The amount of HIV infection among people over 50 is “surprisingly high,” World Health Organization (WHO) officials say, and despite much speculation about why, there’s little definitive information that might shed light on the trend.

HIV prevalence and incidence in the over–50-year-olds seem surprisingly high and the risk factors are totally unexplored,” according to an editorial in the March Bulletin of the World Health Organization. “Understanding the epidemiology of HIV infection in older individuals can lead to interventions to make these years safer and more enjoyable.”

In 2005 people 50 and older accounted for 15 percent of the new HIV/AIDS diagnoses in the U.S. and 24 percent of people living with the virus, according to the U.S. Centers for Disease Control and Prevention (CDC). These figures are up 17 percent since 2001.

That proportion of cases in the 50-something demo is "surprisingly high," says editorial co-author George Schmid, an epidemiologist in WHO's HIV/AIDS branch, because 50-somethings aren’t believed to be at much risk, especially from sex. Whereas a few 50-somethings are contracting HIV in middle age, in the U.S. prevalence has grown in the over-50 demographic partly because people who were infected earlier in life are surviving into middle age, CDC data shows. Schmid didn’t say whether that would also explain the increase in developing countries, where antiretroviral drugs credited with extending the lives of HIV patients are harder to come by.

"To achieve these 'surprisingly high' prevalences, there must be a fair bit of sexual activity, more than we think," Schmid tells "But interest in sex and sexual activity decline for both sexes as we age. Thus, to achieve these 'surprisingly high' prevalences and assuming they are from sex, there must be a relatively high transmission efficiency, for example, thinning of vaginal mucosa. And, studies have shown that older individuals are less likely to use safer sex practices." 

All of that said, the editorial doesn’t distinguish between people who are catching HIV in their 50s and those who were infected when they were younger and are surviving into middle age, making it tough to understand why the proportion of cases in that age group is more than experts originally imagined. "We do not know when these people became infected," Schmid concedes.

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