Prevalence vs. Incidence
The prevalence of HIV/AIDS includes all people living with HIV and AIDS, regardless of when they acquired the infection. The prevalence is expressed as a percentage of the population at a point in time.
HIV incidence, on the other hand, is the number of new HIV infections in a specific population during a specific period.
It is important to remember that a new HIV diagnosis does not necessarily mean a new infection. Many people only find out they are infected years after their initial infection.
Why is the incidence of HIV important?
National HIV/AIDS surveillance had been limited to monitoring HIV prevalence and did not allow us to calculate or track HIV incidence.
With more HIV tests being performed each year, an observed increase in HIV diagnoses may no longer indicate that there may have been an increase in the number of new HIV infections over the same period.
It has become important to distinguish between recent and long-standing HIV infection on the population level and to expand the HIV surveillance system to include both incidence and prevalence.
Accurately measuring incidence can help us understand how HIV is spreading now and where to more effectively focus prevention efforts.
Incidence estimates can also help us measure our progress in reducing the spread of HIV in Florida over time.
How does HIV incidence surveillance work?
A major advance in incidence surveillance has been the development of the Serologic Testing Algorithm for Recent HIV Seroconversion (STARHS). STARHS may distinguish between recent and long-standing HIV infection on a population level.
The STARHS method works by comparing two HIV enzyme-linked immunosorbent assays (EIA). The first EIA is the standard test used for routine diagnosis, which detects very low levels of antibodies.
The second assay is sensitive to the length of time since the infection (i.e., antibody level present). Because a person’s level of antibody gradually increases in the early stages after infection, the result of the second EIA suggests whether they have been infected for a shorter (5 months or less) or longer time.
The assay used is the Calypte HIV-1 BED Capture EIA, which is approved only for surveillance use because it is only accurate at the population level. Under FDA regulations, results of STARHS performed for purposes of HIV incidence surveillance cannot be returned to individuals or their health care providers or used for clinical management.
What specimens are tested with STARHS?
Florida implemented STARHS in public counseling and testing sites in 2004. In November 2006, Florida authorized the use of STARHS testing for HIV surveillance in Chapter 64D-3, Florida Administrative Code (F.A.C). All private and commercial laboratories that provide diagnostic testing for HIV must submit a specimen for STARHS testing.
What else is needed to measure HIV incidence?
Information on testing behavior is needed, such as recency of testing and testing frequency and history of antiretroviral use. Testing and antiretroviral use history information is gathered as part of a comprehensive HIV counseling session.
Estimating HIV incidence
The Stratified Extrapolation Approach developed by the Centers for Disease Control and Prevention (CDC) determines the probability that an individual would have an HIV test during his/her period of recent infection and assigns a ‘weight’ to each case deemed recent or long-standing. Incidence is equal to the sum of the weighted number of infections determined to be recent.
The CDC released new incidence estimates in December 2012 for 2007-2010. In 2010, there were an estimated 47,500 new infections in the United States (95% confidence interval: 42,000-53,000)
Comparing 2008 to 2010, the estimates indicate that the number of new HIV infections remained stable. Even though the analysis shows overall stability in new HIV infections in recent years, the HIV/AIDS epidemic remains at an unacceptably high level.
Florida HIV Incidence Estimates
Using CDC’s methodology, the HIV Surveillance program estimated incidence for 2007-2010 in Florida.
The new estimates show a 29.5% decrease in incidence when comparing 2007 to 2010. The rate of new infections in 2010 was slightly higher than the national rate at 22 per 100,000 population. This is a significant decrease from the rate in 2007, which was 32 per 100,000 population.
Compared to the national estimates, Florida’s most highly impacted groups are similar. From 2007-2010, males accounted for the large majority of new infections, representing 70%, 73%, 73% and 77% of new infections, respectively. Black/African Americans represented 41%, 48%, 44% and 42% of new infections, respectively; and men who have sex with men (MSM) represented 54%, 48%, 56% and 62% of new infections, respectively.
As in the national estimates, Florida’s infection rates per 100,000 demonstrate significant disparities. Most notable is the disproportionate burden of disease among racial/ethnic groups. In 2008, Black/African Americans had an average rate that was 7.5 times that of Whites in Florida; the rate among Hispanics was 2.5 times that of their White peers. Fortunately, in 2010 the gap appears to have closed slightly with Black/African American rates 6.8 times that of Whites; the disparity among Hispanics and whites remained the same. The rate of new infections is consistently highest among individuals aged 25-34 years at the time of infection across all years (61, 65, 50 and 42 per 100,000 population, respectively).
For mode of transmission, MSMs have the highest percentage of new infections across the four years (54%, 48%, 56% and 62%, respectively). Intravenous drug use accounted for the lowest percentage of new infections (9%, 11%, 7%, 6%, respectively) in this category.
The 2007-2010 Florida HIV incidence estimates highlight the populations currently at greatest risk of HIV infection. The estimates will help us focus our prevention strategies and allocate funding accordingly. As more data is collected, it will become easier to track Florida’s HIV incidence trends and adapt our prevention strategies as necessary.
For additional information, please contact Lina Saintus at (850) 245-4444, extension 3376.