Behavioral, Biological and Structural Components of MSM STI Morbidity Steven Goodreau and Matthew Golden University of Washington CFAR

HIV/AIDS in MSM 2002 Diagnoses of HIV/AIDS, by exposure category Source: CDC HIV/AIDS Surveillance Report Vol. 14, 2002

Why? Behavioral explanations: Numbers of partners Condom use

Why? Behavioral explanations: Numbers of partners Condom use Biological explanations: Transmissibility

Why? Behavioral explanations: Numbers of partners Condom use Biological explanations: Transmissibility Structural explanations: Two-sex vs. one sex Population size Versatility Assortative mixing

Thought experiment How large a difference in epidemics might occur between MSM and heterosexual populations that are identical in behavioral characteristics, but differ in structural and biological characteristics? Explore using a mathematical model and data on MSM sexual behavior from UMHS (1995).

Partners in past twelve months All males: NHSLS (random sample of US adults) MSM: UMHS (random sample of MSM in four large US cities) Median = 1

Partners in past twelve months All males: NHSLS (random sample of US adults) MSM: UMHS (random sample of MSM in four large US cities) Median = 1 Median = 3 Mean = 10.9

Partners in past twelve months All males: NHSLS (random sample of US adults) MSM: UMHS (random sample of MSM in four large US cities) Median = 1 Median = 3 Mean = 10.9 Median = 1 Mean = 4.0

Partners in past twelve months All males: NHSLS (random sample of US adults) MSM: UMHS (random sample of MSM in four large US cities) Median = 0 Mean = 1.3 Median = 1 Median = 3 Mean = 10.9 Median = 1 Mean = 4.0

Model basics Sero- negative Subdivided into compartments based on: Activity level No activity Low activity High activity Sero- positive Role Insertive (male) Receptive (female) Versatile Insertive Receptive Deterministic Compartmental heterosexual pop. MSM pop.

Model features Activity levels: none 62.8% low (1 partner per year)15.5 % high (7.7 partners per year)21.7 % Roles:MSM:heterosexuals: versatile50% insertive 50% insertive35% receptive 50% receptive15% Mixing by level: 16.6 times more likely to choose partners of the same activity level than expected by chance UMHS NHSLS Sources:

Model features Transmissibility/unprot. receptive anal act:unprot. insertive anal unprot. receptive vaginal unprot. insertive vaginal Acts/10 (for high-high) partnership: 50 (for high-low) 250 (for low-low) Size:small = 1,000; large = 10,000 Removals:sero-ves: 30 years after entry sero+ves: 7.5 years after seroconversion Arrivals:equal in number to removals Varghese et al UMHS Sources:

Results - HIV prevalence MSM - all versatile small pop, anal transmissibility

Results - HIV prevalence MSM - all versatile small pop, anal transmissibility Heterosexual or MSM-no versatile small pop, anal transmissibility

Results - HIV prevalence MSM - all versatile small pop, anal transmissibility MSM – observed versatility small pop, anal transmissibility Heterosexual or MSM-no versatile small pop, anal transmissibility

Results - HIV prevalence MSM - all versatile small pop, anal transmissibility MSM – observed versatility small pop, anal transmissibility Heterosexual large pop, anal transmissibility Heterosexual or MSM-no versatile small pop, anal transmissibility

Results –HIV prevalence Heterosexual large pop, vaginal transmissibility MSM - all versatile small pop, anal transmissibility MSM – observed versatility small pop, anal transmissibility Heterosexual large pop, anal transmissibility Heterosexual or MSM-no versatile small pop, anal transmissibility

Results (cont.) In order to have the same HIV prevalence as MSM after the first 20 years of the epidemic, heterosexuals would need to have 4.4 times as many partners as MSM on average.

Results (cont.) MSM are more susceptible to high risk behavior by a small subset of the population.

Results (cont.) MSM are more susceptible to high risk behavior by a small subset of the population. If the partnering rates of the high-activity group are doubled:

Results (cont.) MSM are more susceptible to high risk behavior by a small subset of the population. If the partnering rates of the high-activity group are doubled: prevalence at year 20 of the epidemic is changed by 975% in MSM 170% in heterosexuals MSM Het

Implications Structural and biological factors can play major roles in increasing susceptibility of MSM populations to HIV.

Implications Structural and biological factors can play major roles in increasing susceptibility of MSM populations to HIV. The public health community should not assume that persistent differences in HIV/STD levels between MSM and heterosexuals are due only (or even primarily) to differences in individual behavior.

Implications Structural and biological factors can play major roles in increasing susceptibility of MSM populations to HIV. The public health community should not assume that persistent differences in HIV/STD levels between MSM and heterosexuals are due only (or even primarily) to differences in individual behavior. MSM will remain relatively vulnerable to HIV/STD epidemics even with major shifts in behavior.

Implications Structural and biological factors can play major roles in increasing susceptibility of MSM populations to HIV. The public health community should not assume that persistent differences in HIV/STD levels between MSM and heterosexuals are due only (or even primarily) to differences in individual behavior. MSM will remain relatively vulnerable to HIV/STD epidemics even with major shifts in behavior. What about Africa?higher infectivity other structural patterns not modeled here nosocomial

Implications Structural and biological factors can play major roles in increasing susceptibility of MSM populations to HIV. The public health community should not assume that persistent differences in HIV/STD levels between MSM and heterosexuals are due only (or even primarily) to differences in individual behavior. MSM will remain relatively vulnerable to HIV/STD epidemics even with major shifts in behavior. What about Africa?higher infectivity other structural patterns not modeled here nosocomial More broadly, factors other than numbers of unsafe partners can predispose or protect different populations from STD epidemics.

Acknowledgments University of Washington CFAR King Holmes Martina Morris Mark Handcock The entire staff of CAPS’ Urban Men’s Health Study Joseph Catania Ron Stall Tom Coates The participants in the Urban Men’s Health Study