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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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