Objective: To develop and validate a predictive model for acquiring sexually transmitted infections (STIs) in individuals using HIV pre-exposure prophylaxis (PrEP) within the national program providing this strategy.

Methods: Ambispective cohort study, multicentric (23 Spanish hospitals). All PrEP users, with follow-up within the program, will be included. Entry into the program will be considered as the baseline visit. From there, patients are followed quarterly, following the national protocols for monitoring PrEP users in Spain. At each visit, diagnosis of different STIs (serum, pharyngeal swab, urethral or urine sample, rectal swab, endocervical/vaginal swab, ulcer) is performed. Primary outcome variable: development of an STI (Yes/No). The following STIs will be considered: syphilis, Neisseria gonorrhoeae (NG) infection, Chlamydia trachomatis (CT) infection, lymphogranuloma venereum (LGV), Mycoplasma genitalium (MG) infection, genital herpes, hepatitis A, hepatitis B, hepatitis C, HIV infection, and MPOX. Secondary outcome variable: number of diagnosed STIs. In order to determine factors associated with acquiring an STI, multivariate analyses will be conducted using logistic regression. The best models will be analyzed in the validation population. To compare the models, we will follow the Bayesian approach suggested by Benavoli et al.