceived the study. B.T.H., S.K.K., L.A.D., L.T.T., K.E.J., C.M. and M.N. collected data. B.T.H. conducted analyses and drafted the paper. All authors contributed to the writing of this paper by data interpretation and critical revision of drafts. All authors approved the final draft. Merck & Co., Inc (grant number: EPO 8014.033). Merck has been involved in the study design and has approved the decision to submit the paper for publication. The study was approved by the Research Ethics Committee/Data Protection Agency in each country. Women invited to participate received information about the study, and answering the questionnaire was considered informed consent to participation. B.T.H. declares no conflict of interest. S.K.K. has received lecture fees, scientific advisory board fees, and institutional research grants from Merck and Sanofi Pasteur MSD, and scientific advisory board fees from Roche. L.A.D. has received grant support from Merck, Sanofi Pasteur MSD and GlaxoSmithKline.