10/08/08

AIDS in America: More is less?

What’s going on with AIDS in the US? I’ve finally had time to comb through CDC’s estimates of new HIV infections in the US, and I’m frankly little the wiser.

Last week the Centers for Disease Control and Prevention (CDC) suddenly declared that around 55,000 Americans are getting infected with HIV each year (the estimate for the most recent year, 2006, is 56,300). That’s a whopping 40% more people infected each year than CDC previously thought. BUT, says CDC, it doesn’t mean the epidemic is getting worse. For all the years that CDC has trotted out the “40,000 new HIV infections this year” line, and that’s as many years as I can remember, there have actually been another 15,000 or so that didn’t hit the estimates.

In an interview with the wonderous Kaiser Family Foundation, CDC’s HIV epi-boss Kevin Fenton told us not to worry.

“I would like to reassure listeners that the new methods that we have used to calculate incidence are very different to the methods that we use to calculate prevalence and we should not try necessarily to impute one for another.”

Hmm. Because HIV is incurable, there’s a really straightforward relationship between incidence (new infections) and prevalence (total numbers infected at any given point in time). Add up all the incident cases, subtract all the deaths in infected people, and you’re left with the prevalence cases. If you’ve undercounted incidence by 15,000 cases a year for 10 years, then there are 150,000 more people infected with HIV than you previously thought. (Unless people have been dying like flies and you haven’t noticed.) That’s not imputation, that’s arithmetic.

CDC’s last substantial HIV prevalence estimates for the US were based on data up to 2003; it has promised new estimates for a while now, but hasn’t delivered yet. I sympathise with staff who have to try and make national estimates from the mish-mash of data served up by surveillance systems that differ by state (promoters of decentralisation in developing countries take note!). For the record, the new national incidence estimates were extrapolated from data from just 33 states; areas such as California and Washington DC, which contribute disproportionately to the HIV epidemic, do not provide the sort of data that CDC’s Atlanta-based epi-nerds need. No surprises, then, that when the new prevalence estimates do come out they are unlikely to be accompanied by systematic estimates of the number of people in each of the categories at highest risk: gay and bisexual men (who accounted for over half of new infections in 2006), drug injectors and Black Americans with several sex partners. China produces risk population estimates and HIV estimates for every district in the country. So does Indonesia. India, the other country that ranks with the US in the world’s four most populous is trailing, but working on it. Why does the US lag in some of the basics of risk surveillance and HIV prevention planning? (Why it lags in prevention is easier to understand. Even if systems to track risk behaviour and measure infection were delivering the goods, it would be hard to do much about that risk with the money available for prevention: just 4% of federal spending on AIDS within the US went to prevention programmes in 2008.)

CDC’s incidence estimates were broken down only by sex (73% male), risk behaviour (53% gay or bisexual men) and ethnic group (45% black), but not yet by combinations of the above. It is clear that the US has dropped the ball on prevention among gay men. (“Our data suggests that we are seeing sustained increased in HIV incidence in men who have sex with men,” Fenton said). It is equally clear that HIV among Black Americans has been hideously neglected. More on the epidemic in Black America tomorrow.

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This post was published on 10/08/08 in Science.

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