A government report shows that new HIV infections among injection drug users have been cut in half in the past decade, but they continue to engage in risky behaviors such as needle sharing.

The high rate of risky behaviors and a decrease in HIV testing among injection drug users worries health experts, raising concern that the drop in HIV infections may not last, according to Reuters.

A study by the Centers for Disease Control and Prevention (CDC) found 9 percent of intravenous drug users were infected with HIV in 2009, compared with 18 percent in the 1990s. The study found 45 percent of those testing positive in 2009 were unaware of their infection.

The study was based on a survey of 10,000 people from 20 urban areas. More than one-third of participants said they had shared syringes. The study found 69 percent had unprotected vaginal sex, and 46 percent had multiple opposite-sex partners.

According to the CDC, 49 percent of participants said they had been tested for HIV in the past 12 months in 2009, compared with 66 percent in 2006.

“Despite the fact that we’ve seen declines in new HIV infections, a substantial number of IDUs (injection drug users) in major US cities are HIV-infected and their risk behavior remains fairly high,” Dr. Cyprian Wejnert, an epidemiologist at the CDC, told Reuters.

The study authors said public health strategies such as testing and access to new sterile syringes are needed to reduce risk for HIV infection among injection drug users.