Sexual Behavior, Risk Perception & HIV Transmission Can Respond to HIV Antiviral Drugs & Vaccines through Multiple Pathways

There has been growing use of highly active antiretroviral treatment (HAART) for HIV and significant progress in developing prophylactic HIV vaccines. The simplest theories of counterproductive behavioral responses to such interventions tend to focus on single feedback mechanisms: for instance, HAART optimism makes infection less scary and thus promotes risky sexual behavior. 

Here, we develop an agent based, age-structured model of HIV transmission, risk perception, and partner selection in a core group to explore behavioral responses to interventions. We find that interventions can activate not one, but several feedback mechanisms that could potentially influence decision-making and HIV prevalence. In the model, HAART increases the attractiveness of unprotected sex, but it also increases perceived risk of infection and, on longer timescales, causes demographic impacts that partially counteract HAART optimism. Both HAART and vaccination usually lead to lower rates of unprotected sex on the whole, but intervention effectiveness depends strongly on whether individuals over- or under-estimate intervention coverage. Age-specific effects cause sexual behavior and HIV prevalence to change in opposite ways in old and young age groups. 

For complex infections like HIV-where interventions influence transmission, demography, sexual behavior and risk perception-we conclude that evaluations of behavioral responses should consider multiple feedback mechanisms.

Below: Baseline scenario with no interventions: (a) number of HIV—(green) and HIV+ (red) individuals in the population; (b) averageb_t-value for HIV—individuals by age, 15-20-year-olds (red), 20–30 (orange), 30–40 (yellow), 40–50 (green), and 50+ year-olds (blue); (c) average b_t-values for HIV+ individuals by same age groups; (d) number of HIV+ individuals by the same age groups, and with black representing the total number of HIV+ individuals; (e) total average b_t-value for HIV+ (red) and HIV—(green) populations; (f) HIV prevalence (percentage of the population currently infected).

Below:  Figure 2: Baseline scenario with no interventions. (a) total number of −/+ US acts (number of unprotected sex acts between HIV− and HIV+ pairs) per year divided by number of HIV—individuals in that age cohort at the end of the year, for age groups 15–20-year-olds (red), 20–30 (orange), 30–40 (yellow), 40–50 (green), and 50+ year-olds (blue), and cumulative number of individuals across all age groups (black); (b) total number of −/+ US acts per year, for same age groups; (c) number of HIV—individuals in each age cohort at the end of the year, for same age groups.

Below:  Figure 3: Baseline scenario for HAART intervention: (a) number of HIV− (green) and HIV+ (red) in the population; (b) average b_t-value for HIV− individuals by age, 15–20-year-olds (red), 20–30 (orange), 30–40 (yellow), 40–50 (green), and 50+ year-olds (blue); (c) average b_t-values for HIV+ individuals by same age groups; (d) number of HIV+ individuals by the same age groups, and with black representing the total number of HIV+ individuals; (e) total average b_t-value for HIV+ (red) and HIV− (green) populations; (f) HIV prevalence (percentage of population currently infected).

Below:  Figure 4: Baseline scenario for HAART intervention. (a) total number of −/+ US acts (number of unprotected sex acts between HIV− and HIV+ pairs) per year divided by number of HIV—individuals in that age cohort at the end of the year, for age groups 15–20-year-olds (red), 20–30 (orange), 30–40 (yellow), 40–50 (green), and 50+ year-olds (blue), and cumulative number of individuals across all age groups (black); (b) total number of −/+ US acts per year, for same age groups; (c) number of HIV—individuals in each age cohort at the end of the year, for same age groups.

Below:  Figure 5: Baseline scenario for vaccine intervention. (a) number of HIV− (green) and HIV+ (red) in the population; (b) average bt-value for HIV− individuals by age, 15–20-year-olds (red), 20–30 (orange), 30–40 (yellow), 40–50 (green), and 50+ year-olds (blue); (c) average bt-values for HIV+ individuals by same age groups; (d) number of HIV+ individuals by the same age groups, and with black representing the total number of HIV+ individuals; (e) total average bt-value for HIV+ (red) and HIV− (green) populations; (f) HIV prevalence (percentage of population currently infected).

Below:  Figure 6 Baseline scenario for vaccine intervention. (a) total number of −/+ US acts (number of unprotected sex acts between HIV− and HIV+ pairs) per year divided by number of HIV—individuals in that age cohort at the end of the year, for age groups 15–20-year-olds (red), 20–30 (orange), 30–40 (yellow), 40–50 (green), and 50+ year-olds (blue), and cumulative number of individuals across all age groups (black); (b) total number of −/+ US acts per year, for same age groups; (c) number of HIV—individuals in each age cohort at the end of the year, for same age groups.

Full article at: http://goo.gl/kbKoKB

By: Tully S1, Cojocaru M1, Bauch CT1,2.

• 1Department of Mathematics and Statistics University of Guelph, 50 Stone Road East, Guelph, ON, N1G 2W1 Canada.
• 2Department of Applied Mathematics University of Waterloo, 200 University Avenue West, Waterloo, ON, N2L 3G1 Canada.  

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