About Flu

Key Facts about Influenza (Flu) & 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.

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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 1 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).

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.

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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.

Prevent seasonal flu: Get vaccinated

The single best way to prevent the flu is to get a flu vaccine each season.

There are several flu vaccine options for the 2014-2015 flu season.

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

Trivalent flu vaccine protects against two influenza A viruses (an H1N1 and an H3N2) and an influenza B virus. The following trivalent flu vaccines are available:

The quadrivalent flu vaccine protects against two influenza A viruses and two influenza B viruses. The following quadrivalent flu vaccines are available:

(*“Healthy†in this instance refers to children 2 years through 8 years old who do not have an underlying medical condition that predisposes them to influenza complications.)

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.

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Who Should Get Vaccinated This Season?

Everyone 6 months of age and older should get a flu vaccine every season. 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 United States to expand protection against the flu to more people.

Vaccination to prevent influenza is particularly important for people who are at high risk of serious complications from influenza. See People at High Risk of Developing Flu-Related Complications for a full list of age and health factors that confer increased risk.

More information is available at Who Should Get Vaccinated Against Influenza.

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Use of the nasal spray seasonal flu vaccine

Vaccination with the nasal-spray flu vaccine is an option for healthy* people 2 to 49 years of age who are not pregnant. Even people who live with or care for those in a high risk group (including health care workers) can get the nasal-spray flu vaccine as long as they are healthy themselves and are not pregnant. The one exception is health care workers who care for people with severely weakened immune systems who require a protected hospital environment; these people should get the inactivated flu vaccine (flu shot).

Nasal Spray Vaccine in Children Age 2 to 8 Years Old

Starting in 2014-2015, CDC recommends use of the nasal spray vaccine (LAIV) for healthy* children 2 through 8 years of age, when it is immediately available and if the child has no contraindications or precautions to that vaccine. Recent studies suggest that the nasal spray flu vaccine may work better than the flu shot in younger children. However, if the nasal spray vaccine is not immediately available and the flu shot is, children 2 years through 8 years old should get the flu shot. Don’t delay vaccination to find the nasal spray flu vaccine. For more information about the new CDC recommendation, see Nasal Spray Flu Vaccine in Children 2 through 8 Years Old or the 2014-2015 MMWR Influenza Vaccine Recommendations.

(*“Healthy†in this instance refers to children 2 years through 8 years old who do not have an underlying medical condition that predisposes them to influenza complications.)

Who should not be vaccinated against seasonal flu?

Different flu vaccines are approved for use in different groups of people. Factors that can determine a person's suitability for vaccination, or vaccination with a particular vaccine, include a person's age, health (current and past) and any relevant allergies, including an egg allergy.

Flu Shot:

Nasal Spray Vaccine:

For more about preventing the flu, see the following:

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