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Questions and Answers:
Advances in Methods of Measuring Incidence |
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- Why does there appear to be an annual increase in new HIV
infections?
It is
important to note that the new estimates do not show an increase in new
HIV infections. The new estimates are from the first national
surveillance system of its kind in the world that is based on direct
measurement of new HIV infections using technology that can distinguish
recent from long-standing, population-based infections. Using the new
surveillance system, CDC estimates that 56,300 new HIV infections
occurred in the United States in 2006. CDC previously estimated that
approximately 40,000 new HIV infections occurred annually since the
1990s. The 2006 incidence estimate is about 40% higher than the
previous estimates. The new system reveals that the epidemic is—and has
been—worse than previously estimated and underscores the need to expand
HIV prevention to reach those at greatest risk.
What is STARHS?The
CDC-developed, breakthrough technology known as STARHS stands for
Serological Testing Algorithm for Recent HIV Seroconversion. This
technology allows us to know, for the first time, which HIV infections
are new. CDC applied this technology in the first national surveillance
system of its kind in the world to directly measure new HIV infections.
CDC scientists worked for almost a decade to realize the promise of
STARHS technology by combining it with an expanded HIV reporting system
in the U.S. This combined approach will allow CDC to better monitor
the current course of the epidemic, target prevention efforts where
they are needed most, and assess the impact of prevention efforts.
Why is the new national HIV incidence estimate more precise and
more reliable than prior estimates of HIV incidence?CDC’s
new HIV surveillance system is based on an approach known as STARHS
(Serological Testing Algorithm for
Recent HIV Seroconversion), which uses innovative testing technology to
determine at the population level which positive HIV tests represent new
HIV infections (those that occurred within approximately the past five
months). Before the widespread availability of this technology, HIV
diagnosis data provided the best indication of recent trends in key
populations. However, diagnosis data only indicate when a person is
diagnosed with HIV, not when an individual was actually infected, which
can occur many years before a diagnosis.
By
applying this technology to new HIV diagnoses in 22 states with mature
name-based HIV reporting systems, CDC was – for the first time – able to
identify those diagnoses in a given year that represented new
infections. Using a complex statistical model, these data were
extrapolated to the general population to provide the first national
estimates of HIV incidence based on direct measurement.
CDC
researchers also used a separate method called “extended back
calculation” to confirm the official 2006 incidence estimates and to
examine historical trends in HIV infections in the U.S. from 1977 to
2006. The method uses a statistical model that considers all HIV and
AIDS cases diagnosed through 2006 and reported to CDC, as well as HIV
testing patterns. This particular extended back calculation approach has
become possible in the United States because of an expanded name-based HIV reporting system, which provides a
population-based system for identifying new diagnoses. However, the
method is an indirect measure of incidence and is most reliable for
earlier years; data for the most recent years (2003-2006) must be
interpreted with caution. Additionally, extended back-calculation does
not generate single-year estimates, instead providing averages over
multiple-year periods.
The
statistical methods used to develop the 2006 incidence estimates, as
well as the extended back calculation of historical trends, were
developed in consultation with outside experts, and both the methods and
their application underwent rigorous external scientific reviews.
Moving
forward, the STARHS-based surveillance system will provide the most
reliable way to monitor incidence trends. Over time, the picture will
become even more clear as analyses for specific populations are
completed (e.g., black women, young men who have sex with men). Trend
information from this system will allow for improved targeting and
evaluation of prevention efforts for the populations at greatest risk
and improved monitoring of the impact of HIV prevention efforts.
How was incidence extrapolated from 22 states to the entire United States and Washington DC?
- Incidence in 22 states was estimated in almost 70 strata (or subgroups) based on sex, race/ethnicity, age, and transmission category. Using the ratio of the number of new infections to the number of AIDS cases within these strata based on the 22 states, incidence was extrapolated using the number of AIDS cases within these strata for the remaining states and the District of Columbia (all grouped together). This method assumes that the strata-specific ratio of HIV infection to AIDS incidence in the 22 states (all grouped together) is similar to the ratio in other states and DC (all grouped together), an assumption generally supported by historical trends in HIV and AIDS diagnoses in the United States
How can states calculate their own area-specific incidence estimates?
- For states that are not conducting HIV incidence surveillance, local data on HIV diagnoses are best to describe the status of the local epidemic. These states do not have the data that are needed to compute their own incidence estimates.
- Areas that are part of the national HIV incidence estimate can calculate their own estimates. Some areas that participated in HIV incidence surveillance and are not part of the national estimate can also calculate their own incidence estimate. CDC provided training on how to do this and will continue to provide technical assistance.
- To calculate a local incidence estimate, an area must have participated in HIV incidence surveillance and have a sufficient number of BED results and HIV testing histories for their cases. (The numbers required are specified in the CDC technical guidance).
How does CDC know that the higher incidence estimate is not due
to an actual rise in new HIV infections?It is
important to note that the new estimate for national HIV infections does
not represent an actual increase in the annual number of new HIV
infections. A separate CDC historical trend analysis suggests that the
number of new infections was never as low as 40,000 and has been roughly
stable since the late 1990s. Although CDC’s analysis suggests that
overall the epidemic has been stable in recent years, steady increases
in new HIV infections are estimated to have occurred among men who have
sex with men (MSM) since the early 1990s, and the overall number of new
HIV infections remains unacceptably high. These findings underscore the
ongoing challenges in confronting this disease and the urgent need to
expand access to effective HIV prevention programs.
