Sexually transmitted infections (STIs), particularly Human Immunodeficiency Virus (HIV) and syphilis, continue to pose serious public health challenges worldwide, especially among marginalized and high-risk populations such as incarcerated individuals. According to UNAIDS, approximately 39.9 million people were living with HIV globally in 2023, with 1.3 million new infections and 630,000 AIDS-related deaths during the same year [1] . In West and Central Africa, the burden remains significant, with about 5.1 million people living with HIV, reflecting enduring disparities in prevention, testing, and care access [2] .

Syphilis also remains a major concern, with the WHO estimating over 6 million new cases annually among adults aged 15 - 49 [3] . HIV and syphilis interact biologically: syphilitic ulcers can increase susceptibility to HIV, and co-infections complicate both diagnosis and treatment [4] .

Prison settings amplify STI transmission risks due to overcrowding, poor medical infrastructure, high-risk behaviors (e.g., unprotected sex, tattooing, drug use), and limited access to preventive tools and health education [5] [6] . These are further exacerbated by stigma, criminalization, and health system neglect in many correctional environments [7] .

In sub-Saharan Africa, HIV prevalence among prisoners ranges from 2% to over 30%, often far exceeding the general population rates [8] [9] . For instance, studies in Côte d’Ivoire, Nigeria, and South Africa have highlighted elevated HIV and syphilis burdens among inmates, linked to both pre-incarceration risk profiles and intra-prison exposure [10] .

However, data on incarcerated populations in Central Africa—particularly Chad—remain scarce. National HIV/syphilis surveillance rarely includes detainees, creating policy blind spots. Prison health systems in Chad are weakly integrated into national programs, and access to testing and treatment remains limited, despite ongoing HIV response initiatives [11] .

This study aims to fill this gap by estimating the prevalence of HIV and syphilis among inmates in Chad and identifying associated socio-demographic and behavioral determinants. Results are expected to inform targeted public health interventions and promote equitable access to testing and treatment in prison settings.

The findings of this study reveal an HIV prevalence of 5.2% and a syphilis prevalence of 2.9% among incarcerated individuals in Chadian prisons—rates significantly higher than the national HIV estimate of approximately 1.0% [1] . These results confirm the heightened vulnerability of prison populations, consistent with data from other African contexts. For instance, a study conducted at the Abidjan Remand and Correctional Facility (MACA) in Ivory Coast reported an HIV prevalence of 5.9% [12] , while even higher rates have been documented in Iran, with HIV at 14.5% and syphilis at 22.1% [13] .

These elevated rates can be explained by systemic factors inherent to incarceration. Prisons are often overcrowded, unsanitary, and under-resourced in terms of healthcare. High-risk behaviors such as unprotected sex, injecting drug use, and tattooing with non-sterile equipment are common [5] [6] . In low-resource prison settings, the lack of structured screening programs and persistent HIV-related stigma further delay diagnosis and care, facilitating silent transmission in these closed environments [7] [14] [15] .

Multivariate analysis identified three significant predictors of HIV infection. The first was age: individuals aged ≥50 years were eight times more likely to be HIV-positive (aOR = 8.54). This finding aligns with studies in Brazil (aOR = 13.3) [16] and in North Carolina, USA [17] . Several mechanisms may explain this association, including immune senescence, cumulative lifetime sexual exposure, reduced access to or engagement in prevention strategies, and the masking effects of comorbidities common among older prisoners [2] .

The second determinant was marital status. Widowed or divorced inmates had a five-fold higher risk of HIV compared to their married counterparts (aOR = 5.09). This is consistent with research from sub-Saharan Africa where marital disruption is often associated with emotional distress, economic vulnerability, and increased risk behaviors such as transactional sex or exposure to traditional practices like widow inheritance (levirate marriage) [18] [19] .

Drug use emerged as the third key factor, significantly associated with HIV infection (aOR = 4.23). This confirms a well-established global trend: drug use—whether injectable or non-injectable—is a strong predictor of HIV in correctional settings [5] [20] . Behavioral clustering, including multiple sexual partners, low condom use, and unsafe tattooing, may contribute to the elevated risk observed among drug users. In this study, injecting drug use was rarely reported, likely due to underreporting driven by stigma.

Unlike some studies, biological sex was not significantly associated with HIV infection in this population (aOR = 0.86). In other contexts, particularly where women represent a larger share of the prison population, female inmates often experience a higher HIV burden due to previous exposure to sexual violence, transactional sex, and limited access to prevention [21] . In Chad, women represent less than 10% of the prison population, which may limit the statistical power to detect sex-based disparities.

For syphilis, no variable showed a statistically significant association in the multivariate model. Although widowed/divorced status (aOR = 2.28) and drug use (aOR = 1.37) suggested elevated odds, confidence intervals were wide and p-values were non-significant. This may be attributed to limited sample size and low overall prevalence (<3%), reducing the statistical power of the model. Similar challenges in identifying syphilis-related risk factors have been documented in prison studies from Ethiopia and Nigeria, where underreporting and diagnostic limitations hinder accurate assessment [22] [23] .

Several study limitations warrant mention. First, the cross-sectional design precludes conclusions about causality—whether infections were acquired before or during incarceration remains uncertain. Second, the use of secondary data led to considerable missingness for key behavioral variables, such as alcohol and tobacco use. Moreover, social desirability bias may have resulted in underreporting of stigmatized behaviors like same-sex activity or injecting drug use. Finally, syphilis testing was based on rapid diagnostics without confirmatory RPR titers, which may compromise diagnostic sensitivity and specificity.

Prisons in Chad pose heightened risks for HIV and syphilis transmission due to overcrowding, insufficient healthcare services, and the prevalence of high-risk behaviors among inmates. This study identified older age, widowed or divorced marital status, and drug use as significant predictors of HIV infection. While no statistically significant factors were associated with syphilis, its prevalence remains a public health concern.

Addressing these gaps requires targeted screening, harm reduction strategies, and structural reforms. Improving prison health is essential not only for the rights of detainees but also for protecting public health beyond prison walls.

We sincerely thank all the inmates who kindly agreed to participate in this study. We also extend our gratitude to the prison authorities, the CNLS team, and our financial and technical partners for their multifaceted support. We equally acknowledge the contributions of both civilian and military health personnel, whose dedication greatly contributed to the success of this study.