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A Look at Each Vaccine: Influenza Vaccine

The influenza vaccine is given to those who are at high risk of developing severe influenza. For children 6 months to 9 years of age that are receiving the influenza vaccine for the first time, a series of two shots is administered, separated by one month. For children who have already been vaccinated with influenza vaccine, or are over 9 years of age, a single influenza shot is administered yearly.

The Centers for Disease Control and Prevention (CDC) recommends that all children between 6 months and 18 years of age receive the influenza vaccine and that all family contacts of children less than 5 years of age receive the vaccine.

Influenza can be a devastating illness

Every year in the United States, influenza kills about 40,000 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. Learn more about pandemic flu and download a print-ready fact sheet»

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?

Anyone who is at high risk of severe or fatal pneumonia caused by influenza virus should receive the vaccine. Groups at highest risk include:

• Children 6 months to 18 years of age and family members in the home of a child less than 5 years of age
• Children or adults with asthma or anyone living in the home of someone with asthma
• Children or adults with sickle cell disease
• Children or adults with other long-term diseases of the heart, kidney or lungs, or diabetes and family members living in the same home
• Children or adults with weakened immunity from cancers (such as lymphoma or leukemia) or HIV infection
• Children on long-term aspirin therapy
• Women who will be pregnant during the influenza season
• Adults over the age of 50
• Residents in a nursing home or other chronic-care facility
• Those likely to transmit influenza to persons at high-risk, including healthcare workers, caregivers, or household members in contact with persons having high-risk conditions and household contacts or out-of-home caretakers of children 24 to 59 months of age
• Any other person older than 5 years of age wishing to reduce the likelihood of becoming ill with influenza

Recently, the CDC recommended all children between 6 months and 5 years of age to receive the influenza vaccine. This change in recommendations was made because infants and young children are the group most likely to be hospitalized with severe influenza infections. Although elderly adults are the group most likely to die from influenza infections (about 40,000 deaths are caused by influenza virus every year in the United States), infants and young children are the group most likely to be hospitalized by influenza infections (about 200,000 hospitalizations are caused by influenza virus every year in the United States).

In 2001, studies performed in Japan and in an army base in the United States showed that immunization of infants and young children with influenza vaccine not only decreased the incidence of hospitalization from influenza virus in that age group, but also decreased the incidence of hospitalization and death from influenza virus in elderly adults. The decrease in deaths in the elderly following vaccination of young children occurred because adults usually catch influenza from young children.

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). The influenza vaccine given as a shot is a "killed" virus like the polio, hepatitis A, or rabies vaccines. (see How Are Vaccines Made?).

The influenza vaccine is unusual in that each year 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 never immunized before or those who have never experienced influenza before. These symptoms do not mean that you have "the flu." Because the vaccine virus is "inactivated," it does not cause respiratory symptoms, such as congestion and cough, which are common with influenza infections.

Those with severe egg allergies may experience a severe allergic reaction to the vaccine (called anaphylaxis). Symptoms include hives, swelling of the throat, low blood pressure or shock. Because the vaccine virus is grown in eggs, anyone with severe egg allergies could experience this side effect. However, for those at highest risk of severe influenza infection, a method is available to "desensitize" people to the egg proteins in the vaccine (see Egg Allergies).

What is FluMist?

The FluMist vaccine is a live weakened form of influenza virus that is given as a nasal spray. The vaccine protects against serious influenza infection without having to get a shot and is recommended for healthy people between 2 and 49 years of age. However, the vaccine should not be given to people who have asthma or are allergic to eggs.

Do the benefits of the influenza vaccine outweigh the risks?

The influenza vaccine can cause mild side effects. Severe side effects (such as anaphylaxis) do occur rarely. When anaphylaxis does occur, it can be successfully treated. Also, those with egg allergies who need the vaccine can be "desensitized" to the egg proteins contained in the vaccine. On the other hand, influenza hospitalizes and kills more people in this country than any other vaccine-preventable disease — about 200,000 hospitalizations and 40,000 deaths occur every year. Therefore, the benefits of the influenza vaccine clearly outweigh its risks.

Reviewed by: Paul A. Offit, MD
Date: March 2008