What is influenza?

Short Answer

Influenza (the flu) is a contagious respiratory illness caused by viruses that are spread through coughing and sneezing. Germs can be spread a day before symptoms start, often before someone knows they’re sick. People who have the flu might have all of the symptoms or just some of them, including fever, chills, cough, sore throat, runny or stuffy nose, muscle or body aches, headaches, and a sluggish feeling. Children may also have vomiting and diarrhea. The CDC’s Advisory Committee on Immunization Practice (ACIP) reviewed research on the nasal spray and found it did not offer enough protection. All flu vaccines for the 2016-2017 season will be given as a shot. Because influenza viruses can undergo mutations over time, the success of the vaccines can vary from year to year but they are generally 60-75% effective in preventing the flu.


Seasonal influenza (Flu) vaccination information from the CDC

Flu vaccine information for parents from the CDC

ACIP votes down use of LAIV for 2016-2017 flu season

Symptoms & Incubation

Flu symptoms can include the following:

  • Fever (not everyone with the flu has a fever)
  • Chills
  • Cough
  • Sore throat
  • Headache
  • Muscle aches
  • Tiredness

Some children with the flu will vomit or have diarrhea.

Symptoms start about two days after contact with the virus. Some people get better in a few days, while others can be sick for weeks. People can spread the flu from one day before symptoms begin to five to seven days after. This can be longer in children and people who are very sick.


Flu illness can be mild or very serious. Flu seasons also vary in how long and how severe they are from one year to another. Between 1976 and 2006, estimated deaths in the United States from the flu ranged from about 3,000 during the mildest season to 49,000 during the most severe season.

Complications from the flu include:

  • Pneumonia (lung infection)
  • Dehydration (loss of body fluids)
  • Worsening of long-term medical conditions, like asthma and diabetes

People who get these complications are often hospitalized. In the United States, each year an average of 20,000 children younger than 5 years old need hospital care because of flu complications.



Flu spreads when infected people talk, cough, or sneeze, and droplets with virus in them land in the mouths or noses of people nearby. You may also get the flu by touching an object with flu virus on it – like a doorknob or used tissue – and then touching your own eyes, nose, or mouth. People who have the flu should stay home (except to seek medical care) until 24 hours after their fever has disappeared without the use of fever-reducing medicine.

High-risk Populations

The flu causes serious problems for very young children, older people, and people with certain long-term medical conditions like asthma and diabetes.


The “flu shot” is an inactivated vaccine (containing killed virus) that is given with a needle, usually in the arm. The flu shot is approved for use in people older than 6 months, including healthy people and people with chronic medical conditions. There are three different flu shots available:

- a regular flu shot approved for people ages 6 months and older

- a high-dose flu shot approved for people 65 and older

- an intradermal flu shot approved for people 19 to 64 years of age

The flu vaccines is 70-90% effective in preventing the flu.

CDC’s Advisory Committee on Immunization Practices voted that “nasal spray” flu vaccine should not be used during the 2016-2017 flu season.  The Committee continues to strongly recommend everyone 6 months and older get an annual flu vaccination with the flu shot.

How well the flu vaccine works can range widely from season to season, and can be affected by a number of factors.  The reason for the recent poor performance of the nasal spray vaccine is unknown. The change in the recommendation is an example of using new data to ensure public health recommendations offer the best protection. Influenza is a serious disease that causes millions of illnesses, hundreds of thousands of hospitalizations, and thousands of deaths each year. While the protection offered by flu vaccines can vary, millions of people were protected against flu last season.

Safety & Side Effects

Hundreds of millions of Americans have safely received seasonal flu vaccines. Vaccines, like any medicine, can have side effects. Most people who get the flu vaccine have no side effects. Those that do occur are almost always mild, like a sore arm where the shot was given, or a fever. Serious side effects are very rare.


Everyone 6 months and older should get a flu vaccine every year. This is because a new vaccine is made each year to protect against the latest flu viruses and mutations of previous versions of the virus. Also, protection from the flu vaccine wears off after time and needs to be renewed each year.

The body takes about two weeks to build protection against the flu after getting the vaccine, and protection lasts throughout the flu season.

Some children 6 months through 8 years of age may need two doses of the vaccine to be fully protected; ask your pediatrician how many doses are right for your child. The flu vaccine can be given at the same time as other childhood vaccines.

Babies younger than 6 months are too young to get either vaccine, but they can be protected if their mother is vaccinated during pregnancy, and if everyone around them gets the flu vaccine.

Since the Vaccine Was Introduced…

We no longer see devastating influenza pandemics, the worst of which killed an estimated 50 million people and sickened one-fifth of the world’s population in 1918. Both the number and severity of flu cases have significantly decreased since the vaccine has been more widely adopted.