Just How Does AIDS Spread?

By Denise Grady

First there was the news everybody wanted to hear: a New York physician writing in Cosmopolitan reassured women that there is practically no risk of contracting AIDS through ordinary vaginal or oral sex, even with an infected man. The vaginal secretions produced during sexual arousal, he wrote, keep the virus from penetrating the vaginal walls. His explanation: "Nature has arranged this so that sex will feel good and be good for you." Then came the news nobody wanted to hear: Sex Gurus William Masters and Virginia Johnson proclaimed in their new book about AIDS that "the epidemic has clearly broken out into the broader population" of heterosexuals, and that far more people are at great risk than previously thought. Even kissing, they declared, is not safe.

Who is to be believed?

If anything is clear about the AIDS epidemic, it is that anal sex among homosexual men and needle sharing among drug addicts are still the major ways the AIDS virus is transmitted in the U.S. American victims are still overwhelmingly male: 92%. And though there is no doubt that heterosexual intercourse between intravenous drug users or bisexual men and their lovers is contributing to the spread of the disease, the number of AIDS cases traced to sex between men and women not in these high-risk groups is very low -- about 4% -- and has remained stable. But just what is the risk? How contagious is AIDS? What are the odds of picking up the virus from a single sex act if one's partner turns out to be infected?

There are no certain answers to these questions, and that is part of the problem; it is misleading, and perhaps even dangerous, to pretend that there are. The best advice, most AIDS experts agree, is to use condoms and cut down on the number of sex partners. Reason: promiscuity increases the likelihood of encountering the virus as well as other sexually transmitted diseases that may increase susceptibility to AIDS. Some people have picked up the virus from a single sexual encounter, while others have escaped despite hundreds of sexual exposures to an infected spouse. No one knows why. The risk figures that Masters and Johnson offer -- a 1-in-400 risk of a man transmitting the virus to a woman through an act of unprotected vaginal intercourse, and a 1-in-600 risk of a woman to a man -- are supposedly based on a series of assumptions and statistical projections first described in 1987 by Nancy Padian and Jim Wiley of the University of California, Berkeley. The projections are already outmoded. Says Wiley: "A single number cannot describe the rate of transmission. There are too many variables."

And too many unknowns. Research is making it abundantly clear that people differ, often inexplicably, in their vulnerability to the virus and in their tendency to transmit it to others by various routes. An AIDS carrier's infectivity -- his or her ability to pass the virus on -- may vary over time. Only now are researchers beginning to understand these differences and their implications for preventing the spread of the disease.

Striking new research, published last week in the quarterly journal AIDS Research and Human Retroviruses, may help explain why some AIDS carriers can go on having unprotected sex for years without passing the virus to a regular partner. Although it is known that enough of the virus appears in the bloodstream shortly after infection to spread the disease via blood transfusions, sexual transmission is a different matter. The new study, of 24 hemophiliac AIDS carriers, shows that despite repeated sexual contact without condoms, the wives or steady female partners of these men generally remained free of the virus for several years. But when signs of severe immune deficiency began to appear in the men, four of the women became infected.

Although they may have finally contracted the virus simply because of repeated exposure, researchers doubt it. Had that been the case, the women who became infected should have been those who had had sex most often. But frequency of intercourse did not seem to matter. Says Researcher James Goedert of the National Cancer Institute: "The study demonstrates that the infected population gets more infectious as time passes, and that the level of risk increases as time goes on." That led Goedert and his colleagues to speculate that early treatment with AZT, the only approved anti-AIDS drug known to inhibit replication of the virus, may actually make AIDS less contagious. "That's among the most urgent questions we have to answer," says Samuel Broder, director of clinical oncology at the National Cancer Institute.

A study published earlier this year in the Journal of the American Medical Association offered other explanations for why some people become infected after sexual exposure and others do not. Of 25 husbands and 55 wives of patients who acquired the virus from blood transfusions, only two husbands and ten wives became infected in more than two years. None of the couples used condoms. Although a higher proportion of wives than husbands contracted the virus, the difference was not considered statistically significant.

