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Flu Activity Reports
& Surveillance Methods in the United States |
Overview of Influenza Surveillance in the United
States
The Influenza Branch at CDC collects and reports information
on Influenza activity in the United States each week from
October through May. The U.S. Influenza surveillance system
has four separate components that allow the Influenza Branch
at CDC to:
- Find out when and where Influenza is circulating
- Determine what type of Influenza viruses are circulating
- Detect changes in the Influenza viruses
- Track Influenza-related illness
- Measure the impact Influenza is having on deaths in the
United States
The Four Components of Influenza Surveillance:
1. World Health Organization (WHO) and National Respiratory
and Enteric Virus Surveillance System (NREVSS) Collaborating
Laboratories
About 75 WHO and 50 NREVSS collaborating laboratories located
throughout the United States report the total number of respiratory
specimens tested and the number positive for Influenza types
A and B each week. Some laboratories also report the Influenza
A subtype (H1N1 or H3N2) of the viruses they have isolated
and the ages of the persons from whom the specimens were collected.
Some of the Influenza viruses collected by laboratories are
sent to CDC for more testing.
2. U.S. Influenza Sentinel Providers Surveillance
Network
Each week, approximately 900 healthcare providers around
the country report the total number of patients seen and the
number of those patients with Influenza-like illness (ILI)
by age group. For this system, ILI is defined as fever (temperature
of >100°F) plus either a cough or a sore throat.
The percentage of patient visits to sentinel providers for
ILI reported each week is weighted on the basis of state population.
This percentage is compared each week with the national baseline
of 2.5%. The baseline is the mean percentage of visits for
ILI during non-Influenza weeks for the 2000-01 to 2002-03
seasons plus 2 standard deviations. Due to wide variability
in regional level data, it was not possible to calculate region-specific
baselines and it is not appropriate to apply the national
baseline to regional level data.
3. 122 Cities Mortality Reporting System
Each week, the vital statistics offices of 122 cities report
the total number of death certificates filed and the number
of those for which pneumonia or Influenza was listed as the
underlying or as a contributing cause of death. The percentage
of all deaths due to pneumonia and Influenza are compared
with a baseline and epidemic threshold value calculated for
each week.
4. State and Territorial Epidemiologists Reports
State health departments report the estimated level of Influenza
activity in their states each week. States report Influenza
activity as no activity, sporadic, local, regional, or widespread.
These levels are defined as follows:
No Activity: No laboratory-confirmed cases
of Influenza and no reported increase in the number of cases
of ILI.
Sporadic: Small numbers of laboratory-confirmed
Influenza cases or a single Influenza outbreak has been reported,
but there is no increase in cases of ILI.
Local: Outbreaks of Influenza or increases
in ILI cases and recent laboratory-confirmed Influenza in
a single region of the state.
Regional: Outbreaks of Influenza or increases
in ILI and recent laboratory confirmed Influenza in at least
2 but less than half the regions of the state.
Widespread: Outbreaks of Influenza or increases
in ILI cases and recent laboratory-confirmed Influenza in
at least half the regions of the state.
Together, the 4 Influenza surveillance components are designed
to provide a national picture of Influenza activity. Pneumonia
and Influenza mortality is reported on a national level only.
Sentinel physician and laboratory data are reported on a national
level and by Influenza
surveillance region. The state and territorial epidemiologists'
reports of Influenza activity are the only state-level information
reported.
It is Important to Remember the Following About Influenza
Surveillance in the United States:
- All Influenza activity reporting by states and health-care
providers is voluntary.
- The reported information answers the questions of where,
when, and what Influenza viruses are circulating. It can be
used to determine if Influenza activity is increasing or decreasing,
but cannot be used to ascertain how many people have become
ill with Influenza during the Influenza season.
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