Background Concurrent sexual partnerships create higher network connectivity and increase spread of STI through their sexual networks. Recently in our prospective cohort study SWAP (Swingers World Attitude and Practice) swingers were identified as a high transmission population for STI. Swingers, heterosexual couples together having sex with others, by definition are involved in concurrent sexual relationships. Objective of the present paper is to examine indicators for the level of concurrency in sexual relations among swingers predicting high potential of STI transmission.

Methods Participants of the SWAP cohort were followed using (network) questionnaires and STI consultations. Urogenital, oropharyngeal and anorectal samples were tested for Chlamydia trachomatis (CT) and Neisseria gonorrhoeae (NG) by NAAT. STI is defined as positive CT and/or NG diagnosis. We used epidemiological (SPSS) and social network methods for the analysis of the data. Different indicators for level of concurrency were measured: frequency of swinging; types and number of sex partners; and involvement in group sex (sexual intercourse at the same moment in time with multiple partners), all during the past 6 months.

Results Of the 106 respondents, median age 43 years, 13.2% were diagnosed positive for STI. Of the swingers 96% were in a steady relationship, 5% formed a swinging couple with a friend not being their steady partner, 9% also had casual sex partners outside swinging with median 4 sex partners. Median frequency of swinging was 10 times, with median 7 swing sex partners in 6 months. Median number of swing sex partners during an average swinging date was 2; 59% practiced group sex with median 3 sex partners. Median total number of sex partners was 9 in 6 months. Abstract P1-S5.38 table 1 shows frequency of swinging, participating in group sex and total number of sex partners were significantly correlated with having STI.

Conclusions Swingers have concurrent sexual relations with a range of different types and numbers of sex partners and therefore transmission of STI through connected networks is highly plausible. Some of the presented indicators of level of concurrency are associated with STI infection. Further analysis of the concurrency measures, STI related risk behaviours, and the sexual networks of swingers, is necessary to examine how concurrent sexual partnerships form these networks and influence STI transmission through them.