Calculating Disease Rates for Risk Groups: Estimating the National Population Size of Men Who Have Sex with Men

Background: CDC has not previously calculated disease rates for men who have sex with men (MSM) because there is no single comprehensive source of data on the size of this population. For informing HIV and STD prevention planning, CDC is now developing a national population size estimate.

Objectives: Estimate the population size of MSM in the United States to calculate HIV and syphilis rates and compare these rates with rates for other men and women.

Methods: We conducted a systematic literature search and identified seven unique surveys that provided data on same-sex behavior in nationally representative samples of men. Data were pooled by recall period and combined using meta-analytic procedures. We applied the proportion of MSM to U.S. census data to produce a population size estimate. We calculated disease rates using 2007 diagnoses from CDC data systems and the corresponding Census data.

Results: Estimates of the proportion of men who engaged in same-sex behavior differed by recall period: past year = 2.6% (95%CI, 2.2-2.9); past five years = 4.0% (2.8-5.3); ever = 7.0% (4.7-9.2). Based upon the five-year recall period, we calculated an HIV diagnosis rate of 692/100,000 and a syphilis rate of 121/100,000 for MSM in 2007. For HIV and syphilis, respectively, the rate was 60 and 61 times the rate for other men and 54 and 93 times greater than the rate for women.

Conclusions: Estimating the population size for MSM allowed us to calculate the HIV and syphilis rates for this risk group, which greatly improves our understanding of the disproportionate impact of these diseases among MSM nationally.

Implications for Programs, Policy, and/or Research: Understanding the size of risk groups provides important information for assessing disproportionate disease impact and ensuring that prevention efforts and resources are directed to groups most in need.