Similarly, there were no differences in practices such as oral sex and French kissing among the couples; nor did it seem to matter how often they had intercourse. One wife became infected after only one exposure, and another after just eight. Yet eleven women remained uninfected after more than 200 sexual contacts. The researchers speculate that the originally infected spouses may have somehow differed in their ability to transmit the virus. Another possibility: their husbands and wives may have differed in susceptibility.

The J.A.M.A. study's failure to indict specific sexual practices supports laboratory findings that suggest, contrary to Masters and Johnson, that "deep" kissing is safe. The AIDS virus is present in saliva at extremely low levels or not at all. Saliva is a hostile environment for the AIDS virus, explains Jay Levy of the University of California at San Francisco. It will kill half the viruses exposed to it within 30 minutes. Scientists are also skeptical about the danger of oral sex. But that risk is practically impossible to measure because most couples who engage in oral sex also have intercourse, and there is no way to analyze the risks separately.

None of this is to say that the risk of transmitting AIDS through heterosexual intercourse is not a serious one. Exactly how the virus is passed along, though, is still murky. Many researchers strongly suspect that an infected man can more easily pass the virus to a female sexual partner than vice versa. Certainly more women have got the disease from men than men from women: women make up 75% of those who have contracted AIDS through heterosexual intercourse. Researchers have speculated that the virus is more concentrated in semen than in vaginal secretions and that the mucous membranes lining the vagina are especially vulnerable to penetration by the virus.

Dr. Neal Steigbigel, chief of infectious disease at Montefiore Hospital in the Bronx, thinks male-to-female transmission is bound to occur more often simply because of the mechanics of vaginal intercourse. Harvard Virologist Martin Hirsch, however, notes that herpes and syphilis appear to travel equally well in either direction between the sexes. Hirsch thinks the only reason more women have contracted the AIDS virus from men than the other way around is that many more men now have the disease. As more women become carriers, he suspects, they will infect their partners. "There is no doubt," says Dr. Margaret Fischl, an AIDS researcher at the University of Miami School of Medicine, "that this virus, when it comes into contact with any mucous membrane, is going to be transmitted." Men and women, she insists, are equally vulnerable.

In Africa, where it is estimated that more than 2 million people have been infected with the AIDS virus, the disease strikes men and women equally. AIDS is spread among adult Africans primarily by vaginal intercourse, and it is rampant in the large, overcrowded cities of central and western Africa. Most AIDS researchers, however, now dismiss the suggestion that Africa might serve as a model for the heterosexual spread of AIDS in the U.S. For one thing, promiscuity and prostitution are common in parts of Africa, and general standards of hygiene and medical care are low. Scientists believe widespread venereal diseases in Africa also contribute to the spread of AIDS by causing genital ulcerations that make it easier for the virus to enter the bloodstream. Finally, Africans suffer from more nonvenereal viral and parasitic diseases than do Americans, and the Africans' overtaxed immune systems may be more vulnerable to the AIDS virus.

If there is any American parallel to the African experience, it may be developing in some inner cities, where drug addiction and prostitution are inextricably linked to AIDS, where pregnancies among teenagers have become commonplace and where educational programs about safe sex either do not reach their intended audience or cannot cross cultural barriers. In January an article in the New England Journal of Medicine revealed a surprisingly high, 5.2% rate of AIDS virus infection among 4,028 patients attending clinics for sexually transmitted diseases in Baltimore. Most of the patients were black, and their infection rate was notably higher than the rate among whites. Intravenous drug abuse and sexual contact with a drug addict were important risk factors. So too was a history of syphilis in men and virally caused genital warts in women; both venereal problems can cause breaks in the lining of the genital tract that may make it easier for the virus to enter.

The Baltimore researchers were disturbed to find that one-third of the men carrying the AIDS virus and nearly half the women had no idea that they had engaged in any behavior that put them at risk. The proposed solutions: more AIDS screening and personal counseling at clinics for sexually transmitted diseases, greater efforts to eradicate syphilis and other diseases that lead to genital ulcerations, and more education about safe sex and the dangers of drug abuse. With AIDS, there will be no quick fixes or startling innovations, just the desperate, backbreaking efforts required to persuade people to make small but vital changes that may save their lives.

With reporting by Joyce Leviton/Atlanta and Suzanne Wymelenberg/Boston