Background: HIV prevalence and sexual risk have been estimated very high for transgender people. However, the limited sampling and data collection methods used in current research on transgender people potentially led to overrepresentation and generalisation of people at risk for HIV. Current HIV prevalence estimates in transgender populations are generalised from studies mainly focusing on transgender women who engage in sex work. Moreover, current research remains cisnormative, and studies focusing on non-binary people, who identify with a broad range of identities beyond the traditional male and female gender identities, are scarce.

Objective: This study aims to estimate the HIV prevalence rate in the Flemish and Brussels (Belgium) transgender population, including transgender women as well as transgender men and non-binary people, and identifying the associated individual and community-level risk factors.

Methods: In this community-based cross-sectional study, self-identified transgender and non-binary (TGNB) people will be recruited through a two-stage time-location sampling approach to minimize selection bias. In a first part, community settings in which TGNB people gather will be mapped using qualitative and ethnographic research methods, to reveal how the TGNB community in Flanders and Brussels is structured and to develop an accurate sampling frame. In a second part, to select the respondents, a multistage sampling design is applied involving a stratification based on setting type (healthcare facilities vs outreach events), a selection of clusters by systematic sampling and a simple random selection of TGNB people within each cluster. Participants will complete an electronic self-reported survey to measure sociological, sexual and drug-using behaviors (risk factors) and, at the same time, oral fluid aliquots will be collected and tested for HIV antibodies. Logistic regression models will be used to evaluate risk factors independently associated with HIV infection.

Conclusion: To the best of our knowledge, this study will be the first to investigate the HIV prevalence rates and behaviors that increase risk and vulnerability for HIV infection in an accurate representation of the TGNB population in a West European country. The findings of this study will globally serve as a knowledge base for identifying subgroups at risk for becoming infected with HIV within TGNB people and to set up targeted prevention programs on sexual health.

Inclusion Criteria:

• Adult (≥ 18 years) self-identified transgender and non-binary people
• Able to read, write and understand the survey
• Consent to participate in the survey and to self-collect an oral fluid sample

Exclusion Criteria:

• Cisgender people
• People who have no understanding of the multilingual survey

Transgender women and transgender men, as well as non-binary people are included in this study. Transvestite or crossdressing people who identify as transgender are also included.

The study population involves transgender (identifying more with the opposite gender than the gender which was assigned at birth) and non-binary (identifying with both or neither binary genders) people in Flanders and Brussels.