Key facts about influenza (flu) and flu vaccine

What is influenza (also called flu)?

The flu is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and lungs. It can cause mild to severe illness, and at times can lead to death. The best way to prevent the flu is by getting a flu vaccine each year.

Signs and symptoms of flu

People who have the flu often feel some or all of these signs and symptoms:

  • Fever* or feeling feverish/chills
  • Cough
  • Sore throat
  • Runny or stuffy nose
  • Muscle or body aches
  • Headaches
  • Fatigue (very tired)
  • 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 flu spreads

Most experts believe that flu viruses 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. 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, eyes or possibly their nose.

Period of contagiousness

You may be able to pass on the flu to someone else before you know you are sick, as well as while you are sick. Most healthy adults may be able to infect others beginning one day before symptoms develop and up to 5 to 7 days after becoming sick. Some people, especially young children and people with weakened immune systems, might be able to infect others for an even longer time.

How serious is the flu?

Flu is unpredictable and how severe it is can vary widely from one season to the next depending on many things, including:

  • what flu viruses are spreading,
  • how much flu vaccine is available
  • when vaccine is available
  • how many people get vaccinated, and
  • how well the flu vaccine is matched to flu viruses that are causing illness.

Certain people are at greater risk for serious complications if they get the flu. This includes older people, young children, pregnant women and people with certain health conditions (such as asthma, diabetes, or heart disease), and persons who live in facilities like nursing homes.

Flu seasons are unpredictable and can be severe. Over a period of 30 years, between 1976 and 2006, estimates of flu-associated deaths in the United States range from a low of about 3,000 to a high of about 49,000 people.

Complications of flu

Complications of flu can include bacterial pneumonia, ear infections, sinus infections, dehydration, and worsening of chronic medical conditions, such as congestive heart failure, asthma, or diabetes.

Preventing seasonal flu: get vaccinated

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

There are several flu vaccine options for the 2013-2014 flu season.

Traditional flu vaccines made to protect against three different flu viruses (called “trivalent” vaccines) are available. In addition, this season flu vaccines made to protect against four different flu viruses (called “quadrivalent” vaccines) also are available.

The trivalent flu vaccine protects against two influenza A viruses and an influenza B virus.

CDC does not recommend one flu vaccine over the other. The important thing is to get a flu vaccine every year.

When to get vaccinated against seasonal flu

Yearly flu vaccination should begin soon after flu vaccine is available, and ideally by October. However, getting vaccinated even later can be protective, as long as flu viruses are circulating. While seasonal influenza outbreaks can happen as early as October, most of the time influenza activity peaks in January or later. Since it takes about two weeks after vaccination for antibodies to develop in the body that protect against influenza virus infection, it is best that people get vaccinated so they are protected before influenza begins spreading in their community.

Who should get vaccinated?

Everyone six months and older should get a flu vaccine each year. This recommendation has been in place since February 24, 2010 when CDC’s Advisory Committee on Immunization Practices (ACIP) voted for "universal" flu vaccination in the U.S. to expand protection against the flu to more people. While everyone should get a flu vaccine each flu season, it’s especially important that certain people get vaccinated either because they are at high risk of having serious flu-related complications or because they live with or care for people at high risk for developing flu-related complications.

Who is at high risk for developing flu-related complications?

  • Children younger than five, but especially children younger than two years old
  • Adults 65 years of age and older
  • Pregnant women
  • American Indians and Alaskan Natives seem to be at higher risk of flu complications
  • People who have medical conditions including:
    • Asthma (even if it’s controlled or mild)
    • 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 [BMI] of 40 or greater)

Who else should get vaccinated?

Other people for whom vaccination is especially important are:

  • People who live in nursing homes and other long-term care facilities
  • People who live with or care for those at high risk for complications from flu, including:
    • Health care workers
    • Household contacts of persons at high risk for complications from the flu
    • Household contacts and caregivers of children younger than five years of age with particular emphasis on vaccinating contacts of children younger than six months of age (children younger than six months are at highest risk of flu-related complications but are too young to get vaccinated)

Who should not be vaccinated against seasonal flu?

Some people should not be vaccinated without first consulting a physician. 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.
  • Children younger than six months of age (influenza vaccine is not approved for use in this age group).
  • People who have a moderate or severe illness with a fever should wait to get vaccinated until their symptoms lessen.
  • People with a history of Guillain-Barré Syndrome (a severe paralytic illness, also called GBS) that occurred after receiving influenza vaccine and who are not at risk for severe illness from influenza should generally not receive vaccine. Tell your doctor if you ever had Guillain-Barré Syndrome. Your doctor will help you decide whether the vaccine is recommended for you.

If you have questions about whether you should get a flu vaccine, consult your health care provider.

For more about preventing the flu, see the following: