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The Centers for Disease Control and Prevention
(CDC) collects, analyzes, and disseminates
surveillance data on HIV infection and AIDS;
these data are the nation’s source of timely
information on the burden of HIV infection. HIV
surveillance data are used by CDC’s public
health partners in other federal agencies,
health departments, nonprofit organizations, and
academic institutions to help target prevention
efforts, plan for services, and develop policy.
Definitions
This fact sheet contains terms, definitions, and
methods of calculation that are commonly applied
to HIV surveillance data.
Data on HIV infection in the current HIV
Surveillance Report reflect the date of
diagnosis of HIV infection—not the date of
report to CDC. HIV data are displayed by year of
diagnosis.
In the
HIV Surveillance Report, CDC publishes
unadjusted and adjusted—that is, estimated—data
for cases of HIV infection and AIDS.
Unadjusted data include persons who
were diagnosed with HIV infection or AIDS and
have been reported to CDC by state and local
health departments through a given point in
time. As of April 2008, all 50 states, the
District of Columbia, and 6 U.S. dependent areas
(American Samoa, Guam, Northern Mariana Islands,
Puerto Rico, the Republic of Palau, and the U.S.
Virgin Islands) had implemented confidential
name-based HIV infection reporting.
Adjusted (estimated) data are calculated
by applying statistical adjustments to the
unadjusted data, to account for delays in
reporting (of diagnoses and deaths) and for
missing risk factor information. The statistical
adjustments do not account for incomplete
reporting. To allow for stabilization of data
collection and for adjustment of the data to
monitor trends, only areas that have been
conducting confidential name-based HIV infection
reporting for a sufficient length of time are
included in the estimated data. In the 2010 HIV
Surveillance Report, estimated data are shown
for 46 states and 5 U.S. dependent areas that
implemented name-based HIV infection reporting
since at least January 2007 and began reporting
to CDC by June 2007.
Estimated case counts are considered a more
accurate reflection of the burden of disease
than unadjusted case counts because they take
into consideration diagnoses or deaths that have
not yet been reported to the CDC. Therefore, for
planning, resource allocation, and program
evaluation, it is more appropriate to use the
estimated data presented in the HIV Surveillance
Report.
CDC monitors data on HIV infection and AIDS.
Within those categories, CDC monitors
diagnoses, deaths, incidence, stage of disease
at diagnosis, and prevalence.
HIV and AIDS diagnoses: AIDS is diagnosed
when the immune system of a person infected with
HIV becomes severely compromised (measured by
CD4 cell count) and⁄or the person becomes ill
with an opportunistic infection or illness. In
the absence of treatment, AIDS usually develops
8 to 10 years after initial HIV infection; with
early HIV diagnosis and treatment, this may be
delayed by many years. With the release of the
Revised Surveillance Case Definitions for HIV
Infection Among Adults, Adolescents, and
Children Aged <18 Months and for HIV Infection
and AIDS Among Children Aged 18 Months to <13
Years—United States, 2008, CDC has moved
to a staging system of HIV infection that
includes AIDS (HIV infection, stage 3).
AIDS diagnoses and deaths of persons with an
AIDS diagnosis are the number of persons
diagnosed with AIDS and the number of persons
with an AIDS diagnosis who have died in a given
time period, respectively. Note that deaths of
persons with an AIDS diagnosis can be due to any
cause (i.e., the death may or may not be related
to AIDS), and the category is therefore
different from the designation deaths due to
AIDS.
- Unadjusted AIDS diagnoses and deaths of
persons with an AIDS diagnosis are from 50
states, the District of Columbia, and 6 U.S.
dependent areas.
- Estimated AIDS diagnoses and deaths of
persons with an AIDS diagnosis are from 50
states, the District of Columbia, and 6 U.S.
dependent areas. Because all areas implemented
confidential name-based AIDS surveillance in the
early 1980s, their AIDS data can be adjusted to
arrive at the estimations.
