Cutting the Risk of AIDS--a Tale of the Statistics

In the past year the messages to heterosexuals about their risk of AIDS seem confusing and contradictory. This has resulted from an initial denial that heterosexual transmission could occur, followed by an acknowledgement that it could, leading to tremendous fear. So what exactly is the risk?

It must be understood that the danger for heterosexuals, as with the so-called risk groups, varies tremendously. For gay men who have been celibate or have remained in a monogamous relationship since 1977, there is no risk of acquiring AIDS sexually.

For intravenous drug users who have never shared drug equipment, there is no risk of acquiring the AIDS virus through drug use.

Similarly, for heterosexual couples in a monogamous relationship since 1977 who do not use injectable drugs and where neither partner has had contact with blood products, there is no risk of AIDS.

On the other hand, a steady sexual partner of an infected IV drug user is at substantial risk. Thus for heterosexuals with multiple partners or whose partner has multiple partners or might be infected, the risk of infection by sexual contact depends on the frequency and type of sex and the chance that a partner is infected.

Several studies of sexual partners of people infected with the virus show that a single act of unprotected vaginal intercourse has a surprisingly low risk of infecting the uninfected partner--perhaps one in 100 to one in 1,000. For an average, consider the risk to be one in 500. If there are 100 acts of intercourse with an infected partner, the odds of infection increase to one in five.

Statistically, 500 acts of intercourse with one infected partner or 100 acts with five different infected partners lead to a 100% probability of infection (statistically, not necessarily in reality). For 100 acts of intercourse with five different partners, only one of whom is infected, the odds are reduced to one in five.

Condoms are not, of course, 100% protective. However, a consistent use of condoms reduces the risk significantly so that a single act of protected vaginal intercourse with an infected partner might have a risk of one in 5,000, compared with one in 500 without the condom.

A study of heterosexuals where one partner is infected showed that the rate of infection was a little more than twice as high for couples who practice anal and vaginal intercourse as for couples who practice only vaginal intercourse. So the risk for a single episode of anal intercourse would be perhaps one in 200.

In three studies of gay men the risk of infection from oral sex was found to be very low. However, because of a published report of infection of a baby by breast milk, one must assume that the risk from oral sex is low but not zero. It is perhaps one in 50,000--for each episode.

For unprotected vaginal intercourse, studies suggest that certain factors may influence the likelihood of infection. Sexually transmitted diseases, especially those that lead to open sores on the genitals, appear to increase the risk. Some people may have a hereditary predisposition to infection (but nothing suggests that anyone is immune).

Finally, studies show that the longer people are infected, the more infectious they seem to become. This factor may prevent an immediate large spread of the virus among heterosexuals, but the risk will increase with time. The risk will also increase with time because the number of infected people rises each day.

The risks discussed above relate to sex with an infected partner. The other major consideration obviously is what the chance is that one has an infected partner. Studies in the military and of blood donors, although by no means conclusive, show the risk to be under 1% for heterosexuals. Even in these studies most of those infected have been bisexual men or IV drug users.

For heterosexuals the risk that a partner is infected also depends on the city, the number of partners whom a person has had, the likelihood that one or more partners were infected and the number of contacts with that partner.

For people entering a new sexual relationship, the possibility that the partner is infected must be considered. Sadly, the partner may not acknowledge bisexuality or IV drug use because of the social stigma attached. Further, it is not always possible for a person to know if a previous partner was bisexual or an IV drug user, had received blood products or was infected by someone else.

Nevertheless, it should be clear that the risk to heterosexuals is low for a single act of intercourse. However, because of the potential dangers of infection, most people will want to lower their risk as much as possible. The antibody test is useful for monogamous partners who are not intravenous drug users. Two negative tests, three to six months apart, virtually assure freedom from infection. For others who are sexually active, the consistent use of condoms reduces but does not eliminate the risk.

Because the risk is so low that a heterosexual partner is infected and because the virus spreads so poorly through each act of vaginal intercourse, the risk will remain much lower for heterosexuals than for gay and bisexual men for years to come.

But for people with multiple partners, the risk will not be zero. It is the responsibility of society to make people aware of the risk of infection and how to lower that risk, so that people may decide how much risk they are prepared to take.