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Content on this page was developed during the 2009-2010 H1N1 pandemic and has not been updated.

  • The H1N1 virus that caused that pandemic is now a regular human flu virus and continues to circulate seasonally worldwide.
  • The English language content on this website is being archived for historic and reference purposes only.
  • For current, updated information on seasonal flu, including information about H1N1, see the CDC Seasonal Flu website.

NOTE:This page is not intended as a stand-alone Web document and is intended to serve merely as a Section 508-accessible version of the PowerPoint presentation "2009 H1N1: Overview of a Pandemic, April 2009 - August 2010."

2009 H1N1: Overview of a Pandemic

Tracking Influenza

Why do we want to track influenza?

  • Characterize circulating viruses
  • Determine where influenza is spreading
  • Determine to whom influenza is spreading
  • Describe those at risk of severe disease
  • Describe the spectrum of clinical illness
  • Identify viruses with pandemic potential
  • Goal: Make best recommendations for prevention and control

Key Indicators

Influenza activity is monitored by various approaches, primarily with key indicators:

  • Virus characteristics
  • Geographic spread
  • Outpatient illness reports
  • Hospitalizations
  • Deaths

Virus Surveillance

Reports from surveillance partners in clinical settings

  • Participants in the National Respiratory and Enteric Viruses Surveillance System (NREVSS)
  • Generally indicate if influenza A or B

Reports from Public Health Labs participating in CDC’s part of the WHO Global Influenza Surveillance Network. These laboratories report on type and subtype:

  • Influenza B
  • Influenza A
    • Seasonal H1, H3, 2009/H1
    • Unsubtypable A – requires immediate report

  • CDC supports 95 U.S. public health and reference laboratories with staff, equipment, and reagents
  • CDC maintains an "Influenza Reagent Resource" to manufacture and distribute test reagents for U.S. and over 140 countries
  • System monitors for antiviral resistance, genetic mutations, and for vaccine match
  • CDC has continuous development of new and updated diagnostic tests for improved detection

Disease Surveillance

Disease surveillance monitors:

  • Geographic spread of disease in the community
  • Where and when the flu is showing up in clinics and emergency departments
  • In hospitals, the severity, clinical illness, and those at risk
  • Mortality via death reports

Summary of the Geographic Spread of Influenza

  • State and Territorial Epidemiologists Reports – State health departments report the estimated level of spread of influenza activity in their states each week
  • States report influenza activity as no activity, sporadic, local, regional, or widespread

Outpatient Illness Surveillance

  • The Outpatient Influenza-like Illness Surveillance Network (ILINet) consists of more than 3,000 healthcare providers in all 50 states, the District of Columbia and the U.S. Virgin Islands reporting over 25 million patient visits each year
  • Approximately 1,400 outpatient care sites around the country report weekly data to CDC on the total number of patients seen and the number of those patients with influenza-like illness (ILI) by age group

Hospitalization Surveillance – Emerging Infections Program (EIP)

  • The EIP Influenza Project conducts surveillance for laboratory-confirmed influenza related hospitalizations in children and adults 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 and adults with a documented positive influenza test. EIP estimated hospitalization rates are reported every other week during the influenza season, but were reported weekly during the 2009 H1N1 pandemic

Mortality Surveillance

  • 122 Cities Mortality Reporting System Information on patient visits to health care providers for influenza-like illness is collected through the US Outpatient Influenza-like Illness Surveillance Network (ILINet)
  • Surveillance for Influenza-associated Pediatric MortalityInfluenza-associated deaths in children (persons less than 18 years) was added as nationally notifiable condition in 2004. Laboratory-confirmed influenza-associated deaths in children are reported through the Influenza-Associated Pediatric Mortality Surveillance System

Next: Preparing for a Pandemic >

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