A new analysis led by Dr. Michael Campsmith from the Centers for Disease Control and Prevention (CDC) takes a closer look at the problem of undiagnosed HIV infection in the United States by examining the estimated proportion of HIV-positive Americans who are undiagnosed across racial/ethnic populations and risk and age groups.

The analysis builds upon the recent CDC estimates indicating that approximately 1.1 million people (1,106,400) were living with HIV in the United States at the end of 2006, and that about one in five of them (21 percent, or 232,700) were undiagnosed (MMWR, October 3, 2008). Researchers used statistical methods to estimate the number of people living with HIV, as well as the number of people who were infected but undiagnosed, using HIV and AIDS data reported in the U.S. These estimates provide an indication of the size and characteristics of the population who is unaware of their infection and may be unknowingly transmitting HIV to others.

The analysis finds significant gaps in knowledge of infection across populations, with the greatest gaps in knowledge among racial/ethnic minorities, males exposed through sexual contact, and youth. Further, the study quantifies the total burden of undiagnosed infection by racial/ethnic and risk groups, indicating that substantially greater numbers of African Americans and gay and bisexual men are living with undiagnosed HIV infection than other populations.

The findings of this analysis point to substantial unmet prevention needs and can be used to focus HIV testing and other prevention initiatives where they will have the greatest impact. These data will also allow CDC to measure the nation's progress in decreasing the proportion of people unaware of their infection over time.

Racial/Ethnic Minorities Less Likely To Be Aware of Their HIV Infections

The new study finds that, among those living with HIV, African Americans and Latinos were less likely to be diagnosed compared to their white counterparts (22.2 percent of blacks and 21.6 percent of Hispanics living with HIV were undiagnosed, vs. 18.8 percent of whites). Because of the substantially greater burden of HIV infection among African Americans, the rate of undiagnosed HIV infection among blacks was nine times that among whites (380.3 per 100,000 population vs. 42.2) and the rate for Hispanics (126.4 per 100,000 population) was nearly three times that among whites. Overall, blacks accounted for nearly half (48.6 percent) of HIV-positive persons unaware of their status -- or an estimated 113,100 individuals living with undiagnosed infection. By comparison, whites accounted for 30.9 percent (72,000) and Hispanics accounted for 18.0 percent (41,900) of individuals living with undiagnosed infection. This analysis adds to an already substantial body of data showing the tremendous toll that HIV takes on African American and Latino communities.

Large proportions of HIV-infected American Indians/Alaska Natives (26.1 percent) and Asians/Pacific Islanders (29.8 percent) were also estimated to be undiagnosed, though these groups represent a relatively small proportion of the total population living with HIV in the United States. The rate of undiagnosed infection among American Indians/Alaska Natives was 60.4 per 100,000 population, and the rate among Asians/Pacific Islanders was 38.6.

Knowledge of HIV Infection Status Differs Substantially by Risk Group

By risk group, men infected through heterosexual contact were most likely to be undiagnosed (an estimated 26.7 percent), followed by MSM (23.5 percent), and women infected through heterosexual contact (21.1 percent). Although the reasons for the high percentage of heterosexual men who are undiagnosed are uncertain, this may be due in part to a low perception of HIV risk among these men or other barriers to accessing HIV testing services.

In terms of the total burden of undiagnosed infection, MSM still account for substantially greater numbers of undiagnosed individuals than any other transmission category, representing more than half (53.7 percent) of HIV-infected persons unaware of their status -- or an estimated 124,900 individuals living with undiagnosed infection. By comparison, heterosexual men accounted for 12.0 percent (27,900) and heterosexual women accounted for 18.3 percent (42,700) of individuals living with undiagnosed infection. These data suggest that expanded HIV testing among MSM remains critical to further reductions in HIV infection in the U.S.

The analysis also indicates that injection drug users (IDUs) were least likely to be undiagnosed (14.5 percent of male IDUs; 13.7 percent of female IDUs). While the reasons for this are unknown, increased knowledge of status among this group may be due in part to frequent interaction with needle exchange, drug treatment, and/or community outreach programs where HIV testing may be offered.

Young People Least Likely to Have Been Diagnosed

Researchers found that nearly half of young people (13-24) living with HIV were undiagnosed (47.8 percent) -- a much greater proportion than any other age group. By comparison, the proportion of HIV-infected individuals that were undiagnosed in the older age groups was in the range of 15 to 30 percent, depending on the specific age group. The high percentage of undiagnosed youth is likely due to lack of knowledge and misperceptions about risk in this group, as well as the lower likelihood that younger people would have been infected long enough to exhibit symptoms of the disease that may prompt an HIV test.

Because older individuals are much more likely to be living with HIV, the greatest burden of undiagnosed infection is among the older age groups. Individuals between 35 and 44 years of age account for the greatest number of undiagnosed individuals, representing an estimated 76,100 undiagnosed infections. By comparison, 54,300 individuals between the ages of 45 to 54, and 49,700 individuals between the ages of 25 to 34 were living with undiagnosed HIV infection. Young people between the ages of 13 to 24 accounted for an estimated 23,100 undiagnosed infections.

HIV Testing Plays a Key Role in Prevention Efforts

As the number of people living with HIV continues to increase, reaching this population with HIV testing, treatment, and prevention services is critical. Testing is the essential first step in linking people with HIV to medical care and ongoing support to help them establish and maintain safer behaviors. A substantial proportion of new infections in the U.S. are believed to be transmitted by those who are unaware of their infection, and studies show that once people learn they are HIV-infected, most take steps to protect their partners. Additionally, data suggest that many people with HIV are diagnosed late in the course of infection, when it may be too late to fully benefit from life-extending treatments.

This study demonstrates the continued importance of HIV testing to ensure that all Americans know their HIV-infection status to protect themselves and their partners from infection. To increase knowledge of serostatus, CDC recommends that all persons likely to be at high risk for HIV be tested for HIV at least annually; further, CDC recommends that HIV screening become a routine part of health care for all individuals between the ages of 13 and 64, regardless of risk.