title: Probability of HIV-1 transmission per coital act in monogamous, heterosexual, HIV-1-discordant couples in Rakai, Uganda.
abstract: BACKGROUND
  The probability of HIV-1 transmission per coital act in representative African populations is unknown. We aimed to calculate this probability overall, and to estimate how it is affected by various factors thought to influence infectivity.
  
  
  METHODS
  174 monogamous couples, in which one partner was HIV-1 positive, were retrospectively identified from a population cohort in Rakai, Uganda. Frequency of intercourse and reliability of reporting within couples was assessed prospectively. HIV-1 seroconversion was determined in the uninfected partners, and HIV-1 viral load was measured in the infected partners. Adjusted rate ratios of transmission per coital act were estimated by Poisson regression. Probabilities of transmission per act were estimated by log-log binomial regression for quartiles of age and HIV-1 viral load, and for symptoms or diagnoses of sexually transmitted diseases (STDs) in the HIV-1-infected partners.
  
  
  RESULTS
  The mean frequency of intercourse was 8.9 per month, which declined with age and HIV-1 viral load. Members of couples reported similar frequencies of intercourse. The overall unadjusted probability of HIV-1 transmission per coital act was 0.0011 (95% CI 0.0008-0.0015). Transmission probabilities increased from 0.0001 per act at viral loads of less than 1700 copies/mL to 0.0023 per act at 38 500 copies/mL or more (p=0.002), and were 0.0041 with genital ulceration versus 0.0011 without (p=0.02). Transmission probabilities per act did not differ significantly by HIV-1 subtypes A and D, sex, STDs, or symptoms of discharge or dysuria in the HIV-1-positive partner.
  
  
  CONCLUSIONS
  Higher viral load and genital ulceration are the main determinants of HIV-1 transmission per coital act in this Ugandan population.
authors.affiliation: Departments of Population and Family Health Sciences, School of Hygiene and Public Health, Johns Hopkins University, Baltimore, MD 21205, USA. rgray@jhsph.edu
authors.forename: R H
authors.initials: RH
authors.lastname: Gray
authors.forename: M J
authors.initials: MJ
authors.lastname: Wawer
authors.forename: R
authors.initials: R
authors.lastname: Brookmeyer
authors.forename: N K
authors.initials: NK
authors.lastname: Sewankambo
authors.forename: D
authors.initials: D
authors.lastname: Serwadda
authors.forename: F
authors.initials: F
authors.lastname: Wabwire-Mangen
authors.forename: T
authors.initials: T
authors.lastname: Lutalo
authors.forename: X
authors.initials: X
authors.lastname: Li
authors.forename: T
authors.initials: T
authors.lastname: vanCott
authors.forename: T C
authors.initials: TC
authors.lastname: Quinn
mesh_terms: D000293:Adolescent; D000328:Adult; D003075:Coitus; D018562:Disease Transmission, Infectious; D005260:Female; D015658:HIV Infections / Q000453:epidemiology / Q000635:transmission*; D006679:HIV Seropositivity; D015497:HIV-1 / Q000276:immunology; D006801:Humans; D008297:Male; D008875:Middle Aged; D011336:Probability; D012044:Regression Analysis; D014454:Uganda / Q000453:epidemiology; D019562:Viral Load