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BACKGROUND:Using primary human papillomavirus (HPV) testing for cervical screening increases detection of high-grade cervical intraepithelial neoplastic lesions and invasive cancer (cervical intraepithelial neoplasia grade 2+ [CIN2+]) compared to cytology, but no evaluation has been conducted in a population previously offered HPV vaccination. We aimed to assess colposcopy referral and CIN2+ detection rates for HPV-screened versus cytology-screened women in Australia's HPV-vaccinated population (by 2014, resident women ≤33 years had been age-eligible for HPV vaccination, with 3-dose uptake across age cohorts being about 50%-77%). METHODS AND FINDINGS:Compass is an open-label randomised trial of 5-yearly HPV screening versus 2.5-yearly liquid-based cytology (LBC) screening. In the first phase, consenting women aged 25-64 years presenting for routine screening at 47 primary practices in Victoria, Australia, provided a cervical sample and were randomised at a central laboratory at a 1:2:2 allocation to (i) image-read LBC screening with HPV triage of low-grade cytology ('LBC screening'), (ii) HPV screening with those HPV16/18 positive referred to colposcopy and with LBC triage for other oncogenic (OHR) types ('HPV+LBC triage'), or (iii) HPV screening with those HPV16/18 positive referred to colposcopy and with dual-stained cytology triage for OHR types ('HPV+DS triage'). A total of 5,006 eligible women were recruited from 29 October 2013 to 7 November 2014 (recruitment rate 58%); of these, 22% were in the group age-eligible for vaccination. Data on 4,995 participants were analysed after 11 withdrawals; 998 were assigned to, and 995 analysed (99.7%) in, the LBC-screened group; 1,996 assigned to and 1,992 analysed (99.8%) in the HPV+LBC triage group; and 2,012 assigned to and 2,008 analysed (99.8%) in the HPV+DS triage group. No serious trial-related adverse events were reported. The main outcomes were colposcopy referral and detected CIN2+ rates at baseline screening, assessed on an intention-to-treat basis after follow-up of the subgroup of triage-negative women in each arm referred to 12 months of surveillance, and after a further 6 months of follow-up for histological outcomes (dataset closed 31 August 2016). Analysis was adjusted for whether women had been age-eligible for HPV vaccination or not. For the LBC-screened group, the overall referral and detected CIN2+ rates were 27/995 (2.7% [95% CI 1.8%-3.9%]) and 1/995 (0.1% [95% CI 0.0%-0.6%]), respectively; for HPV+LBC triage, these were 75/1,992 (3.8% [95% CI 3.0%-4.7%]) and 20/1,992 (1.0% [95% CI 0.6%-1.5%]); and for HPV+DS triage, these were 79/2,008 (3.9% [95% CI 3.1%-4.9%]) and 24/2,008 (1.2% [95% CI 0.8%-1.6%]) (p = 0.09 for difference in referral rate in LBC versus all HPV-screened women; p = 0.003 for difference in CIN2+ detection rate in LBC versus all HPV-screened women, with p = 0.62 between HPV screening groups). Limitations include that the study population involved a relatively low risk group in a previously well-screened and treated population, that individual women's vaccination status was unknown, and that long-term follow-up data on disease detection in screen-negative women are not yet available. CONCLUSIONS:In this study, primary HPV screening was associated with significantly increased detection of high-grade precancerous cervical lesions compared to cytology, in a population where high vaccine uptake was reported in women aged 33 years or younger who were offered vaccination. It had been predicted that increased disease detection might be associated with a transient increase in colposcopy referral rates in the first round of HPV screening, possibly dampened by HPV vaccine effect; in this study, although the point estimates for referral rates in women in each HPV-screened group were 41%-44% higher than in cytology-screened women, the difference in referral rate between cytology- and HPV-screened women was not significant. These findings provide initial support for the implementation of primary HPV screening in vaccinated populations. TRIAL REGISTRATION:Australian New Zealand Clinical Trials Registry ACTRN12613001207707.