aHarvard School of Public Health, Boston, MA, USA; bCenter for AIDS Prevention Studies, UCSF, San Francisco, CA 94105, USA; and cSan Francisco Department of Public Health, San Francisco, CA 94102, USA.
Received: 6 December 2000; accepted: 20 December 2000.
*The YWS Study Investigators Group includes (in alphabetical order): Jeffrey Klausner, MD, MPH, San Francisco Department of Public Health, San Francisco, CA; George Lemp, PhD, University of California Universitywide AIDS Research Program, Oakland, CA; Scott Morrow, MD, San Mateo County Department of Public Health, San Mateo, CA; Juan Ruiz, MD, Dr PH, California Department of Health Services, Sacramento, CA; Fred Molitor, PhD, University of California at Davis, Center for Health Services Research in Primary Care, Davis, CA, USA.
Receptive anal intercourse has long been identified as a strong predictor of HIV transmission between male sexual partners. However, this sexual practice is rarely discussed as a primary risk factor for heterosexual transmission, despite studies demonstrating that anal intercourse carries a higher risk than vaginal intercourse [1,2]. Cultural taboos may influence the self-reporting of anal intercourse, resulting in an underestimation of the sexual practice in heterosexual populations . The studies that have documented anal intercourse as an independent risk factor for HIV acquisition have shown that condoms are rarely used for heterosexual anal intercourse [3,4]. Furthermore, relatively few studies [3,4] have estimated the prevalence of anal intercourse in heterosexual populations, or considered HIV prevention interventions specifically targeting this practice among women.
The Young Women's Survey (YWS) presented an opportunity to estimate the prevalence and characterize the correlates of anal intercourse in a population-based study of sexually transmitted diseases and HIV in young women, aged 18–29 years, residing in low-income neighborhoods in northern California. The YWS study design and methods have been described in previous publications . Briefly, young women from five counties were interviewed from 4 April 1996 to 6 January 1998 on sexual and risk behaviors and were tested for HIV, syphilis, gonorrhea, and chlamydia. A total of 2547 women were enrolled in the study. All results are adjusted for the survey sampling design.
The median age of enrolled women was 23.9 years [interquartile range (IQR) 21–27]. Almost all (95.8%) women reported having had sexual intercourse (defined as oral, vaginal or anal penetration) with another person. The median number of years they had been sexually active was 7.3 (IQR 4.7–10.4). Overall, 21.7% of sexually active women reported ever having had anal intercourse. Of respondents who reported ever having had anal intercourse, 29.6% of women with a steady partner and 7.3% of women with a casual partner had engaged in anal intercourse in the previous 2 months. Condom use during the most recent anal sex act was reported by 19.3% of women with a steady partner and 68.2% of women with a casual partner. Condom use was more frequently reported for vaginal sex with steady partners: 30.6% reported condom use at last vaginal sex (P < 0.01 for comparison with steady partners and anal intercourse); 62.8% of women reported condom use during the last vaginal sex with the most recent casual partner.
Table 1 shows the prevalence of anal intercourse by demographic and behavioral characteristics. A history of anal intercourse was associated with race/ethnicity, higher income, higher education, a history of injection drug use, partner history of injection drug use, and the receipt of money or drugs in exchange for sex (P < 0.001). Variables found to be independently associated with a history of anal intercourse in multiple logistic regression were: trading sex for money or drugs [odds ratio (OR) 4.3; 95% confidence interval (CI) 3.0–6.4]; having sex with an injection drug user (OR 2.9, 95% CI 2.2–4.0), and increased monthly income (compared with < US$500): [US$500–999 (OR 1.2 95%CI 0.9–1.7); US$1000–2999 (OR 1.7, 95% CI 1.2–2.3); ≥ US$3000 (OR 2.9, 95% CI 1.7–4.8)].
Heterosexual intercourse is often understood to refer to vaginal intercourse only; few studies of male-to-female HIV transmission risk elaborate on the differences between vaginal and anal intercourse, perhaps because some believe that the attributable risk is low. By disregarding anal intercourse in previous analyses, models may have overestimated the infectivity of vaginal sex. The only published study  that has made a distinction between anal and vaginal intercourse reports the infectivity of unprotected receptive anal intercourse to be 20 to over 500 times greater than receptive vaginal intercourse, depending on the stage of infection in the index case. Our findings show that a substantial percentage of heterosexual women engaged in anal intercourse, and that condoms are less likely to be used with steady partners during anal intercourse than vaginal intercourse. Furthermore, we found that women with a history of anal intercourse were also more likely to report other risk behaviors that put them at risk of HIV infection, confirming other studies that have shown similar links [3,5]. Anal intercourse may account for a higher proportion of HIV transmission to women than commonly believed, and needs to be specifically addressed in prevention messages among all women at risk of HIV infection.
and the YWS Study Investigators Group*
1. Leynaert B, Downs AM, de Vincenzi I. Heterosexual transmission of human immunodeficiency virus: variability of infectivity throughout the course of infection. European Study Group on Heterosexual Transmission of HIV.
Am J Epidemiol 1998, 148: 88 –96.
2. Padian NS, Shiboski SC, Glass SO, Vittinghoff E. Heterosexual transmission of human immunodeficiency virus (HIV) in northern California: results from a ten-year study.
Am J Epidemiol 1997, 146: 350 –357.
3. Halperin DT. Heterosexual anal intercourse: prevalence, cultural factors, and HIV infection and other health risks, Part I.
AIDS Patient Care STD 1999, 13: 717 –730.
4. Erickson PI, Bastani R, Maxwell AE, Marcus AC, Capell FJ, Yan KX. Prevalence of anal sex among heterosexuals in California and its relationship to other AIDS risk behaviors.
AIDS Educ Prev 1995, 7: 477 –493.
5. Ruiz JD, Molitor F, McFarland W. et al
. Prevalence of HIV infection, sexually transmitted diseases, and hepatitis and related risk behavior in young women living in low-income neighborhoods of northern California.
West J Med 2000, 172: 368 –373.