Assortative sexual mixing patterns in male-female and male-male partnerships in Melbourne, Australia: Implications for HIV and sexually transmissible infection transmission

Background: Assortative mixing patterns have become a new and important focus in HIV/sexually transmissible infection (STI) research in recent years. There are very limited data on sexual mixing patterns, particularly in an Australian population. 

Methods: Male-female and male-male partnerships attending the Melbourne Sexual Health Centre (MSHC) between 2011 and 2014 were included. Correlation of age between two individuals within a partnership was examined by using Spearman's rank correlation. The Newman's assortativity coefficient was used as an aggregate quantitative measurement of sexual mixing for number of partners and condom use. 

Results: 1165 male-female and 610 male-male partnerships were included in the analysis. There was a strong positive correlation of age in both male-female (rho≤0.709; P<0.001) and male-male partnerships (rho≤0.553; P<0.001). The assortative mixing pattern for number of partners was similar in male-female (r≤0.255; 95% CI: 0.221-0.289) and male-male partnerships (r≤0.264; 95% CI: 0.218-0.309). There was a stronger assortative mixing pattern for condom use in male-male (r≤0.517, 95% CI: 0.465-0.569) compared with male-female (r≤0.382; 95% CI: 0.353-0.412) partnerships. 

Conclusion: Male-female and male-male partnerships have a high assortativity mixing pattern for age, number of partners and condom use. The sexual mixing pattern is not purely assortative, and hence it may lead to increased HIV and STI transmission in certain risk groups.