Disparities in HIV

Have you ever wondered why 67% of all people infected with HIV live in Sub-Saharan Africa? Why is the disease such an epidemic there and not in the US? Interestingly, despite being only 13% of the US population, African Americans bear nearly 50% of the disease burden in the United States and as a group rank #16 in the world for prevalence of HIV (CDC fact sheet).


Why is there such a large disparity? Many studies have shown that traditional risk factors (numbers of sexual partners, use of alcohol and sex, smoking, IV drug use, men who have sex with men, etc.) do not account for differences in HIV and other sexually transmitted infections (STIs) between African Americans and whites even after adjusting for baseline characteristics (age at first intercourse, sex, education, marital status, etc.), but this begs the question "Why?"

Some suggest there are genetic differences that may account for the higher prevalence of disease but this theory is quickly disproved as the prevalence of a variety of sexually transmitted pathogens (viruses, bacteria, protozoans) show the same qualitative disparities (similar distribution patterns and transmission networks) for both groups. In addition, variations of STI prevalence within and between African countries suggest no common genetic link. The the rapid, dramatic declines in the disparities for curable STIs are clearly due to changes in behavior or the environment, and they show how powerful these effects can be. Biological differences suggesting the STIs themselves are the reason are also not likely related to this difference since clinical trial results regarding the impact of STI treatment on HIV incidence have been disappointing.

Racial disparities in HIV and other STIs cannot be explained by differences in behavior but that does not mean that there is no behavioral basis for these disparities, just that it is not adequately captured at the individual level. On the population level, epidemic effects operate though networks that can only be seen when individual data is linked though a simulation program, such as the one referenced below). The theory of concurrency, even when it is low grade (having two sexual partners rather than one) can rapidly connect a population and generate overall connectivity that is similar in magnitude to but more robust than that generated by super spreaders or core groups (sex workers or promiscuous individuals).

This 5-minute video demonstrates the relationship of concurrency (number of current sexual partners) with the reachability of a disease to spread to other individuals in the network.



Concurrency and Reachability video at statnetproject.org.

Background: Squares are men, circles are women, and the lines mean sexual relationship. Concurrency means a relationship that is not monogamous at any given time. At the start of the video, the middle 10 are the HIV infected individuals who in theory will pass the disease on with every sexual contact (beta of 1, which we can argue about later). Red lines mean that one of individuals was infected (with HIV) and blue means mutually monogamous.

In conclusion, the theory of concurrency (individuals having more than one sexual partners) is an alternative way to think about risk groups and risk behaviors and may lead to a more productive understanding of HIV transmission, disparities, and prevention and why Uganda and African Americans in the US have an HIV epidemic. The sexual network perspective is not only an individuals behavior that defines his or her risk; it is his or her partner's behavior and (ultimately) his or her position in a sexual network. This is demonstrated by the "trees" at the end of the video: 1. in each generation more people are infected and 2. more generations are created since the velocity has been increased in these trees. A small reduction in concurrent relationships, not necessarily promoting abstinence but simply monogamy in a relationship, can have a dramatic effect on transmission rates (as well as condom use and circumcision but I'll leave that for another blog entry).

Thanks for a great talk Dr. Martina Morris! This was one of the most interesting talks I've ever had.

Reference: Link to Morris et al. (2009) Journal of Public Health article.