I have an article in the current issue of The New Republic about how the AIDS epidemic has been more severe in the United States than other developed countries. When I started my reporting, I assumed that the experience of AIDS was basically the same in Western Europe, Australia and North America, and I was all primed to write a little dispatch about how each country’s different reporting and surveillance systems just made it look different.
But that wasn’t what I found. The differences in the epidemic are real. AIDS arrived in America earlier than other countries, spread faster and has lingered longer. History, demographics, and politics account for the difference better than methodological quirks do.
But that doesn’t mean the methodology doesn’t matter. AIDS is a complex disease—easy to contract, invisible to spread, expensive to treat—and each country has a slightly different way of tracking the virus’s transmission through its population. The question became how—and whether—we can make comparisons between countries in spite of these differences.Why Did AIDS Ravage the U.S. More Than Any Other Developed Country?
The first thing you notice about HIV statistics is how slippery they are. The Centers for Disease Control and Prevention’s AIDS surveillance says there were 46,268 diagnoses of HIV in 2010. The online Atlas provided by the CDC’s National Center for HIV/AIDS says there were 46,043.
It’s the same in Europe. Each country reports its own HIV statistics independently, then they’re gathered and re-reported by the European Centers for Disease Control. The Robert Koch Institute (Germany’s equivalent of the CDC) says 3,034 people were infected with HIV in Germany in 2008. The ECDC says it was 2,850.
These discrepancies are understandable given the nature of the virus and the systems we use to track it. Every country has its own national surveillance system. In the U.S. and the U.K., clinics that provide HIV testing and care sends their numbers up the chain (“we diagnosed five people with HIV this month, three of them were gay men, two were drug users…”). Germany, by contrast, doesn’t track every clinic, just a sample, and produces the nationwide figures by adjusting them for population every year.
Because people are often diagnosed with HIV years after they’re infected—if at all—and because it takes time for numbers to arrive at the central authorities, the statistics are constantly being updated (“backcalculated,” which sounds cooler, is what the epidemiologists call it) to reflect the number of infections, as opposed to the number of diagnoses, that occurred in any given year.
It gets even trickier when it comes to counting AIDS deaths. An “AIDS death,” as defined by the U.S. and the U.K., is “someone with AIDS who dies,” whether they die from AIDS-induced pneumonia or are hit by a bus. People who are never diagnosed with AIDS, who die in the up-to-10-years-long incubation period, aren’t included in the numbers. Germany only counts deaths that are actually caused by AIDS, but they have the same problem of non-diagnosis. All three countries produce estimates on top of the raw case data.
So how do they know how many people have HIV but aren’t diagnosed? Every once in a while, epidemiologists take 10,000 or so random blood samples and test them all. In the U.S., the U.K., and Germany, it turns out an estimated 20 percent of people infected with HIV don’t know it.
Even AIDS itself has a different definition depending on where you’re standing. In the U.S., AIDS is someone whose little virus-fighters, their CD4 cells, are below 200 per cubic mm of blood (normal is 500-1,500) or who is already showing symptoms. The WHO definition is anyone with a CD4 count below 350. In most European countries, you need to have an AIDS-related infection to be diagnosed with AIDS.
So there are a lot of good reasons why comparing the AIDS epidemic between countries isn’t perfect. The numbers keep getting tweaked and updated, and the assumptions and definitions plugged into them differ by time and place. Epidemiologist after epidemiologist tells me that comparing any statistic directly to another is probably unfair. You need to think of each number as a range, a window that takes into account the vicissitudes of measuring an invisible disease with differently calibrated surveillance systems, much of it before digital record-keeping. UNAIDS, which has data for every country from Andorra to Zimbabwe, simply rounds all the numbers to the nearest hundred and puts a ‘circa’ (~) at the beginning of them.
Caroline Sabin, a professor of medical statistics and epidemiology at University College London and my sherpa through these methodological Himalayas, is the first to tell me not to make apples to apples comparisons. ‘It’s more like Red Delicious to Granny Smith,’ she says.
So why did I just spend a whole article comparing these numbers to each other?
The first reason I feel comfortable doing this is the consistency and scale of the differences between countries. In 2010, the lowest reported number of AIDS deaths in the United States was 15,529. The highest estimate of German AIDS deaths was 550. The rates per 100,000 population are 6.1 and 0.5, respectively. Those numbers have been steady since the early 2000s.
The three countries under comparison—Britain, Germany and the United States—have functioning governments and dedicated systems for tracking and reporting statistics. Their epidemiologists attend the same conferences, they apply the same statistical methods and rigor to their back-calculations. The calibration and strength of these systems differ, I’m sure, but we’re not comparing Monaco and Malawi here. Even accounting for the different methodologies, and accepting that each number is a range rather than a precise figure, no one gave me any reason to believe that the British and German systems are under-estimating their AIDS statistics by hundreds of percentage points.
It’s also worth noting that the differences in the epidemic show up beyond the national surveillance systems. Sabin points me to cohort studies, surveys of people living with HIV, that also found higher death rates in the U.S. than the rest of the rich world. In the story I mention one that reported mortality rates in the United States four times those in Europe, and roughly equal to South Africa.
A 2013 review comparing the AIDS epidemic across the 33 richest countries in the world—the U.S., the E.U., Japan, etc.—found that one-third of people living with HIV in the developed world were in the United States. This is the story the data seems to tell, no matter how you ask it.
“The scale of the epidemic in the U.S. is much greater,” Sabin told me. “We can’t say precisely by how much, but that’s what the numbers seem to say.”