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Disease information

Virus or bacteria

Influenza is caused by influenza virus, which infects the nose, throat, windpipe (trachea) and lungs.

How it spreads

• Coughing
• Sneezing
• Touching contaminated surfaces and then touching your eyes, nose or mouth

If 10 susceptible people are exposed to someone who has influenza, 3 of 10 will typically become infected.

Symptoms

• High fever
• Chills
• Muscle aches
• Headache
• Runny nose
• Cough

Typical disease timeline

Influenza typically lasts 2 to 3 days.

Possible complications

• Pneumonia
• Encephalitis
• Heart disease
• Guillain-Barré syndrome (GBS)
• Death (Thousands to tens of thousands of people every year in the U.S.)

Other things to know

• There are four types of influenza virus: A, B, C and D.
• Influenza A virus infects people, birds, pigs, horses, cats, dogs, sea mammals and aquatic birds. Influenza B virus only infects people. Influenza C virus infects people and pigs, although infection with influenza C virus is rarely reported in people. Influenza D viruses infect cows, but they have not been found to infect people.
• Influenza A viruses can cause epidemics and pandemics. Influenza B viruses contribute to epidemics but do not cause pandemics. Pandemics occur when different influenza viruses combine and create a new version of influenza virus. Because the version is new, most of the world's population is vulnerable.
• Influenza viruses tend to change enough that the influenza vaccine is updated each year.

Vaccine information

The influenza vaccine is used to prevent influenza. 

Vaccine type

• Live, “weakened” vaccines
• Killed viral vaccines
• Recombinant vaccines

Additional information about type of vaccine

Most influenza vaccines are given as a shot and are killed viral vaccines. However, one version of the shot is made using recombinant DNA technology. The nasal spray version of the influenza vaccine is a live, weakened vaccine.

Vaccine side effects

Shot

Common

• Redness, soreness or hardening at the injection site (15 to 20 of 100 recipients)
• Muscle aches (Fewer than 1 of 100 recipients)
• Headache (About 10 of 100 recipients)
• Low-grade fever (About 10 of 100 recipients)

Rare

• Allergic reaction (i.e., hives) (1 of 10,000 recipients)

Very rare

• Guillain-Barré syndrome (GBS) (1 person out of 1 million recipients)
• Serious allergic reaction (fewer than 1 of 1,000,000 recipients)

Nasal spray

Common

• Runny nose (50 to 60 of 100 recipients)
• Mild congestion (50 to 60 of 100 recipients)
• Sore throat (10 of 100 recipients)

Rare

• None

Very rare

• Serious allergic reaction (about 3 of 1,000,000 recipients)

Other things to know

Previously, it was recommended that people with egg allergies not receive the seasonal influenza vaccine. However, studies have shown that the quantities of egg proteins found in the vaccine are not sufficient to cause an allergic reaction. To be on the safe side, people who have egg allergies can stay at the doctor's office for about 15 minutes after receiving the vaccine.

The version made using recombinant technology does not require eggs and, therefore, is free of egg proteins. A second, egg-free version is made using mammalian cells.

Some people with a history of GBS think they cannot get the influenza vaccine; however, the influenza vaccine is only a precaution for people whose case of GBS developed within 6 weeks of receipt of influenza vaccine. Most others with a history of GBS can get the influenza vaccine.

Reasons to get the vaccine

• Influenza is a common infection.
• Children can easily spread influenza.
• Influenza viruses often change each year, so getting the vaccine one year doesn't mean you'll be protected the next year.
• Influenza can lead to other infections, like pneumonia.
• Every year, more people in the U.S. are hospitalized or die from influenza than from all other vaccine-preventable diseases combined.
• The influenza vaccine is safe.