Collaboration between SHM and ECDC to improve estimates of HIV numbers
SHM recently initiated a new project together with the ECDC to develop methods that yield reliable estimates of the number of people with HIV. Such estimates, including those of people as yet unaware that they are infected, are necessary to yield more accurate estimates of trends in new infections and test behaviour within at-risk groups. As part of this project, two methods were examined in detail and developed further. Both have now been extensively tested using data from a number of pilot countries in Europe.
Although HIV has now become a chronic disease, life-long treatment is required to keep the virus under control. Since new HIV patients are diagnosed each year, the number of people on treatment continues to increase. As a result, HIV remains a significant problem for public health care in Europe. To estimate the true extent of this problem, it is important to have reliable estimates of the number of people with HIV, including those who do not yet know they are infected with HIV because they have never, or not recently, been tested. Reliable figures on trends in the number of new infections each year and on the test behaviour within at-risk groups are also extremely important to assess the effectiveness of campaigns to reduce the number of infections.
In 2010, the European Centre for Disease Prevention and Control (ECDC) in Stockholm therefore initiated project to develop methods to yield reliable estimates of the number of people with HIV. Within this project, Stichting HIV Monitoring (SHM) played a leading role, and other partners included University College London, Imperial College in London, the Medical Research Council Biostatistics Unit in Cambridge and the University of Bern.
As part of this project, two methods were investigated in detail and further developed. The first method used the annual number of HIV diagnoses and information about CD4 cell counts to retrospectively calculate the number of infections in preceding years. This method is based on observational cohort data on the rate at which CD4 cell counts drop in HIV patients who have not yet been treated with combination therapy. As well as estimating the annual number of HIV infections, this method also provides an estimate of the time between infection and HIV diagnosis, along with information about the as yet undiagnosed group of HIV-infected people.
The disadvantage of this first method is that it requires extensive historical data, which are not available in all European countries. A second method was therefore developed that does not require historical data, although it does yield less information. The method is based on new HIV diagnoses with AIDS symptoms at the time of diagnosis. Since we know the rate at which HIV-infected people develop AIDS based on their CD4 counts, we can use it to estimate the number of non-diagnosed HIV-infected people with a CD4 count below 350 cells/mm3; in other words, those people who should receive immediate treatment to avoid the onset of AIDS.
Both methods have been extensively tested using data from a number of pilot countries in Europe. These data were taken from the TESSy surveillance database that is managed by the ECDC and with which SHM, in collaboration with the National Institute for Public Health and the Environment (RIVM), shares data from the Netherlands. At present, both methods are being implemented in a user-friendly software package that will be available for general use in Europe during the course of next year.