title: Men having sex with men serosorting with casual partners: who, how much, and what risk factors in Switzerland, 2007-2009.
abstract: BACKGROUND
  Serosorting is practiced by men who have sex with men (MSM) to reduce human immunodeficiency virus (HIV) transmission. This study evaluates the prevalence of serosorting with casual partners, and analyses the characteristics and estimated numbers of serosorters in Switzerland 2007-2009.
  
  
  METHODS
  Data were extracted from cross-sectional surveys conducted in 2007 and 2009 among self-selected MSM recruited online, through gay newspapers, and through gay organizations. Nested models were fitted to ascertain the appropriateness of pooling the datasets. Multiple logistic regression analysis was performed on pooled data to determine the association between serosorting and demographic, lifestyle-related, and health-related factors. Extrapolations were performed by applying proportions of various types of serosorters to Swiss population data collected in 2007.
  
  
  RESULTS
  A significant and stable number of MSM (approximately 39% in 2007 and 2009) intentionally engage in serosorting with casual partners in Switzerland. Variables significantly associated with serosorting were: gay organization membership (aOR = 1.67), frequent internet use for sexual encounters (aOR = 1.71), having had a sexually transmitted infection (STI) at any time in the past 12 months (aOR = 1.70), HIV-positive status (aOR = 0.52), regularly frequenting sex-on-premises venues (aOR = 0.42), and unprotected anal intercourse (UAI) with partners of different or unknown HIV status in the past 12 months (aOR = 0.22). Approximately one-fifth of serosorters declared HIV negativity without being tested in the past 12 months; 15.8% reported not knowing their own HIV status.
  
  
  CONCLUSIONS
  The particular risk profile of serosorters having UAI with casual partners (multiple partners, STI history, and inadequate testing frequency) requires specific preventive interventions tailored to HIV status.
authors.affiliation: Institute of Social and Preventive Medicine (IUMSP), Lausanne University Hospital, Biopôle 2, Route de la Corniche 10, 1010 Lausanne, Switzerland. Francoise.Dubois-Arber@chuv.ch.
authors.forename: Stéphanie
authors.initials: S
authors.lastname: Lociciro
authors.forename: André
authors.initials: A
authors.lastname: Jeannin
authors.forename: Françoise
authors.initials: F
authors.lastname: Dubois-Arber
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D003430:Cross-Sectional Studies; D015658:HIV Infections / Q000097:blood / Q000453:epidemiology* / Q000635:transmission*; D018023:HIV Seronegativity; D006679:HIV Seropositivity; D018451:Homosexuality, Male / Q000706:statistics & numerical data*; D006801:Humans; D016015:Logistic Models; D008297:Male; D008875:Middle Aged; D015999:Multivariate Analysis; D015995:Prevalence; D018570:Risk Assessment; D012309:Risk-Taking; D012725:Sexual Behavior; D012747:Sexual Partners; D012749:Sexually Transmitted Diseases / Q000453:epidemiology / Q000517:prevention & control; D013557:Switzerland / Q000453:epidemiology; D055815:Young Adult
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