Do the new HIV incidence estimates indicate that the number of
new HIV infections among women is increasing?No.
CDC’s historical analysis indicates that the number of infections among
women has remained relatively stable since the early 1990s.
Where can I find the data for the new HIV incidence estimatesThe new
incidence estimates were released by The Journal of the American
Medical Association (JAMA) on August 3, 2008. The new
estimates are also available on this Web site.
How do the 2006 HIV incidence estimates affect future HIV prevention
efforts?The new
technology provides a critical missing piece in tracking the U.S. HIV
epidemic and, over time, will allow us to better direct and measure
progress in prevention efforts among populations at highest risk for
HIV. The current HIV incidence estimates, as well as future annual
estimates, will become one of CDC’s most sensitive measures for
monitoring progress toward reducing HIV transmission in the U.S.
Although the 2006 national incidence estimate may appear different than
suggested by previous HIV incidence studies, the overall data are
consistent with CDC’s HIV prevention priorities. CDC will strive to
expand the reach of prevention services for at-risk populations and
remain committed to its core prevention priorities: reducing new
infections, increasing knowledge of HIV status, linking persons infected
with HIV to care and prevention services, and strengthening the ability
to monitor the epidemic.
Will the 2006 HIV incidence estimates cause CDC’s priority
populations to shift?Not at
this time. Gay and bisexual men of all races and African Americans
continue to be hardest hit by the epidemic; and about 25% of people
living with HIV remain unaware of their infections. The new estimates
also show that more HIV infections occur among young people under age 30
than any other age group. More detailed information about trends in HIV
incidence among youth and other at-risk populations will become
available over time. As these data become available, CDC priorities may
change over time in accordance with these trends.
Which populations
experienced the greatest burden of HIV infections in 2006?The 2006 HIV incidence
estimates show: -
Gay and bisexual men of all races remain the group most heavily
affected by HIV, accounting for 53% of all new infections.
- The impact of HIV is greater among blacks than any other racial
or ethnic group, with an HIV incidence rate that is 7 times higher
than whites (83.7/100,000 for blacks compared to 11.5/100,000 for
whites) and almost 3 times higher than Latinos (29.3/100,000).
What does the new
HIV incidence estimate imply about HIV prevention efforts?We have
significant evidence that prevention works. Infection rates for people
who inject drugs have declined dramatically over time. Infection rates
in women of all races have been relatively stable since the early 1990s.
Infection rates in heterosexuals who engage in high-risk sexual
behaviors have been relatively stable since the early 1990s. While
stability and declining infection rates in certain populations is an
important sign of progress, the new estimates underscore the need to
expand access to HIV prevention. Many populations at risk for HIV are
not being reached by our collective prevention efforts, and much more
must be done. For example, recent data show that over the past year, 80%
of MSM are not being reached by the intensive interventions we know to
be most effective. This underscores the challenge of reaching new
generations while adapting to the evolving epidemic. Additionally, far
too many undiagnosed individuals remain unaware of their infections. We
must ensure that resources and prevention efforts are commensurate with
the scale of the epidemic.
Will CDC be looking at new interventions and communication
strategies to prevent HIV among populations hardest hit by the epidemic?
Yes, CDC
is currently working to expand the use of new scientifically proven
interventions for a range of populations at risk and testing a number of
innovative behavioral interventions for key risk groups, particularly
African American men who have sex with men (MSM). CDC remains committed
to expanding HIV prevention and care services to all at-risk populations
and those hardest hit by the epidemic, such as African Americans and MSM. It is clear that there is an urgent need to accelerate progress,
and as a nation we must continue to invest in prevention and extend the
reach of HIV prevention services.
Why is the new method viewed as a breakthrough in HIV incidence
monitoring?This new method is viewed as a breakthrough in HIV incidence monitoring
because it can distinguish recent from long-standing HIV infections. The new method provides the clearest picture of the U.S. epidemic to
date, and over time, will enable CDC to better target and evaluate
prevention efforts for the populations at greatest risk. Accurately tracking the HIV epidemic is essential to CDC’s HIV
prevention efforts.
How will STARHS influence CDC’s HIV/AIDS surveillance and
prevention activities?The 2006
HIV incidence estimates provide the clearest picture of the U.S.
epidemic to date, and over time will enable CDC to better target and
evaluate prevention efforts for the populations at greatest risk. By
applying the Serological Testing Algorithm for Recent Seroconversion (STARHS)
to new HIV diagnoses in 22 states with name-based HIV reporting systems,
CDC is able to identify new diagnoses that represent new infections. These data are extrapolated to the general population to provide the
first national estimate of HIV incidence based on direct measurement. The STARHS-based surveillance system will provide the most
reliable way to monitor incidence trends and result in more accurate
and timely HIV incidence estimates to better direct prevention
programs, mobilize communities and inform resources decisions.
Will the 2006 estimate of annual new HIV infections cause a
corresponding increase in the often-cited figure of more than 1 million
people living with HIV/AIDS in the United States? What about the 25%
estimate of those who don’t know they are infected?No. The 2006 HIV incidence estimate does
not affect the previous HIV prevalence estimate of more than 1 million
people living with HIV/AIDS. New estimates of national HIV prevalence
and the proportion of persons who know their HIV status in 2006 will be
generated later in 2008.
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