HIV surveillance data among Men who have Sex with Men (MSM) and Injection Drug Users (IDU)
Surveillance is the foundation of the nation’s HIV prevention efforts. It is critical to have accurate population estimates in order to better understand the HIV burden among risk groups as well as to make a substantial impact and save lives through better HIV prevention program planning. Without census estimates available for the number of injection drug users (IDU) or men who have sex with men (MSM) in the U.S., it has been difficult to quantify disease rates and disparities in disease rates among IDU and MSM compared to their population sizes in the same way we do for other factors such as sex, race/ethnicity, and age.
While there have been a variety of methods and estimates of the population sizes of these groups, CDC was interested in developing single estimates to use in interpreting HIV surveillance data. Because meta-analysis quantitatively combines estimates from multiple studies, it provides a strong basis for a national estimate of the proportions of MSM and IDU in the U.S. CDC recently published estimates of the populations’ sizes of MSM and IDU in the U.S. using meta-analysis of multiple national, population-based surveys.
Since the beginning of the HIV epidemic in the United States, men have accounted for the largest proportion of cases, and the large majority of those men have been gay, bisexual, and other MSM. In 2010, MSM accounted for 63% of all new HIV infections, and MSM with a history of injection drug use (MSM-IDU) accounted for an additional 4% of new infections and IDU (male and female) accounted for approximately 9% of all new infections. At the end of 2009, an estimated 56% of persons living with an HIV diagnosis in the U.S. were MSM or MSM-IDU and 16% were IDU.
Estimates for MSM and IDU populations
Using meta-analysis with behavioral data from population-based surveys, CDC found that men who engaged in same-sex behavior in the past five years comprised 3.9% (3.5 – 4.4) of the U.S. adult male population. Men who recall ever engaging in same-sex behavior comprised 6.9% (5.1-8.6) of the U.S. adult male population. In a recent publication, CDC used the 5-year estimate as a conservative, mid-range estimate of the size of the MSM population. Compared to other men and women, HIV rates among MSM were 38 to 75 times as high, and syphilis rates were 63 to 109 times as high.
Similar to the MSM population size estimate, CDC conducted a meta-analysis of behavioral data from national surveys to estimate the number of persons in the U.S. who have injected drugs to use as a denominator to calculate HIV diagnosis and prevalence rates for IDU.
The meta-analysis estimated that lifetime IDU comprise 2.6% (95% confidence interval (CI): 1.8% – 3.3%) of the U.S. population age 13 years or older; the past-year IDU estimate was 0.30% (95% CI: 0.19 % – 0.41%). Comparing black IDU to white IDU, the rate of HIV diagnosis was nearly 15 times as high; comparing Hispanic/Latino to white IDU, the rate was more than 9 times as high.
CDC continues to work on developing national estimates of population sizes for other transmission categories (MSM/IDU and heterosexual contact). Given the need for comparability across groups, it is expected that disease rates by transmission category will be calculated based on the ‘ever’ behavioral recall period for all groups, including MSM.
National estimates of population sizes can be used to provide a broader understanding of the HIV epidemic among those at risk for transmission and acquisition of HIV. These estimates and disease rate calculations also provide important tools for monitoring and characterizing the HIV epidemic in the United States as well as planning and optimizing the allocation of resources to programs serving disproportionately affected populations and addressing health inequities. As such, they can be helpful in implementing and monitoring the National HIV/AIDS Strategy.