Background: Prior studies have suggested sexually transmitted infections, such as syphilis, are associated with an increased risk of transmission of the HIV virus. However, no study has measured the population-level prevalence and predictors of co-infection in Zambia.
Methods: We use data from the 2016 ZAMPHIA, a national household survey that included the DPP® Syphilis Screen & Confirm Assay (Chembio) for active syphilis and Alere Determine? and Uni-Gold? rapid HIV tests, per national algorithm. Bivariate and multivariate logistic regression models were developed to assess associations between co-infection and selected socio-demographic and sexual behavior variables. For the multivariate analysis, those infected with both active syphilis and HIV were classified as co-infected, and the comparison group is those who had no infection. Due to the complex survey design, all reported figures account for the survey design and are weighted.
Results: A total of 19,114 individuals aged 15-59 responded to the individual interview and had a valid syphilis and HIV test. Among this sample were 10, 972 females and 8, 142 males. The national prevalence of active syphilis was 3.0%. Prevalence of active syphilis was highest among females (3.4%), those ≥25 years of age (3.9%), and respondents who reported ≥2 sexual partners in the last 12 months (5.3%). Sexually active HIV-positive persons had a higher prevalence of active syphilis compared to those who were HIV-negative (9.6% vs. 2.1%). The national prevalence of HIV/syphilis co-infection was 1.3% (95% CI 1.1, 1.5). Co-infection was higher among females living in urban areas (aOR =3.1, 95% CI=1.9, 5.0) and females who had an early sexual debut (before age 15 years) (aOR =1.7, 95% CI=1.0, 3.0). Co-infection was also high among both men and women who engaged in transactional sex in the past 12 months (aOR=2.3 (95% CI= 1.1, 5.2) and 4.1 (95% CI= 1.7, 9.8), respectively).
Conclusions: These findings show high prevalence of HIV/syphilis co-infection in Zambia and the need for the scale up of syphilis screening and treatment, particularly among HIV-infected adults. Additionally, these results can be used by syphilis and HIV prevention programs to target populations most at risk for co-infection, specifically those engaging in transactional sex.

Table 1. Active Syphilis Infection and HIV and Syphilis Co-infection among ZAMPHIA 2016 respondents aged 15-59
[Table 1. Active Syphilis Infection and HIV and Syphilis Co-infection among ZAMPHIA 2016 respondents aged 15-59 ]