HIV in numbers
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Hello and welcome to More or Less on the BBC World Service. I'm Ruth Alexander. The news that a baby has been cured of HIV has got us thinking about how the data on HIV cases are gathered. The disease has been one of modern medicine's greatest challenges since it first appeared about 30 years ago, so it seemed encouraging to see that in recent years the number of new cases has been falling. But there's a lot more uncertainty about these figures than the headlines suggest. Wesley Stevenson's been looking at this.
Yes, according to the Joint United Nations Programme on HIV-AIDS, or UNAIDS as it's known, in 2011 there were 34 million people living with HIV, and 2.5 million new cases in that year. That's 20% fewer new infections than was the case 10 years earlier.
So you'd think that tells a simple and strong story. Incident rates are falling, prevention programmes must be working.
But this might not be the entire story. New methods of estimating these figures are being introduced all the time in order to improve the accuracy of these figures, and sometimes this can have a dramatic effect. For example, in 2007, UNAIDS reduced their estimates of the numbers of people living with HIV worldwide by 6 million. Why? I asked Ron Brookmeyer, professor of biostatistics at the University of California in the US.
What does the recent 'cured' baby story tell us about how HIV cases are measured?
Many of the statistics were based on women attending antenatal clinics. Obviously, surveys among pregnant women in antenatal clinics are not representative necessarily. of the whole country, because first, there's no data from that that reflects men. These are women who have been sexually active. They're women in a certain age range. And often there are differences between urban and rural areas who are attending these antenatal clinics. And in fact, what we have seen is when estimates based on antenatal clinic data compared to what comes out of more rigorously designed nationally representative probability-based surveys, is that the prevalence of HIV infection appears to be higher when it's based on the antenatal clinic data than when we look at the nationally representative surveys.
So where are we now? Well, Professor Brookmeyer says there's still uncertainty in the figures.
The reason has to do with the quality and reliability of the statistics that we have to work with. The statistics are based on a combination of... surveys, longitudinal studies and computer models. And the completeness and accuracy of the data sources varies a lot by country.
So along with the past changes to the way these figures are calculated, this adds up to a problem.
Do those changes in the figures reflect real underlying changes in the epidemic, or do they simply reflect changes to new data sources that are coming online, or changes to the inputs into the various computer models? It's, in a sense, comparing apples to oranges.
Now, efforts are made by NGOs such as the World Health Organization and UNAIDS to work these changes backwards and recalculate the figures in the past to say what they would have been had they had the benefit of better models in previous years.
But all the same, should we be suspicious that the falling numbers of new infections are painting an artificial picture of success and are actually down to, not progress in prevention, but changes in the way new cases are counted?
Well, actually, Professor Brookmeyer doesn't think so. He says that although the data are a bit messy, there is other evidence that prevention programmes are making a difference.
We've had a lot of prevention successes, including antiretrovirals for infected persons that decrease their risk of transmitting to others, antiretrovirals for high-risk uninfected persons, which reduce their risk of acquiring infections, increases in testing and counselling and condom usage, circumcision. All of these have been shown in rigorous epidemiological studies and clinical trials to be effective.
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Chapters
4 chapters
1
What is the main topic discussed in this episode?
0:00–1:35
2
What does the recent 'cured' baby story tell us about how HIV cases are measured?
1:35–5:30
3
How has the reported global number of people living with HIV changed in recent years?
5:30–8:17
4
Why did UNAIDS cut its HIV estimates by 6 million in 2007?
8:17–9:26