Changing Dynamics of HIV Transmission in Côte d'Ivoire: Modeling Who Acquired and Transmitted Infections and Estimating the Impact of Past HIV Interventions (1976-2015)

Abstract

Introduction: Understanding the impact of past interventions and how it affected transmission dynamics is key to guiding prevention efforts. We estimated the population-level impact of condom, antiretroviral therapy (ART), and prevention of mother-to-child transmission activities on HIV transmission and the contribution of key risk factors on HIV acquisition and transmission.

Methods: An age-stratified dynamical model of sexual and vertical HIV transmission among the general population, female sex workers (FSW), and men who have sex with men was calibrated to detailed prevalence and intervention data. We estimated the fraction of HIV infections averted by the interventions, and the fraction of incident infections acquired and transmitted by different populations over successive 10-year periods (1976-2015).

Results: Overall, condom use averted 61% (95% credible intervals: 56%-66%) of all adult infections during 1987-2015 mainly because of increased use by FSW (46% of infections averted). In comparison, ART prevented 15% (10%-19%) of adult infections during 2010-2015. As a result, FSW initially (1976-1985) contributed 95% (91%-97%) of all new infections, declining to 19% (11%-27%) during 2005-2015. Older men and clients mixing with non-FSW are currently the highest contributors to transmission. Men who have sex with men contributed ≤4% transmissions throughout. Young women (15-24 years; excluding FSW) do not transmit more infections than they acquired.

Conclusions: Early increases in condom use, mainly by FSW, have substantially reduced HIV transmission. Clients of FSWs and older men have become the main source of transmission, whereas young women remain at increased risk. Strengthening prevention and scaling-up of ART, particularly to FSW and clients of female sex workers, is important.

Figure 1. Observed (points, mean with 95% confidence intervals) and predicted HIV prevalence in A) the general population (15-49 years old, and B) among female sex workers in Côte d'Ivoire

In A) FSW are not assumed to be sampled in the household-based HIV prevalence surveys and were excluded in calculating model-based overall prevalence estimates. Incremental Mixture Importance Sampling (IMIS) resulted in 41,146 unique sets of parameters labelled samples from posterior (grey curves). One thousands of these curves were resampled with probability proportional to their importance weights to derive the median (red) and 95% credible intervals (shaded blue area).

Figure 2. Historical trends in condom use and estimated averted fractions (AF) of HIV infections over 1987-2015 in Cote d'Ivoire, according to different counterfactual scenarios

The proportion of sex acts protected by condom estimated from data (Table S5) is shown for A) the general heterosexual population excluding women engaged in sex-work (displayed trends are a weighted average of each age-class specific trends), B) female sex workers (FSW), and C) men who have sex with men (MSM). Dashed horizontal lines represented the three counterfactuals scenarios of condom use trends assumed to derive the AF, corresponding to constant levels from 1987, 1990, and 1993 onward. Panel D) shows AF estimates of increases in condom use by all risk groups for each counterfactual. In panel E) AF due to condom use increases in specific risk group are presented (general heterosexual population not engaging in sex work only, by FSW only, and by MSM only). For these, each AF estimates assumed that condom has not increased in the other risk groups (i.e. condom use remained at their counterfactual levels). Median and 95% credible intervals of the posterior estimates are shown.

Figure 3. Predicted coverage and population-level impact of antiretroviral therapy (ART) and of prevention of mother-to-child (PMTCT) activities in Cote d'Ivoire during 2000-2015

Panels shows A) coverage of ART, B) coverage of PMTCT and C) averted fractions (AF) of all new HIV infections among adults due to scaling-up of ART, and D) averted fractions of new infants infections due to PMTCT. AFs were calculated over the following time periods: 2000-2005, 2005-2010, 2010-2015, and 2000-2015. Medians and 95% credible intervals of the posteriors are presented. The counterfactual scenarios used to estimate the AF assumed the absence of ART and PMTCT interventions.