County- and State-level Estimates of Population Sizes of Men Who Have Sex With Men Across the United States.

Background: Accurate estimates of the population sizes of men who have sex with men (MSM) are essential for evaluating HIV-related interventions. We aimed to estimate MSM populations at county and state levels using recent data from the decennial census and the National Health and Nutrition Examination Survey. Methods: We used 2020 Decennial Census data to calculate a weight for each U.S. county, reflecting the proportion of male-male partner households relative to counties of similar urbanicity based on the 2013 National Center for Health Statistics Urban-Rural Classification Scheme. We applied these weights to urbanicity-stratified estimates of the prevalence of adult men who reported sex with a man in the past 12 months, derived from National Health and Nutrition Examination Survey (2015-2020). Multiplying these percentages by adult male populations produced county estimates, which were aggregated to state levels. Results: We estimated approximately 2.1 million MSM in the United States, representing 1.7% of adult males. State-level estimates ranged from 0.3% in Wyoming (n = 675) to 3.7% in the District of Columbia (n = 9709). California had the largest MSM population (n = 318 612; 2.1%), followed by Florida (n = 191 199; 2.3%) and Texas (n = 173 751; 1.6%). At the county level, Los Angeles County, CA, had the largest MSM population (n = 82 513; 2.1%), followed by Cook County, IL (n = 39 580; 2.0%), and Broward County, FL (n = 34 321; 4.7%). Broward County, FL (4.7%), and San Francisco County, CA (4.6%), had the highest proportions relative to their male populations. Conclusions: County- and state-level MSM estimates provide crucial denominators for calculating disease rates and informing public health interventions.