Background
Long acting preexposure prophylaxis (PrEP) can address periods of partial adherence among individuals using oral PrEP. Using PrEP dispensing records, we determined the proportion of partially adherent men-who-have-sex-with-men (MSM) in Thailand who return within ten weeks and could benefit from cabotegravir as long-acting PrEP (CAB-LA). We then used a mathematical HIV transmission model to study the epidemiological impact of CAB-LA in Thai MSM.
Methods
PrEP dispensing records were analyzed from 12 key population-led clinics in Thailand (2019-2022). We included individuals whose first visit was at least one year before the last recorded visit in 2022. For those who received three or more PrEP prescriptions, we calculated the ratio of days covered by PrEP (assuming daily use) to the total days between prescriptions. A ratio >75% indicated high adherence to oral PrEP (effectiveness of 93%), and a ratio <75% was classified as partial adherence (effectiveness 60%, Jourdain Lancet Public Health 2022). Individuals who returned within 10 weeks were assumed protected by CAB-LA (34% higher effectiveness). Based on literature, we modeled the tail phase of CAB-LA, estimating reduced effectiveness of 0-26%, 49-76%, and 82-95% during the first, second, and third 12-week periods following the last CAB-LA injection. An HIV transmission model was calibrated to the Thai MSM HIV epidemic, including a discrete choice experiment showing that 14% prefer CAB-LA. As the number of PrEP users is unknown, we estimated this population to range between 10,000 and 100,000 in 2024-2034.
Results
A number of 34,843 PrEP prescriptions were dispensed to 7,257 MSM, with common discontinuations (45% did not return within six months after start). Among individuals filling at least 3 prescriptions, 52.6% had periods of partial adherence to oral PrEP that could not be covered by CAB-LA, 14.0% had periods of partial adherence to oral PrEP that could be covered by CAB-LA, and 33.0% were consistently protected by both PrEP modalities. Compared to a scenario when only oral PrEP is available, adding CAB-LA will avert between 323 (0.7%, 10,000 users in 2024-2034) and 1316 (4.4%, 100,000) new infections in the next decade. The incidence of HIV in 2034 is expected to decrease in the next decade by 1.3% and 9.1% for 10,000 and 100,000 PrEP users, respectively.
Conclusions
Although discontinuation of PrEP is a challenge, offering both CAB-LA and oral PrEP as HIV prevention options can further reduce HIV incidence.