HIV infection is significantly more common among non-Hispanic blacks than it is among any other young adult racial or ethnic group in the United States, according to the first study drawn from the nation’s general youth population.

The infection rate for young non-Hispanic blacks ages 19 to 24 is 4.9 per 1,000 people compared to a rate of 0.22 per 1,000 for all other races. The overall HIV infection rate for young adults was 1 per 1,000, a figure that is lower than other estimates of HIV prevalence that relied on different reporting methods. It also was much lower than reported rates of other sexually transmitted diseases. The HIV infection rate among young adult men was slightly higher than that for young women.

The new study appears in the current issue the American Journal of Public Health and draws on the National Longitudinal Survey of Adolescent Health and uses data from more than 13,000 young adults who agreed to be screened for HIV infection.

“The infection rate for non-Hispanic blacks is 20 times greater than the remainder of the population and this disparity begins early in life,” said Martina Morris, lead author of the paper and a sociologist who directs the University of Washington’s Center for the Studies in Demography and Ecology.

The study differed from previous ones in that it had a large representative sample of young adults and did not rely on data of HIV cases reported to the Centers for Disease Control (CDC). The new study sample included some people who were injection drug-users, in jail and men who have sex with men – groups that are known to have higher HIV rates. However, the study was not designed to estimate HIV prevalence in these specific groups.

Morris noted that earlier studies have estimated that half of all HIV infections in the early 1990s were acquired before age 25. However, data from the new study suggest that by 2000 the fraction of infection acquired before age 25 had dipped to between 15 percent and 30 percent.

Overall, of the nearly 13,200 individuals who were screened, 15 tested positive for HIV, eight were indeterminate and the remainder were negative. Twelve of the 15 positive tests were among non-Hispanic blacks.

Although whites, Hispanics, Asians and American Indians were represented in the sample, federal and university human subjects guidelines protecting confidentiality prevented HIV infection rates from being calculated for these groups. Because of the small number of cases – three – it might have been possible to uncover the identity of those who tested positive.

Morris said the results of the study parallel the racial disparities for other sexually transmitted diseases seen in CDC data. For example, rates of gonorrhea are about 17 times greater among black youth, and rates of syphilis are 12 times greater and rates of chlamydia are about five times greater.

“We need to understand the mechanism that controls the spread of these pathogens. The persistent differentials across a wide range of disease suggests a common mechanism,” she said.

Morris added that recent research suggests disparities in the prevalence of sexually transmitted diseases are shaped by differences in the underlying networks that spread infection. Segregation along racial, economic and geographic lines can lead to differences in the pool of potential sex partners. In addition, when sexual partnerships overlap in time the connectivity of a sexual network increases.

“Think of it as a group of people holding hands in a circle,” she said, “Everyone has only two partners, but the whole population is connected. This can amplify the transmission of HIV and other diseases.”

This pattern of having concurrent partners is more common among non-Hispanic blacks, so it may boost the spread of disease within this network. Segregation then keeps the disease from spreading to other groups.

“We need to better understand the way networks influence the transmission dynamics of HIV, because small difference in behavior can have large impacts on network connectivity,” said Morris. “Behavior still matters, but people need to know what behaviors to change.”

She said one of the lessons learned from studying partnership networks and behavior in sub-Saharan Africa, which has nearly 80 percent of the world’s cases of HIV, is that prevention programs that stress having one partner at a time, as in Uganda, have been effective in reducing transmission.

The study was supported by the National Institute of Child Health and Human Development and a number of cooperating agencies to Carolina Population Center at the University of North Carolina at Chapel Hill. Co-authors were Mark Handcock of the UW and Drs. William Miller, Carol Ford and Myron Cohen and John Schmitz, Marcia Hobbs, Kathleen Harris and J. Richard Udry of UNC.