Background:
Blacks in the US suffer disproportionally high rates of sexually transmitted infections (STIs). Differences in STI behavioral risk factors, such as number of partners and condom use, do not fully explain these disparities. Instead, these disparities may be caused by higher rates of mixing between core and non-core among Blacks compared to other racial/ethnic groups.

Objective:
To determine to what extent racial/ethnic differences in mixing between core and non-core could contribute to racial/ethnic STI disparities.

Method:
A deterministic compartmental mathematical model was used to simulate gonorrhea transmission in two racial/ethnic subpopulations that were identical except for the pattern of mixing between core and non-core. The model population was stratified into eight groups based on race/ethnicity (Black or White), core status, and disease status (susceptible or infected/infectious). Ordinary differential equations described the change in each population compartment as a function of sexual contact rates, mixing patterns between compartments, transmission probability, duration of infection, and entry into and exit from the population. Mixing by core status was varied from fully assortative (like with like) in both racial/ethnic groups to assortative in one group and random (proportional to group sizes) in the other. Mixing by race/ethnicity was varied from assortative to random.

Result:
When mixing by race/ethnicity was random, the endemic prevalence of GC in the group with random mixing between core and non-core was twice that in the assortatively mixing group. When the two racial/ethnic groups did not intermix, prevalence in the randomly mixing group was three times higher than in the assortatively mixing group.

Conclusion:
Lower levels of mixing between core and non-core in Whites compared to Blacks could partly account for observed racial/ethnic STI disparities.

Implications:
In order to address racial/ethnic STI disparities, it is important to investigate patterns of mixing by sexual activity.