Fast start: sex readily spreads HIV in infection's first weeks.

People with the AIDS virus are many times more infectious to their sexual partners in the weeks or months just after they acquire the virus than they are later on, researchers in Uganda have determined. The study confirms the long-standing hypothesis that, compared with those infected for years, people recently infected with HIV contribute to the spread of the virus in excess proportion to how often they have unprotected sex.

Identifying people with highly infectious, early-stage HIV--and treating them immediately or at least encouraging them to avoid unsafe behaviors--could prevent more new infections than do current practices, which primarily target people with advanced HIV, say Maria J. Wawer of Columbia University and her U.S. and Ugandan collaborators.

In western Uganda, where people with HIV infection survive an average of 8 to 10 years, Wawer and her team recruited some 15,000 volunteers, with or without HIV. At 10-month intervals from 1994 to 1999, study participants provided blood samples and answered questions about sexual activity and other behaviors that can spread HIV. To understand how a person's stage of infection influences the virus' transmissibility, the team identified heterosexual couples that had named each other as regular sex partners.

The analysis focused on 239 couples in which an initially uninfected person had frequent sex with an infected partner. In 72 cases, the partner at risk became infected during the study, and blood tests linked most such infections to the other member of the pair. No one in these relationships reported homosexual activity, anal intercourse, or needle use, and no one used condoms more than occasionally.

Overall, HIV's transmission rate is about 0.1 percent per act of heterosexual intercourse, Wawer's data suggest. That estimate agrees with past findings for populations that rarely use condoms. However, a person infected for less than 5 months had a 0.8 percent chance per act of intercourse of spreading HIV, Wawer and her colleagues report in the May 1 Journal of Infectious Diseases. That elevated risk was evident because some people acquired HIV, presumably from extra partners, and quickly infected their main partners.

The new study provides "the first empirical evidence ... that people with brand-new infections are very infectious," says Myron Cohen of the University of North Carolina at Chapel Hill.

Past studies suggested that HIV concentrations in bodily fluids peak within the first month after infection, so Wawer and her collaborators speculate that transmissibility could be as high as 2 percent per act of unprotected intercourse in the first weeks of infection. After that, they say, the risk greatly diminishes for most of the rest of the person's life. The risk appears to rise slightly in the final 3 years of life.

HIV tests that identify new infections have recently become available. Those diagnostic tools make it possible to treat early-stage infections. If early treatment proves beneficial and becomes common, patients' sex partners could also benefit, says Cohen.