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A Look at Each Vaccine: Influenza Vaccine

Influenza can be a devastating illness

Every year in the United States, influenza kills thousands to tens of thousands of people. Probably the best example of exactly how devastating influenza can be was the influenza pandemic in 1918 — this worldwide outbreak killed 21 million people in a single influenza season. 

What is influenza?

Commonly known as the flu, influenza is a virus that infects the trachea (windpipe) or bronchi (breathing tubes). Symptoms come on suddenly and include high fever, chills, severe muscle aches and headache. The onset of shaking chills is often so dramatic that many people will remember the exact hour that it started. The virus also causes runny nose and a cough that can last for weeks.

Complications of influenza include severe, and occasionally fatal, pneumonia.

Who should get the influenza vaccine?

The Centers for Disease Control and Prevention (CDC) recommend that everyone 6 months of age and older receive the influenza vaccine each year.

Children 6 months to 8 years of age require two doses of influenza vaccine separated by four weeks if they:

For the first time this year, the CDC has also expressed a preference for healthy children between 2 and 8 years of age to get the nasal spray version rather than the shot if it is available when they report for vaccination. However, the priority is getting them vaccinated as soon as is feasible, so if the nasal spray is not available, they should be given the shot.

How is the influenza vaccine made?

The influenza vaccine is made by growing influenza virus in hen's eggs, purifying it, and completely killing it with a chemical (like formaldehyde). (see How Are Vaccines Made?).

The vaccine can include live or “killed” influenza viruses:

The influenza vaccine is unusual in that most years a different vaccine is made. Because strains of influenza virus that circulate in the community can differ from one season to the next, the vaccine must change to best protect against those different strains. Every year in the United States, the Centers for Disease Control and Prevention (CDC) determines what strains of influenza are circulating, and makes sure that all the influenza vaccines that are made that season contain viruses that would protect against the circulating strains. For this reason, the influenza vaccine is probably the hardest vaccine to make.

Does the influenza vaccine have side effects?

Side effects from the influenza vaccine are extremely rare. Fever or muscle aches generally occur in those who never had influenza or the influenza vaccine before. These symptoms do not mean that you have "the flu." Because the vaccine virus in the shot is "inactivated," it cannot cause respiratory symptoms, such as congestion and cough, which are common with influenza infections; however, people who receive the nasal spray may experience mild congestion or runny nose.

Although the influenza vaccine is made in eggs and some people are severely allergic to eggs, the quantity of egg proteins in the vaccine is insufficient to cause a severe allergic response. But just to be sure, people with severe egg allergies should remain at their provider's office for about 30 minutes after receiving the influenza vaccine.

Do the benefits of the influenza vaccine outweigh the risks?

The influenza vaccine can cause mild side effects. On the other hand, influenza hospitalizes and kills more people in this country than any other vaccine-preventable disease — about 200,000 hospitalizations and thousands to tens of thousands of deaths occur every year. Therefore, the benefits of the influenza vaccine clearly outweigh its risks.

Virtually everyone 6 months of age and older
Disease Risks Vaccine Risks
  • High fever and chills
  • Severe muscle aches
  • Headaches
  • Pneumonia
  • Runny nose and coughing for weeks
  • Disease can be fatal
  • Pain, redness and swelling at the injection site
  • Fever or muscle aches
  • Mild congestion or runny nose (following nasal spray version)


Plotkin SA, Orenstein W, and Offit PA. Inactivated influenza vaccines and influenza vaccine - live in Vaccines, 6th Edition. 2012, 257-311.

Reviewed by: Paul A. Offit, MD
Date: August 2014

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