Tracing of Patients Lost to Follow-up and HIV Transmission: Mathematical Modeling Study Based on 2 Large ART Programs in Malawi

Objective: Treatment as prevention depends on retaining HIV-infected patients in care. We investigated the effect on HIV transmission of bringing patients lost to follow-up (LTFU) back into care. Design: Mathematical model. Methods: Stochastic mathematical model of cohorts of 1000 HIV-infected patients on antiretroviral therapy, based on the data from 2 clinics in Lilongwe, Malawi. We calculated cohort viral load (sum of individual mean viral loads each year) and used a mathematical relationship between viral load and transmission probability to estimate the number of new HIV infections. We simulated 4 scenarios: "no LTFU" (all patients stay in care), "no tracing" (patients LTFU are not traced), "immediate tracing" (after missed clinic appointment), and "delayed tracing" (after 6 months). Results: About 440 of 1000 patients were LTFU over 5 years. Cohort viral loads (million copies/mL per 1000 patients) were 3.7 95% prediction interval (PrI), 2.9-4.9] for no LTFU, 8.6 (95% PrI, 7.3-10.0) for no tracing, 7.7 (95% PrI, 6.2-9.1) for immediate, and 8.0 (95% PrI, 6.7-9.5) for delayed tracing. Comparing no LTFU with no tracing, the number of new infections increased from 33 (95% PrI, 29-38) to 54 (95% PrI, 47-60) per 1000 patients. Immediate tracing prevented 3.6 (95% PrI, -3.3 to 12.8) and delayed tracing 2.5 (95% PrI, -5.8 to 11.1) new infections per 1000. Immediate tracing was more efficient than delayed tracing: to 116 and 142 tracing efforts, respectively, were needed prevent 1 new infection. Conclusions: Tracing of patients LTFU enhances the preventive effect of antiretroviral therapy, but the number of transmissions prevented is small.