Influenza Pandemic Preparation and Response: A Citizen's Guide

The U.S. influenza surveillance system has seven different components, including four that operate year-round that allow CDC to:

• Find out when and where influenza activity is occurring
• 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 Seven Components of Influenza Surveillance:

1. World Health Organization(WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) Collaborating Laboratories About 80 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.

Most of the U.S. WHO collaborating 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.

1. U.S. Influenza Sentinel Providers Surveillance Network Each week, approximately 1,200 health-care 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 (37.8 C)) and a cough and/or a sore throat in the absence of a KNOWN cause other than influenza.

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 2.2%. The baseline is the mean percentage of patient visits for ILI during non-influenza weeks for the previous three seasons plus two standard deviations.

Due to wide variability in regional level data, it is not possible to calculate region-specific baselines, and it is not appropriate to apply the national baseline to regional data.

# 122 Cities Mortality Reporting System Each week, the vital statistics offices of 122 cities report the total number of death certificates received and the number of those for which pneumonia or influenza was listed as the underlying or as a contributing cause of death by age group.

The percentage of all deaths due to pneumonia and influenza are compared with a seasonal baseline and epidemic threshold value calculated for each week.

1. 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:

1. No Activity: No laboratory-confirmed cases of influenza and no reported increase in the number of cases of ILI.
2. Sporadic: Small numbers of laboratory-confirmed influenza cases or a single laboratory-confirmed influenza outbreak has been reported, but there is no increase in cases of ILI.
3. 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.

1. Widespread: Outbreaks of influenza or increases in ILI cases and recent laboratory-confirmed influenza in at least half the regions of the state.

# Influenza-associated pediatric mortality Influenza-associated pediatric mortality is a newly added nationally notifiable condition. Laboratory-confirmed influenza-associated deaths in children less than 18 years old are reported through the Nationally Notifiable Disease Surveillance System.

# Emerging Infections Program (EIP) The EIP Influenza Project conducts surveillance for laboratory-confirmed influenza related hospitalizations in persons less than 18 years of age in 60 counties covering 12 metropolitan areas of 10 states (San Francisco CA, Denver CO, New Haven CT, Atlanta GA, Baltimore MD, Minneapolis/St. Paul MN, Albuquerque NM, Las Cruces, NM, Albany NY, Rochester NY, Portland OR, and Nashville TN). Cases are identified by reviewing hospital laboratory and admission databases and infection control logs for children with a documented positive influenza test (viral culture, direct/indirect fluorescent antibody assay (DFA/IFA), reverse transcription-polymerase chain reaction (RT-PCR), or a commercial rapid antigen test) conducted as a part of routine patient care.

EIP estimated hospitalization rates are reported every two weeks during the influenza season.

1. New Vaccine Surveillance Network (NVSN) The New Vaccine Surveillance Network (NVSN) provides population-based estimates of laboratory-confirmed influenza hospitalization rates for children less than 5 years old residing in three counties: Hamilton County OH, Davidson County TN, and Monroe County NY. Children admitted to NVSN hospitals with fever or respiratory symptoms are prospectively enrolled and respiratory samples are collected and tested by viral culture and RT-PCR. NVSN estimated rates are reported every two weeks during the influenza season.

Together, the seven 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 provider 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.

Both the EIP and NVSN data provide population-based, laboratory-confirmed estimates of influenza-related pediatric hospitalizations but are reported from limited geographic areas.

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.