HIV surveillance, US style: don’t try this at home

For a world-renowned centre of epidemiological excellence, the US CDC can do some pretty shonky work. This week, the second week of April, 2009, they have finally published some results from surveillance among drug injectors carried out more than three years ago. And the analysis is so simplistic that it tells us virtually nothing about HIV-related risk.

Leave aside the fact that they used Respondent Driven Sampling (RDS), a fashionable but deeply unsatisfactory method that could more honestly be named GSS — Glorified Snowball Sampling. [An aside for nerds: if you want to know why RDS is a bad idea even for drug injectors read this fabulous paper by Greg Scott. You’ve just got to love the young lady who has organised a training scheme to help non-injectors fool study staff into letting them participate. Ah, the land of enterprise…] Leave aside the fact that they inexplicably excluded from analysis the 8% of the sample who knew they were HIV infected. (This is the agency that invented Positive Prevention, for God’s sake. They ought to be supremely interested in the ongoing risk behaviours of people who are already infected.) Leave aside the three year time lag between collection of routine public health surveillance data and publication of the results –a delay that borders on the unethical. Even if those problems didn’t exist, you’d still have almost nothing of interest to write home about, because the analysis is so uninformative.

The headline figure is needle sharing — close to a third of injectors who think they are HIV negative used a needle that might already have been used by another injector at some time in the preceding 12 months. We’re talking about smack and crack users here, people who inject toxic substances into their bodies perhaps as often as three times a day. In most countries, we ask injectors to think back over the last week, and even then we get some dodgy results. How US injectors can remember what they did 11 months earlier I can’t imagine.

Then there’s information about sex. For some reason, they only report vaginal sex, although anal sex between men carries a far higher risk of HIV infection in the United States than any other activity. We’re told, wide-eyed, that 62% of injectors had unprotected vaginal sex in the last year, and that 42% had more than one partner in that time. But we don’t know if the sex with multiple partners was protected, we don’t know whether the unprotected sex was with injecting partners. Despite the CDC’s pronouncement that “a substantial proportion of IDUs are at risk for acquiring HIV infection through their sexual behavior in addition to their drug use”, we actually have no way of knowing whether the sex added to their risk or not.

I could rant on with more examples of missed opportunities for useful analysis, but I guess I’ve made my point. I do want to congratulate CDC, though, on their “honesty box”. They don’t try to fudge data loss and missing values:

“During May 2005–February 2006, a total of 13,519 persons were recruited to participate; of these, 1,563 (11.6%) were found ineligible, and 46 had missing recruitment information. Among the remaining 11,910 participants, data for 1,609 were excluded: 881 who already knew they had HIV infection, 334 whose data were lost during electronic upload, 288 initial participants (whose responses were excluded as part of the respondent-driven sampling methodology), 68 who could not be identified as either male or female, and 38 who gave responses with questionable validity.”

Interesting that CDC doesn’t allow for transgender injectors. Interesting that they provide “technical assistance” on HIV surveillance to countries such as Cambodia and Vietnam who have in the past made a far better fist of the work than CDC does at home.

This post was published on 14/04/09 in Science, War on drugs.

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  1. Comment by Doreen Hardy, 14/04/09, 04:55:

    I was equally disheartened to see that they are still only counting vaginal sex, because both men and women engage in anal sex which has long been known to be the riskiest means of sexual transmission. Until anal sex is included in their surveillance, do any of these numbers mean anything? Is there data that teases apart transmission from used needles and anal sex? The CDC is too big to be nimble with data. I can’t help wondering where the 334 participants who vanished on upload landed.

  2. Comment by Anonymous, 19/04/09, 04:15:

    “Glorified Snowball Sampling”? As in
    en.wikipedia.org/wiki/Snowballing_(sexual_practice) ??
    My goodness, what won’t these statisticians get up to next.

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