Diagnoses of HIV infection and deaths of persons
with a diagnosis of HIV infection are the number
of persons diagnosed with HIV infection and the
number of persons with a diagnosis of HIV
infection who have died in a given time period,
respectively. Note that diagnoses of HIV
infection are regardless of stage of disease at
diagnosis (that is, persons diagnosed with HIV
infection who have not progressed to AIDS;
persons who were diagnosed with HIV infection
and AIDS at the same time; and persons who were
diagnosed with HIV infection and later received
an AIDS diagnosis). Also note that deaths of
persons with a diagnosis of HIV infection may be
due to any cause (i.e., the death may or may not
be related to HIV infection). Other systems
provide data on HIV infection as a cause of
death in the general population. To provide the
reader with a more accurate understanding of the
number of persons diagnosed with HIV infection
who have died, CDC includes in its surveillance
report data on persons diagnosed with HIV
infection regardless of the stage of disease at
death, which includes persons who may have had
an AIDS diagnosis at time of death.
-
Unadjusted
diagnoses of HIV infection and deaths of persons
with a diagnosis of HIV infection are reported
to CDC by 50 states and 6 U.S. dependent areas
that implemented confidential name-based HIV
infection reporting surveillance by April 2008.
- Estimated diagnoses of HIV infection and
deaths of persons with a diagnosis of HIV
infection are from 46 states and 5 U.S.
dependent areas that have had confidential
name-based HIV infection reporting since at
least January 2007 and began reporting to CDC by
June 2007.
Uses of these data: Estimated diagnoses
of HIV infection, AIDS diagnoses, and death data
provide trends of the burden of disease and are
useful for tracking the time from a diagnosis of
HIV infection to an AIDS diagnosis or death.
Discrepancies between populations in the time
from HIV infection diagnoses to AIDS diagnoses
or time to death underscore inequities in access
to testing and care; this knowledge can help
direct resource allocation.
HIV incidence: In general, HIV incidence
is expressed as the estimated number of persons
newly infected with HIV during a specified time
period (e.g., a year), or as a rate calculated
by dividing the estimated number of persons
newly infected with HIV during a specified time
period by the number of persons at risk for HIV
infection.
- Example: An estimated 15,600 whites were
newly infected with HIV in the U.S. in 2009.
- Example: The estimated rate of new HIV
infections among blacks/African Americans was
69.9 per 100,000 population in the U.S. in 2009.
It is important to understand the difference
between HIV incidence and new diagnoses of HIV
infection. HIV incidence refers to persons newly
infected with HIV, whereas individuals newly
diagnosed with HIV may have been infected years
before being diagnosed.
Uses
of these data: Incidence estimates are useful
for planning and allocation of funds, as well as
evaluating the impact of prevention programs.
Persons living with a diagnosis of HIV
infection or AIDS: These terms denote the
number of persons in the 46 states and 5 U.S.
dependent areas with long-term, confidential
named-based HIV infection reporting who have
received a diagnosis of HIV infection and are
still alive, or the number of persons in the 50
states, District of Columbia, and 6 U.S.
dependent areas who have received an AIDS
diagnosis and are still alive.
The data in the HIV Surveillance Report
represent the number of persons living with a
diagnosis of HIV infection who have been
diagnosed, have been reported to the HIV
surveillance system, and have not been reported
as deceased.
HIV prevalence: The number of persons
living with HIV disease at a given time
regardless of the time of infection, whether the
person has received a diagnosis (aware of
infection), or the stage of HIV disease.
Although prevalence does not indicate how long a
person has had a disease, it can be used to
estimate the probability that a person selected
at random from a population will have the
disease. CDC reports prevalence as the number of
persons living with HIV infection in a given
population at a given time and also reports
prevalence rates, calculated per 100,000
population.
- Example: At the end of 2008, an estimated
1,178,350 adults and adolescents were living
with HIV infection in the United States.
- Example: In 2008, the estimated HIV prevalence
rate for blacks/African Americans (1,819 per
100,000 population) was 7.6 times the rate for
whites (238 per 100,000 population).
In 2011, CDC published
HIV Surveillance–United States; 1981-2008.
The estimates include persons infected with HIV
who have not been diagnosed nor reported to the
HIV surveillance system.
Uses
of these data: Prevalence is useful for
planning and resource allocation, as it reflects
the number of people currently needing care and
treatment services for their HIV infection.
Prevalence rates are useful for comparing HIV
disease between populations and for monitoring
trends over time.
