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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

Background on Influenza

What is Influenza?

  • Influenza (the flu) is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness, and at times can lead to death

What are the Symptoms of Influenza?

Symptoms of influenza include the following:

  • Fever* or feeling feverish/chills
  • Cough
  • Sore throat
  • Runny or stuffy nose
  • Muscle or body aches
  • Headaches
  • Fatigue (tiredness)
  • Some people may have vomiting and diarrhea, though this is more common in children than adults.
* It's important to note that not everyone with flu will have a fever

How Influenza Spreads

  • People with flu can spread it to others up to about 6 feet away
  • Most experts think that flu viruses are spread mainly by droplets made when people with flu cough, sneeze or talk
  • These droplets can land in the mouths or noses of people who are nearby or possibly be inhaled into the lungs. (large droplet transmission, small particle droplet nuclei)
  • Less often, a person might also get flu by touching a surface or object that has flu virus on it and then touching their own mouth or nose (contact transmission)

Influenza Incubation Period

  • 1- 7 days (typically 2-3 days)
  • Viral shedding can begin 1 day before illness onset
  • This means that people can be contagious to others from the day before illness
  • Most people will shed virus and possibly able to spread flu to others for 5-7 days after illness onset
  • Children, severely ill persons, and those with weakened immune systems may shed virus for longer than 7 days

What is the Impact of Seasonal Influenza in the United States?

  • Flu seasons vary substantially from year to year
  • Difficult to predict severity or timing
  • 5% - 20% of U.S. population infected
    • Highest illness rates in children
    • Highest complication rates in elderly
  • Annual estimated flu-associated deaths from 1976-2007 ranged from a low of about 3,000 deaths (1986-1987 season) to nearly 49,000 deaths (2003-2004 season)1
  • About 90% of deaths occur among people 65 years of age and older2
  • Annual average of 220,000 hospitalizations – about 50% in 65 and older3
  • Influenza results in substantial economic impact – estimated at ~$37.5 billion4

1-2. CDC. Estimates of deaths associated with seasonal influenza --- United States, 1976—2007. MMWR 2010; 59:1057-1062.
3. Thompson WW, Shay DK, Weintraub E, et al. Influenza-associated hospitalizations in the United States. JAMA 2004; 292:1333-1340.
4. Molinari NM, Ortega-Sanchez, IR, Messonnier ML, et al. The annual impact of seasonal influenza in the US: Measuring disease burden and costs.
Vaccine. 2007; 25: 5086–5096.

Groups at Increased Risk of Severe Influenza

  • Children younger than 5, but especially children younger than 2 years old
  • Adults 65 years of age and older
  • Pregnant women
  • Last flu season, American Indians and Alaskan Natives seemed to be at higher risk of flu complications
  • People who have medical conditions including:
    • Asthma
    • Neurological and neurodevelopmental conditions [including disorders of the brain, spinal cord, peripheral nerve, and muscle such as cerebral palsy, epilepsy (seizure disorders), stroke, intellectual disability (mental retardation), moderate to severe developmental delay, muscular dystrophy, or spinal cord injury].
    • Chronic lung disease (such as chronic obstructive pulmonary disease [COPD] and cystic fibrosis)
    • Heart disease (such as congenital heart disease, congestive heart failure and coronary artery disease)
    • Blood disorders (such as sickle cell disease)
    • Endocrine disorders (such as diabetes mellitus)
    • Kidney disorders
    • Liver disorders
    • Metabolic disorders (such as inherited metabolic disorders and mitochondrial disorders)
    • Weakened immune system due to disease or medication (such as people with HIV or AIDS, or cancer, or those on chronic steroids)
    • People younger than 19 years of age who are receiving long-term aspirin therapy
    • People who are morbidly obese (Body Mass Index, or BMI, of 40 or greater)

Influenza Viruses

  • There are three main types of influenza (flu) virus: Types A, B and C
  • Human influenza A and B viruses cause seasonal epidemics of disease almost every winter in the United States
  • Influenza type C infections cause a mild respiratory illness and are not thought to cause epidemics
  • Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: the hemagglutinin (H) and the neuraminidase (N)
  • Influenza B viruses are not divided into subtypes. Influenza B viruses also can be further broken down into different strains

Key Influenza Viral Features

this illustration provides a 3D graphical representation of a generic influenza virion’s ultrastructure, and is not specific to a seasonal, avian or 2009 H1N1 virus
Surface proteins (major antigens)

Hemagglutinin (HA)

  • HA attaches virus to host cells
  • Antibody to HA is protective

Neuraminadase (NA)

  • Helps release virions from cells
  • Antibody to NA can help modify disease severity

M2 Ion Channel

  • Proton-selective ion channel
  • Lowers the pH inside of the virus resulting in dissociation of the RNPs from the matrix protein (M1)

Ribonucleoprotein (RNP)

  • RNA segment packaged in nucleoprotein
  • An independent transcription active unit (containing a polymerase complex, consisting of PA, PB1 and PB2

Influenza Viruses

  • Current subtypes of influenza A viruses found in people are influenza A (H1N1) and influenza A (H3N2) viruses
  • Influenza A (H3N2) viruses cause the greatest morbidity, mortality
  • Influenza A (H1N1), A (H3N2), and influenza B viruses are included in each year’s influenza vaccine

Influenza: An Ever-Changing Virus

Influenza (flu) viruses can change in two different ways: antigenic drift and antigenic shift

Antigenic drift:

  • Refers to small changes in influenza viruses that happen continually over time
  • Is one of the main reasons why people can get the flu more than one time
    • A person infected with a particular flu virus strain develops antibody against that virus. As newer virus strains appear, the antibodies against the older strains no longer recognize the "newer" virus, and re-infection can occur
  • In most years, one or two of the three virus strains in the influenza vaccine are updated to keep up with the changes in the circulating flu viruses

The other way influenza viruses change is through "antigenic shift"

Antigenic shift:

  • Is an abrupt, major change in the influenza A viruses
  • Results in a new influenza A subtype or a virus with a hemagglutinin or a hemagglutinin and neuraminidase combination that has emerged from an animal population that is so different from the same subtype in humans that most people do not have immunity to the new (e.g. novel) virus
  • An example of a "shift" occurred in the spring of 2009, when a new H1N1 virus with a new combination of genes (from American pigs, Eurasian pigs, birds and humans) emerged in people and quickly spread, causing a pandemic
  • Although influenza viruses are changing by antigenic drift all the time, antigenic shift happens only occasionally

Next: Tracking Influenza >

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