Zambia CAB-LA cohort of 3,755 people: baseline syphilis 4.57%, incidence 11.76 per 100 person-years

A retrospective cohort study in Zambia has measured syphilis prevalence and incidence among people receiving long-acting injectable cabotegravir (CAB-LA) for HIV prevention. According to the authors, writing in BMC Public Health, the analysis used routinely collected programmatic data on 3,755 individuals who started CAB-LA across several provinces during the national introduction period, October 2023 to September 2024 biomedcentral.comBMC Public Healthbiomedcentral.comOpen source ↗.

Baseline prevalence and geography

Valid baseline syphilis results were available for 2,755 participants. Of these, 126 tested reactive, a baseline prevalence of 4.57% (95% CI: 3.85–5.42).

Prevalence differed by province. It was highest in Copperbelt Province (11.36%), followed by Northwestern Province (8.57%) and Western Province (7.27%).

Participants who reported intimate partner violence had a baseline prevalence of 8.70%, against 4.51% among those who did not. Alcohol users had a prevalence of 7.76%, against 4.31% among non-users biomedcentral.comBMC Public Healthbiomedcentral.comOpen source ↗.

Incident infections and gaps in follow-up testing

Only 1,318 participants had at least one repeat syphilis test during follow-up. The remaining 2,437 (65%) had no documented repeat test.

Incident syphilis was assessed among baseline non-reactive participants with at least one repeat test. Among 1,117 such participants, 31 incident infections occurred. This gives a cumulative incidence of 2.78% (95% CI: 1.96–3.91) and an incidence rate of 11.76 per 100 person-years (95% CI: 7.99–16.69).

Southern Province recorded the largest number of incident infections, 21 cases, with an incidence rate of 14.50 per 100 person-years biomedcentral.comBMC Public Healthbiomedcentral.comOpen source ↗.

The authors used Poisson regression with a person-time offset. In the adjusted model, residence in Western Province was associated with higher incident syphilis (adjusted IRR 6.24; 95% CI: 1.02–38.31; p=0.048). Sex, age, depression, intimate partner violence and alcohol use were not statistically significant.

The authors conclude that the syphilis burden among CAB-LA users in Zambia remains substantial, with infections concentrated in high-mobility areas. They identify poor follow-up syphilis testing during CAB-LA care as the main programmatic gap. They recommend routine syphilis screening and treatment at every CAB-LA visit, stronger STI monitoring systems, a reliable supply of benzathine penicillin and targeted interventions in high-risk areas.