A focused multi-state model to estimate the pediatric and adolescent HIV epidemic in Thailand, 2005-2025

Abstract

Background: We estimated the magnitude of the HIV epidemic among children and youth living with HIV (CYHIV) aged 0-25 years in Thailand, projecting forward from 2005 to 2025, and identified underreported input parameters that influence epidemic projections, in order to inform future public health and research priorities.

Methods: We developed a focused multi-state transition model incorporating perinatally-acquired HIV and non-perinatally-acquired HIV, stratified by population, including men who have sex with men (MSM), female sex workers (FSW), people who inject drugs (PWID), and the remainder of the population ("other"). We populated the model with published and programmatic data from the Thai national AIDS program when available. We projected the period from 2005-2025 and compared model results to programmatic data and projections from other models. In a scenario analysis, we projected the potential impact of pre-exposure prophylaxis (PrEP) for MSM from 2018-2025.

Results: The initial 2005 cohort was comprised of 66,900 CYHIV; 8% CYHIV were <5 years, 21% were 5-14 years, and 71% were 15-25 years of age. By 2020, 94% were projected to be >15 years and infections among MSM constituted 83% of all new HIV infections. The numbers of CYHIV decreased over time, projected to reach 30,760 by 2020 (-54%) and 22,640 by 2025 (-66%). The proportion of all CYHIV aged 0-25 who were diagnosed and on ART increased from 37 to 60% over the 2005-2025 period. Projections were sensitive to variations in assumptions about initial HIV prevalence and incidence among MSM, PWID, and "other" youth.

Conclusions: More data on incidence rates among sexual and gender minority youth and PWID are needed to characterize the role of specific exposures and key populations in the adolescent HIV epidemic. More accurate estimates will project shifts in population and inform more targeted interventions to prevent and care for Thai CYHIV.

Fig 1. Model diagram.

(1) At the beginning of each year i, those already in care the previous year $V_{j,k}^0$ have a probability of becoming lost-to-follow-up (pLTFU(i)) for each age j and population k. Those who were LTFU the previous year Z_j,k, can return to care (pRTC(i)). (2) Among the population at risk Y_j,k(i), newborns have a probability pMTCT(i) to acquire HIV perinatally (X₀), and children and youth have a probability pI_j,k(i) of acquiring non perinatal HIV (C_j,k). (3) Those who have never accessed care (new infections and those already known as infected but not in care), Z_j,k, have a probability of being diagnosed pDIAG_j,k(i) and subsequently accessing ART pART_j,k(i). (4) Survival rates ($S_k^{care}$ and $S_k^{nocare}$) are applied to each cohort. Following these steps, children and youth age up into the next year and a new cycle begins. Those who age >25 years exit the model. CYHIV: children and youth living with HIV, HEU: HIV-exposed, uninfected infants, CYPHIV: children and youth living with perinatally-acquired HIV, YNPHIV: youth living with non-perinatally-acquired HIV, MTCT: mother-to-child transmission, ART: anti-retroviral therapy.

Fig 2. Model-projected distribution of children and youth living with HIV by age group, 2005–2025.

The vertical axis shows the proportion of all CYHIV in Thailand who fall into each age category in each modeled calendar year. The horizontal axis shows each calendar year from 2005 to 2025. Children and youth aged 0–4, 5–9, 10–14, 15–19, 20–24, and 25 years are shown in different shades of blue and green.

Fig 3. ART coverage among CYHIV aged 0–25 years in Thailand, 2005–2025.

The vertical axis shows the proportion of all CYHIV aged 0–25 years in Thailand who fall into each category of ART coverage: on ART (green), returned to care (blue), stopped ART (yellow), and never initiated ART (red). The horizontal axis shows each calendar year from 2005 to 2025. Each panel represents either all CYHIV (A), only CYPHIV (B), or only CYNPHIV at the age of 13–25 years (C). ART: antiretroviral therapy, CYHIV: children and youth living with HIV, CYPHIV: children and youth living with perinatally acquired HIV, CYNPHIV: children and youth living with non-perinatally acquired HIV.