NEW YORK — The number of people diagnosed with HIV in the United States dropped 33 percent in the last decade, but new cases among young homosexual and bisexual men doubled, according to researchers who say prevention programs need to be expanded.

Better screening and prevention have driven the decline among women and people ages 35 to 44, a study by the U.S. Centers for Disease Control and Prevention found. The two-fold increase was seen among young bisexual and homosexual men age 13 to 24, according to a study published in the Journal of the American Medical Association.

The findings, released Saturday to coincide with the International AIDS Conference in Melbourne, Australia, are the first to use data from all 50 U.S. states to examine long-term trends in diagnosis, said study author Amy Lansky. Though strides are being made to cut HIV, prevention and screening efforts aren’t adequately reaching a younger group, she said.

“There’s a new generation that comes up and many don’t have first-hand experience with the devastation we saw in the earlier years,” said Lansky, the CDC’s deputy director for Surveillance, Epidemiology and Laboratory Science in the Division of HIV/AIDS Prevention in Atlanta.

There is also a higher prevalence of HIV among gay and bisexual men, so “they face a greater risk of being exposed to HIV with each sexual encounter,” she said. Lack of access to health care as well as substance abuse also raises the risk, she said.