Nearly one in five gay and bisexual men in 21 major U.S. cities are infected with HIV, and nearly half of them do not know it, U.S. health officials said on Thursday.

Young men, and especially young black men, are least likely to know if they are infected with HIV, according to a study by the U.S. Centers for Disease Control and Prevention.

We need to reinvigorate our response to preventing HIV among gay and bisexual men, Dr. Jonathan Mermin, director of CDC's Division of HIV/AIDS Prevention, said in a telephone interview.

We can't allow HIV to continue its devastating toll among gay and bisexual men, and in particular, among young black men.

Mermin's comments echoed an AIDS policy rolled out in July by the White House that asked states and federal agencies to find ways to cut new HIV infections by 25 percent.

Researchers at the CDC studied 8,153 men who have sex with men in 21 U.S. cities. The men were taking part in the 2008 National HIV Behavioral Surveillance System, which looked at prevalence and awareness of the human immunodeficiency virus or HIV, the virus that causes AIDS.

Overall, they found that 19 percent of gay men are infected with HIV.

The study found that 28 percent of gay black men infected with HIV, compared with 18 percent of Hispanic men and 16 percent of white men.

Black men in the study were also least likely to be aware of their infection, with 59 percent unaware of their infection compared with 46 percent of Hispanic men and 26 percent of white men.

Age also plays a role. Among 18 to 29-year-old men, 63 percent did not know they were infected with HIV, compared with 37 percent of men aged 30 and older, the team reported in the CDC's weekly report on death and disease.

The CDC recommends that gay and bisexual men of all ages get an HIV test each year, and men at highest risk -- those who have multiple sex partners or use drugs during sex -- get tested every three to six months.

This alarming new data provides further evidence that prevention efforts for gay men have not been adequate to meet the growing epidemic and should be dramatically scaled up, said Carl Schmid of the nonprofit AIDS Institute.

The severity of the impact of HIV in the gay community is nothing new. What has been missing is an appropriate response by our government, at the federal, state and local levels, and the gay community itself, he said in a statement.

Mermin said some studies had shown that there was less urgency and fear associated with HIV infections than in the past, which may be due to the effectiveness of AIDS treatment.

While not a cure, drug cocktails can keep patients healthy and can reduce the risk that they will infect other people. Companies that make HIV drugs include Gilead Sciences Inc, Bristol-Myers and Abbott Labs.

Aids