Americans between the ages of 13 and 24 accounted for more than a quarter of new HIV infections in 2010 -- about 12,000 cases -- but only a third of that age group had ever been tested for the virus, the CDC reported.
"This is our future generation, and the bottom line is that every month, 1,000 youth are becoming infected with HIV," said Dr. Thomas Frieden, director of the agency.
The "shocking" data, reported in a Vital Signs article from Morbidity and Mortality Weekly Report, detail the prevalence, incidence, and risk factors of HIV among youths, Frieden said in a teleconference with reporters.
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One implication of the new incidence data is a growing future healthcare burden, Frieden said.
Noting that the lifetime cost of care for a person with HIV is about $400,000, he said: "Every month we are accruing about $400 million of healthcare costs -- and every year $5 billion -- from preventable infections in youth."
"It is just unacceptable that young people are becoming infected at such high rates," Frieden said.
CDC researchers used surveillance data to analyze 2009 prevalence rates of diagnosed HIV among youths and the number of new infections in the 13 to 24 age group in 2010.
They also assessed the prevalence of risk factors and HIV testing among youths, both those still in high school and those 18 through 24.
They found that in 2009, the prevalence of HIV among youth was 69.5 per 100,000 population, with a state-by-state range from 2.3 to 562.8 per 100,000.
The rates were higher in southern and northeastern states compared with the West and Midwest.
Also, of the estimated 47,500 new HIV infections in 2010, 12,200 (25.7 percent) were among youths.
More than four-fifths of the new infections in 2010 (82.8 percent) were acquired by males.
Among newly infected youth, 57.4 percent were African American, 19.6 percent were Hispanics, and 19.5 percent were white.
Male-to-male sexual contact accounted for 72.1 percent of infections, while 19.8 percent were because of heterosexual contact. Injection drug use accounted for 4 percent, and 3.7 percent of infections were due to a combination of male-to-male sex and injection drug use.
Among males, 87.1 percent of infections were attributed to male/male sex, while among females, 85.7 percent were attributed to heterosexual contact.
Overall, youths with HIV made up 6.7 percent of the 1.1 million HIV-positive people in the U.S., the agency reported, and 59.5 percent of those did not know they were infected.
"That's a much higher proportion than the less than the 20 percent we estimate overall don't know they are HIV-infected," Frieden said.
The agency used data from 12 states and nine large urban school districts, collected in 2009 and 2011, to analyze risk behaviors among male and female students in grades 9 through 12.
Males who reported sexual contact with other males, the CDC found, reported more risky behavior than other youths.
For instance, they were more likely to report sexual intercourse with four or more persons during their lifetime (39.4 percent versus 26.9 percent) and to have ever injected any illegal drug (20.4 percent versus 2.9 percent).
Importantly, they were also significantly less likely to have used a condom during last their sexual intercourse (44.3 percent versus 70.2 percent), the agency reported.
They were less likely to report having ever been taught in school about AIDS or HIV infection (74.6 percent versus 86.3 percent), the CDC found.
Overall, in 2011, 12.9 percent of all students in grades 9 through 12 reported that they had ever been tested for HIV, but the proportion reached 22.2 percent among those who reported being sexually active (49.2 percent of males and 45.6 percent of females).
In the older group – those 18 through 24 -- 34.5 percent reported ever having been tested for HIV.
The CDC has recommended for several years that HIV testing should be part of routine medical care, but Frieden said many doctors still haven't bought into the idea.
"You have a very, very small proportion of who refuse testing," he said, "but unfortunately a relatively large proportion of doctors who don't make it routine."