Influenza-Associated Hospitalizations Surveillance (FluSurv-NET)

Influenza virus. Image courtesy

Influenza virus. Image courtesy


CEIP conducts active, laboratory-based surveillance for all laboratory-confirmed influenza cases hospitalized in the three-county catchment area. A CDC-generated case report form is completed for every case identified. These data are entered into a CDC-generated database and de-identified data are transmitted to CDC on a monthly basis. Data from all FluSurv-NET sites are aggregated and presented on CDC’s weekly FluView website.


  • Characterize the burden of and risk factors for laboratory-confirmed, influenza-associated hospitalizations in a timely manner
  • Describe clinical and epidemiologic characteristics of hospitalized case-patients
  • Conduct special studies (such as influenza vaccine effectiveness) using the surveillance catchment area as a platform

Main Components

  • Active, laboratory-based surveillance
  • Special study: Evaluating the Effectiveness of Influenza Antiviral Treatment in Hospitalized Children
  • Special study: Evaluating the Influenza Burden Estimation Project


Case Definition

Case-patients must be a resident of Alameda, Contra Costa, or San Francisco County and must be admitted to a catchment area hospital between October 1 and April 30. Furthermore, case-patients must either (1) be admitted to a hospital three or fewer days before or within 14 days after a positive influenza test (community associated), or (2) admitted for a non-respiratory illness then subsequently develop respiratory symptoms at least three days after hospital admission and test flu positive more than three days after admission (hospital-acquired). In order to meet the surveillance case definition there must also be evidence of a positive influenza test, including: a viral culture, immunofluorescence antibody staining (Direct [DFA] or indirect [IFA]), reverse transcription polymerase chain reaction (RT-PCR), or a commercially available rapid diagnostic test for influenza.


1. Special Study: Evaluating the Effectiveness of Influenza Antiviral Treatment in Hospitalized Children


Influenza causes thousands of hospitalizations and deaths annually in the United States. Since 2009, antiviral treatment has been recommended for hospitalized patients of all ages with suspected influenza. However, antiviral treatment of hospitalized children with laboratory-confirmed influenza has declined by 27% since the 2009 flu pandemic. The lack of data specific to pediatric populations is thought to have contributed to the observed decline in antiviral use.


This study seeks to evaluate the current antiviral treatment recommendations for hospitalized children with influenza and determine the effectiveness of influenza antiviral treatment to reduce the length of hospitalization.

2. Special Study: Evaluating the Influenza Burden Estimation Project


Surveillance for influenza is conducted annually due to the substantial variability from year to year in the timing, severity, and affected age and risk groups. CEIP’s surveillance activities include surveillance for all known influenza-related hospitalizations. However, not every person truly hospitalized with influenza in the three-county catchment area is tested for influenza and therefore not all cases are reported. Data collected through existing surveillance represent only a fraction of the true annual burden of cases, hospitalizations, and deaths due to influenza. In the spring wave of the 2009 H1N1 pandemic, CDC developed a “pyramid” multiplier model to better estimate the population burden of influenza.

A 2010/2011 influenza season pilot project was initiated to recalculate the multipliers in the pyramid model for a non-pandemic season. However, it remains unclear what potential variation in multipliers there is on a season-by-season basis, as health-care seeking behavior and influenza testing practices may differ as awareness and attention to influenza varies. Thus, in order to better characterize the multiplier methods for estimating the burden of influenza hospitalization on an ongoing basis, it is important to evaluate and refine the multipliers previously developed during a second non-pandemic influenza season.


  • To better estimate the true number of influenza-associated hospitalizations represented by the existing EIP influenza surveillance system
  • To assess potential season-to-season variation in the correction-factor for under-ascertainment of persons hospitalized with influenza



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For questions about influenza surveillance and projects, please contact:

Pamala Daily Kirley, CACLS, MPH
Project Coordinator, Influenza and Pertussis