ObjectiveTo estimate the prevalence and describe the patterns of concurrent human papillomavirus (HPV) and STIs and associated factors among HIV-negative young Western Cape, South African women participating in the Efficacy of HPV Vaccine to Reduce HIV Infection (EVRI) trial.MethodsHIV-negative women aged 16–24 years old were enrolled in the EVRI trial (NCT01489527) and randomised to receive the licensed four-valent HPV vaccine or placebo. At study entry, participants were clinically evaluated for five STIs: herpes simplex virus type 2 (HSV-2), chlamydia, gonorrhoea, syphilis and disease-causing HPV genotypes (6/11/16/18/31/33/35/39/45/51/52/56/58/59/68). Demographic and sexual history characteristics were compared among women with STI co-infections, single infection and no infection using Pearson χ²and Mann-Whitney tests. ORs were calculated to evaluate factors associated with STI co-infection prevalence.ResultsAmong 388 young women, STI co-infection prevalence was high: 47% had ≥2 concurrent STIs, 36% had a single STI and 17% had none of the five evaluated STIs. HPV/HSV-2 (26%) was the most prevalent co-infection detected followed by HPV/HSV-2/Chlamydia trachomatis(CT) (17%) and HPV/CT (15%). Co-infection prevalence was independently associated with alcohol use (adjusted OR=2.01, 95% CI 1.00 to 4.06) and having a sexual partner with an STI (adjusted OR=6.96, 95% CI 1.53 to 30.08).ConclusionsAmong high-risk young women from underserved communities such as in Southern Africa, a multicomponent prevention strategy that integrates medical and behavioural interventions targeting both men and women is essential to prevent acquisition of concurrent STI infections and consequent disease.Trial registration numberNCT01489527; Post-results.