Rate: A measure of the frequency of an
event compared with the number of persons at
risk for the event. Rates are calculated by
dividing the number of events (numerator) by the
size of the population (denominator) and
including a measure of time. When comparing
rates between populations, it is typical to
standardize the denominator in order to make
direct comparisons. This standardization will
depend on the magnitude of the local
surveillance data—for national data, the
population size is most often standardized to
100,000.
- Incidence rate: a measure of the frequency
with which new cases of illness, injury, or
other health condition occur, expressed
explicitly per a time frame. Incidence rate is
calculated as the number of new cases over a
specified period divided either by the average
population (usually mid-period) or by the
cumulative person-time the population was at
risk.
- Prevalence rate: the proportion of a
population that has a particular disease,
injury, other health condition, or attribute at
a specified point in time or during a specified
period.
Percentage: A proportion of the whole, in
which the whole is 100.
Proportion: A portion of a population or a
data set, usually expressed as a decimal fraction
(e.g., 0.2), a fraction (1/5), or a percentage of
the population (20%) or of the data set.
Stage of disease: In December 2008, CDC published Revised Surveillance Case Definitions for HIV Infection Among Adults, Adolescents, and Children Aged <18 Months and for HIV Infection and AIDS Among Children Aged 18 Months to <13 Years — United States, 2008.
For adults and adolescents (i.e., persons aged
≥13 years), the surveillance case definitions for
HIV infection and AIDS were revised into a single
case definition for HIV infection that includes AIDS
and incorporates the HIV infection staging
classification system. In addition, the HIV
infection case definition for children aged <13
years and the AIDS case definition for children aged
18 months to <13 years were revised. No changes were
made to the HIV infection classification system, the
24 AIDS-defining conditions for children aged <13
years, or the AIDS case definition for children aged
<18 months. These case definitions are intended for
public health surveillance only and not as a guide
for clinical diagnosis.
A confirmed case meets the laboratory criteria for
diagnosis of HIV infection and one of the four HIV
infection stages (stage 1, stage 2, stage 3, or
stage unknown).
- HIV infection, stage 1: No AIDS-defining
condition and either CD4+ T-lymphocyte count of
≥500 cells/μL or CD4+ T-lymphocyte percentage of
total lymphocytes of ≥29.
- HIV infection, stage 2: No AIDS-defining
condition and either CD4+ T-lymphocyte count of
200–499 cells/μL or CD4+ T-lymphocyte percentage
of total lymphocytes of 14-28.
- HIV infection, stage 3 (AIDS): CD4+
T-lymphocyte count of <200 cells/μL or CD4+
T-lymphocyte percentage of total lymphocytes of
<14, or documentation of an AIDS-defining
condition. Documentation of an AIDS-defining
condition supersedes a CD4+ T-lymphocyte count
of ≥200 cells/μL and a CD4+ T-lymphocyte
percentage of total lymphocytes of ≥14.
- HIV infection, stage unknown: No information
available on CD4+ T-lymphocyte count or
percentage and no information available on
AIDS-defining conditions.
Transmission category: The term for
summarizing the multiple risk factors that a
person may have had by selecting the one most
likely to have resulted in HIV transmission. For
surveillance purposes, persons with more than
one reported risk factor are classified in the
transmission category listed first in the
hierarchy and therefore counted only once. The
exception is men who report sexual contact with
other men and injection drug use; this group
makes up a separate transmission category.
- Male-to-male sexual contact: Persons whose
transmission category is classified as
male-to-male sexual contact include men who
noted sexual contact with other men (i.e.,
homosexual contact) and men who noted sexual
contact with both men and women (i.e., bisexual
contact).
- Heterosexual contact: Persons whose
transmission category is classified as
heterosexual contact are persons who noted
heterosexual contact with a person known to
have, or to be at high risk for, HIV infection
(e.g., an injection drug user or man who has sex
with men).
- Injection drug use: Persons whose
transmission category is classified as injection
drug use are persons who noted receiving an
injection, either self-administered or given by
another person, of a drug that was not
prescribed by a physician for this person. The
drug itself is not the source of the HIV
infection, but rather the sharing of syringes or
other injection equipment (e.g., cookers, and
cottons), which can result in transmission of
bloodborne pathogens, such as HIV.
- Male-to-male sexual contact and injection
drug use: Persons whose transmission category is
classified as male-to-male sexual contact and
injection drug use include men who noted
injecting drugs as well as sexual contact with
other men or sexual contact with both men and
women.
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