Overview of Influenza Surveillance in the United States

Overview of Influenza Surveillance in the United States

The Epidemiology and Prevention Branch in the Influenza Division at CDC collects, compiles and analyzes information on influenza activity year round in the United States and produces FluView, a weekly report from October through mid-May. The U.S. influenza surveillance system is a collaborative effort between CDC and its many partners in state, local, and territorial health departments, public health and clinical laboratories, vital statistics offices, healthcare providers, clinics and emergency departments. Information in five categories is collected from eight different data sources that allow CDC to:

Five Categories of Influenza Surveillance

1. Viral Surveillance — About 80 U.S. World Health Organization (WHO) Collaborating Laboratories and 70 National Respiratory and Enteric Virus Surveillance System (NREVSS), located throughout the United States participate in virologic surveillance for influenza. All state public health laboratories participate as WHO collaborating laboratories along with some county public health laboratories and some large tertiary care or academic medical centers. Most NREVSS laboratories participating in influenza surveillance are hospital laboratories. The WHO and NREVSS collaborating laboratories report the total number of respiratory specimens tested and the number positive for influenza types A and B each week to CDC. Most of the U.S. WHO collaborating laboratories also report the influenza A subtype (H1 or H3) of the viruses they have isolated and the ages of the persons from whom the specimens were collected. The majority of NREVSS laboratories do not report the influenza A subtype. Reports from both sources are combined and the weekly total number of positive influenza tests, by virus type/subtype, and the percent of specimens testing positive for influenza are presented in the weekly influenza update, FluView. Some of the influenza viruses collected by U.S. WHO collaborating laboratories are sent to CDC for further characterization, including gene sequencing, antiviral resistance testing and antigenic determination. This information is presented in the antigenic characterization and antiviral resistance sections of the FluView report.

Surveillance for Novel Influenza A Viruses –??In 2007, human infection with a novel influenza A virus became a nationally notifiable condition. Novel influenza A virus infections include all human infections with influenza A viruses that are different from currently circulating human influenza H1 and H3 viruses. These viruses include those that are subtyped as nonhuman in origin and those that are unsubtypable with standard methods and reagents. Rapid reporting of human infections with novel influenza A viruses will facilitate prompt detection and characterization of influenza A viruses and accelerate the implementation of effective public health responses.

2. Outpatient Illness Surveillance — Information on patient visits to health care providers for influenza-like illness is collected through the US Outpatient Influenza-like Illness Surveillance Network (ILINet).

3. Mortality Surveillance — Rapid tracking of influenza-associated deaths is done through two systems:

4. Hospitalization Surveillance — Laboratory confirmed influenza infections in children and adults are monitored through the Emerging Infections Program (EIP).

5. Summary of the Geographic Spread of Influenza — State health departments report the estimated level of spread of influenza activity in their states each week through the State and Territorial Epidemiologists Reports. States report influenza activity as no activity, sporadic, local, regional, or widespread. These levels are defined as follows:

Together, the five categories of influenza surveillance are designed to provide a national picture of influenza activity. Pneumonia and influenza mortality is reported on a national level only. Outpatient illness and laboratory data are reported on a national level and by influenza surveillance region.

The state and territorial epidemiologists"e; reports of influenza activity are currently the only state-level information reported publicly. EIP data provides population-based, laboratory-confirmed estimates of influenza-related hospitalizations but are reported from limited geographic areas.

It is important to maintain a comprehensive system for influenza surveillance for several reasons:

It is important to remember the following about influenza surveillance in the United States: