NEW YORK, Aug 27 (Reuters) - New Yorkers are contracting HIV at three times the national rate, the city health department said on Wednesday, attributing the difference to New York's large population of high-risk groups such as gay men and blacks.

In 2006, 72 in every 100,000 New Yorkers became infected with HIV, the virus that causes AIDS, compared with the national average of 23 infections, the health department said.

Some 4,800 people contracted HIV in 2006 in New York, long considered the epicenter of the U.S. HIV/AIDS epidemic. About 100,000 New Yorkers are believed to be infected with the virus, officials said.

Blacks and men who have sex with men have the highest rates of new infections and are represented in large numbers in New York City.

Half of the city's 2006 infections occurred among men who have sex with men, the city's health report said.

Black men and women were infected at three times the rate of whites. Some 17 percent of the total number of new infections were among black men who have sex with men and blacks in general accounted for nearly half of the city's new infections.

The data was gathered using a new formula developed by the U.S. Centers for Disease Control and Prevention, which involves testing blood samples found to be HIV-positive to determine when the infection occurred.

Previous data did not distinguish recent infections from those that occurred years earlier, the department said.

Monica Sweeney, the city health department's assistant commissioner for HIV prevention and control, said the numbers will help New York better allocate resources to fight the epidemic. (Reporting by Edith Honan; Editing by Michelle Nichols and Xavier Briand)

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