HIV Epidemic in London spread quickly and in Clusters

The HIV/AIDS epidemic in London in the late 1990s was driven by transmission of the AIDS virus within clusters of sexual contacts in short periods of time, according to a study published in the open-access journal PLoS Medicine in March 2008.

To determine transmission patterns for sexually transmitted diseases, scientists sometimes construct sexual contact networks among the infected individuals. The information about these sexual contact networks can additionally be used for various health initiatives, including identifying, treating, and advising potentially unknowingly infected individuals. The information on the network structures can also be used to implement community-based prevention strategies.

Traditionally, sexual contact networks are constructed by interviewing infected persons. However, this may not be completely appropriate when studying HIV infection for several reasons. Namely, the period of transmission is long, and the risk of transmission from a single sexual contact is relatively low.

To learn more about the HIV/AIDS epidemic in London in the 1990s, Andrew Leigh Brown and colleagues at the University of Edinburgh and London's Chelsea and Westminster Hospital examined sexual contact networks among men who have sex with other men. However, rather than the traditional oral method of collecting information, they used phylogenetics to examine the level of genetic relation between the viruses obtained by different individuals.

Collecting genetic data on HIV in individual patients is a part of determining an effective treatment regimen, so the scientists were able to compare the sequences of genes in HIV from over 2,000 patients, largely men who have sex with men, who attended a large HIV clinic in London between 1997 and 2003. The analyzed sequences showed 402 that very closely matched at least one other viral sequence in the group. After more extended analysis, it was found that patients whose viruses matched largely arranged into six clusters each with ten or more individuals, with additional smaller clusters outside of these.

Then, based on the time of sample collection, and using information about what rates changes occur in the viral genes, the scientists found dated phylogenies, or family trees, for the clusters. Most of the transmissions within each cluster occurred in 3-4 year periods in the late 1990s, with at least one in four transmissions in the large clusters occurred within 6 months following the infection of the transmitting partner.

The results indicate that the growth of the HIV epidemic in London among men who have sex with men often occurred in short, rapid episodes rather than slowly over a longer time frame. Frequently, it seems that individuals passed on the virus to others just months after becoming infected themselves. This implies that transmission of the virus in the early stages of HIV likely drove the epidemic forward considerably.

It is likely, but not yet proven, that these results apply generally to HIV infections in larger populations of men having sex with men. If this is confirmed in additional studies, then this quantitative description of HIV transmission using phylogenetics can help design future strategies to strengthen HIV prevention in this population.

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