The Effectiveness of Condoms in Reducing Heterosexual Transmission of HIV.(Statistical Data Included)

Heterosexual intercourse is the primary mode of HIV infection worldwide.[1] In the United States, male homosexual contact and intravenous drug use account for the majority of HIV infections, but transmission via heterosexual contact continues to increase. Heterosexual contact with an infected partner is the greatest risk factor for women and, consequently, for their newborn children. In 1988, 2% of male AIDS cases and 30% of female cases reported in the United States were attributed to heterosexual contact.[2] By 1998, this percentage had risen to 7% for men and 38% for women.[3]

Although new treatments appear promising for retarding the progression of HIV-related disease, prevention remains the most effective weapon against the epidemic. Recommendations for the prevention of sexually transmitted HIV infection include abstinence, long-term monogamy with a seronegative partner, a limited number of lifetime sexual partners and condom use for each and every act of intercourse.[4] The use of condoms is recommended for individuals who have multiple partners, who have a primary partner who is infected, or who have a partner whose serostatus is unknown,[5] although the absolute amount of protection they provide has not been accurately established.

The effectiveness of the condom as a contraceptive provides insight into its usefulness as a barrier device capable of preventing HIV transmission. Defined as the proportionate reduction in pregnancies caused by use of a contraceptive method, effectiveness is estimated as one minus the ratio of two failure rates. The failure rate ratio is calculated by dividing the pregnancy rate associated with use of a contraceptive method by the rate related to no method use for a given time period.[6] The likelihood of becoming pregnant during the first year of condom use ranges from 2.6% to 15.8%.[7] The likelihood of pregnancy in a population not practicing contraception is estimated from groups such as the Hutterites, and is often assumed to be 85%.[8] These probabilities can be transformed into rates,(*) providing an estimate of condom effectiveness for preventing pregnancy of 90.7% to 98.6%.

The effectiveness of condoms in reducing HIV may be estimated in the same way as for contraception. For HIV, the failure rate ratio is calculated by dividing the seroconversion rate among couples always using condoms by the rate among couples never using condoms. A comparison group of condom nonusers is essential to determine the reduction in HIV incidence that is due to condom use. The best measure of condom efficacy is obtained by comparing monogamous, serodiscordant couples (i.e., those who differ in their HIV infection status) who always use condoms during penetrative vaginal intercourse with those who never do. Since HIV serodiscordant couples cannot ethically be assigned at random to "always" and "never" use condoms, estimates must be obtained from observational studies. Unfortunately, observational studies may be biased by an unequal distribution of HIV risk factors across study categories.

For both contraception and HIV prevention, condom failure has two sources: user failure and method failure. User failure includes nonuse and incorrect use, and is attributed to the person using the condom. Method failure is the absolute, theoretical failure inherent in the device, and is independent of the user. User failure varies per person and per contact, while method failure is assumed to be constant. It is impossible to measure absolute method failure, since it is confounded with user failure.

Condom failure due to nonuse, incorrect use, breakage and slippage may occur for both HIV prevention and birth control.[9] In several recent in vivo trials measuring failure due to breakage and slippage, rates have varied from 0.5% to 6.7% for breakage and 0.1% to 16.6% for slippage,[10] Quality control standards set by the Food and Drug Administration allow four out of 1,000 condoms in any given batch to leak water.[11] In vitro trials have reported HIV leakage in 0-100% of the condoms tested,[12] with all but one brand[13] between 0.0% and 54%.

Various estimates of the condom's effectiveness at reducing heterosexual transmission of HIV are available from studies of serodiscordant couples. In order to obtain a single overall estimate of effectiveness, we present a meta-analysis of those results. An initial attempt[14] to do so was flawed because it aggregated studies with varying definitions of condom use, directions of transmission, study designs and types of index cases. A subsequent report[15] controlled for the direction of transmission, but did not remove the sometimes or occasional users of condoms from among the never-users, and also did not control for study design.