An unusually detailed study of people newly infected with H.I.V. in the United States has confirmed that the majority of new cases occur among gay and bisexual men and that blacks are most at risk. But the data show that whites and blacks tend to be infected at different times in their lives.
Most new infections of white gay and bisexual men occur when the men are in their 30s and 40s, the study found, while black gay and bisexual men were more likely to be infected in their teens and 20s. The results were reported on Thursday by the Centers for Disease Control and Prevention.
The centers reported last month that the study found that the virus is spreading faster in the United States than had been thought. In 2006, the study found, 56,300 people were newly infected with H.I.V., 40 percent higher than the agency’s previous estimate of roughly 40,000 new cases a year. The study was performed using new technology that allows researchers to distinguish between new and older infections.
The study’s findings “serve as a powerful reminder that the U.S. epidemic of H.I.V. disease is far from over,” said Dr. Kevin Fenton of the centers.
The details of the center’s demographic analysis were released on Thursday in the hope that knowledge of the age, race and other characteristics of the newly infected would better direct prevention efforts.
“The data really confirm what we had suspected and known before,” said Dr. Fenton, who emphasized the disease’s “disproportionate impact on gay and bisexual men and on blacks and Latinos.”
Black people, who make up about 12 percent of the population, accounted for more than 45 percent of the new infections, the study found, and the disparity was particularly acute among women.
Black women are nearly 15 times as likely to be infected with H.I.V. as white women; Hispanic women are 4 times as likely to be infected as white women. Black men have six times the H.I.V. incidence rate of white men.
Among those newly infected with the virus, black men were no more likely to be drug users or to engage in risky sex than white men were, according to the study. More research is needed to explain why young black men are at such greater risk for contracting the disease, but there are several hints in the data, researchers said.
First, the fact that proportionally more blacks than whites are already infected would tend to produce higher transmission rates among blacks, said Dr. Richard Wolitski, acting director of the center’s division for H.I.V. and AIDS prevention. Young black men are much more likely to be in prison, where H.I.V. infections risks are particularly high. Young black gay and bisexual men also tend to have partners who are older than their white counterparts, and thus are more likely to already have been infected, Dr. Wolitski said.
In one of the most dismal statistics provided by the centers, researchers said that 80 percent of those found to be newly infected by the disease had not been reached by prevention efforts, which are often sponsored by federal, state and local health officials.