How many people get HIV?

If you heard my story this morning about HIV infection rates (“How many people get HIV?”), you might have wondered, What’s changed? Why are the estimates different from in the past?

The issue has to do with when people first became infected with HIV. Here’s the problem: Many people walk into a clinic and get diagnosed as HIV-positive long after they were first infected (a year later, or many years later). Therefore, the number of “newly diagnosed HIV cases this year” does not tell you how many people became infected this year.

Maria Courogen, of the Washington Department of Health, explained to me that a new, two-stage HIV test has been developed. The second stage can distinguish between a recent infection and an older infection. When you’re first infected, your body mounts a big immune response, and the number of antibodies spikes way up. After a few weeks, the antibodies begin to subside.

A federally-funded study took this two-stage test to 22 communities across the U.S. and compared how many people were newly-infected vs. infected earlier. They used this data to extrapolate and estimate the number of new infections per year.

It doesn’t change much. For most people, you won’t do anything differently whether there are 570 new cases or 700 new cases per year in Washington state. I think there are two reasons we sat up and paid attention. Because we haven’t heard any really new news about HIV in America And, since the new estimate is in fact an increase, it served as a reminder that the epidemic isn’t going away.