Influenza (Flu)

Revised: February 2009

What Everyone Should Know About Flu and the Flu Vaccine

What is the flu?

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. Some people, such as older people, young children, and people with certain health conditions, are at high risk for serious flu complications.

Every year in the United States, on average:

  • 5% to 20% of the population gets the flu;
  • more than 200,000 people are hospitalized from flu complications, and;
  • about 36,000 people die from flu.

The best way to prevent this illness is by getting a flu vaccination.

What are the symptoms of the flu?

The flu usually starts suddenly and may include these symptoms:

  • fever (usually high)
  • headache
  • extreme tiredness
  • dry cough
  • sore throat
  • runny or stuffy nose
  • muscle aches
  • stomach symptoms, such as nausea, vomiting, and diarrhea, may occur in children but are rare in adults

Are some people at higher risk for complications than others from getting the flu?

Yes. People at high risk for serious flu complications include older people, young children, and people of any age with certain health conditions.

What are the complications associated with the flu?

Some of the complications caused by flu include pneumonia, dehydration, and worsening of chronic medical conditions, such as congestive heart failure, asthma or diabetes. Children may get sinus problems and ear infections.

How is the flu spread?

The flu is spread in respiratory droplets released by coughing and sneezing. It usually spreads from person to person, though occasionally people may be infected by touching something with virus on it and then touching their mouth or nose.

When and for how long is a person able to spread the flu?

People with flu are contagious (able to infect others) beginning one day before getting symptoms. Adults remain contagious up to seven days after getting sick and children can remain contagious for even longer. That means that you can give someone the flu before you know you're sick as well as when you are sick.

Cold Versus Flu

What is the difference between a cold and the flu?

The flu and the common cold are both respiratory illnesses but they are caused by different viruses. Because colds and flu share many symptoms, it can be difficult (or even impossible) to tell the difference between them based on symptoms alone. Special tests can be carried out, when needed, to tell if a person has the flu; these tests usually must be done within the first few days of illness.

What are the symptoms of the flu versus the symptoms of a cold?

In general, the flu is worse than the common cold, and symptoms such as fever, body aches, extreme tiredness and dry cough are more common and intense. Colds tend to develop gradually, while the flu tends to start very suddenly. Colds are usually milder than the flu. People with colds are more likely to have a runny or stuffy nose. Colds generally do not result in serious health problems, such as pneumonia, bacterial infections, or hospitalizations.

Preventing the Flu

What can I do to protect myself against the flu?

The single best way to prevent the flu is to get a flu vaccination each fall. There are two types of vaccines:

  • The "flu shot" is an inactivated vaccine (containing killed virus) that is given with a needle. The flu shot is approved for use in people older than 6 months, including healthy people and people with chronic medical conditions.
  • The nasal-spray flu vaccine is a vaccine (sometimes called LAIV for "Live Attenuated Influenza Vaccine") made with live, weakened flu viruses that do not cause the flu. LAIV is approved for use in healthy people 2 years to 49 years of age who are not pregnant.
  • Children aged 6 months-8 years should receive 2 doses of vaccine if they have not been vaccinated previously at any time with either the flu shot or the nasal-spray flu vaccine. Children aged 6 months-8 years who received only 1 dose in their first year of vaccination should receive 2 doses the following year.

About two weeks after vaccination, antibodies develop that protect against influenza virus infection. Flu vaccines will not protect against illnesses caused by other viruses, such as the common cold.

Who should get the flu vaccine?

All persons, including school-aged children, who want to reduce the risk of becoming ill with influenza or of transmitting influenza to others should get the flu vaccine. In other words, when there is an adequate supply, everyone should get the flu vaccine.

Those people at greatest risk for complications of the flu and those most likely to get or spread the flu should be vaccinated with the flu vaccine as soon as it is available. These include:

  • Children aged 6 months up to their 19th birthday;
  • Persons aged ≥ 50 years;
  • Children and adolescents (aged 6 months--18 years) who are receiving long-term aspirin therapy and who therefore might be at risk for experiencing Reye syndrome after influenza virus infection;
  • Women who will be pregnant during the influenza season;
  • Adults and children who have chronic lung, heart, kidney, liver, blood, or metabolic disorders (including diabetes);
  • Adults and children who have immune system suppression (including immune system suppression caused by medications or by HIV);
  • Adults and children who have any condition (for example, cognitive dysfunction, spinal cord injuries, seizure disorders, or other neuromuscular disorders) that can compromise respiratory function or the handling of respiratory secretions or that can increase the risk for aspiration;
  • Residents of nursing homes and other chronic-care facilities;
  • Health care personnel;
  • Healthy household contacts (including children) and caregivers of children aged ≤ 59 months (i.e., aged < 5 years) and adults aged ≥ 50 years; and
  • Healthy household contacts (including children) and caregivers of persons with medical conditions that put them at higher risk for severe complications from influenza.

For a full list of recommendations, see http://www.cdc.gov/flu/professionals/acip/persons.htm .

Who should NOT be vaccinated?

There are some people who should not be vaccinated. They include:

  • People who have a severe allergy to chicken eggs;
  • People who have had a severe reaction to an influenza vaccination in the past;
  • People who developed Guillain-Barré syndrome (GBS) within six weeks of getting an influenza vaccine previously;
  • Children less than 6 months of age;
  • People who are sick with a fever. (These people can get vaccinated once their symptoms lessen. People with a mild illness can usually get the vaccine.)

Can antiviral medications prevent the flu?

Four antiviral drugs (amantadine, rimantadine, oseltamivir, and zanamivir) are licensed by the U.S. Food and Drug Administration (FDA) for treatment and prevention of the flu. Antiviral drugs are not a substitute for influenza vaccination. All of these drugs are available only by prescription and are different in terms of who can take them, how they are given, dosages based on age or medical conditions, and side effects. In addition, some influenza virus types or subtypes may be resistant to certain antiviral drugs. Your doctor can help decide whether you should take an antiviral drug and which one you should use.