Influenza vaccine is an optional vaccine offered each year at the beginning of the flu season (usually between October and December). It is not part of the routine childhood immunization schedule and is not required by schools for admission. Since 2004, however, it has been strongly recommended by both the CDC and the AAP for all children ages 6 to 23 months.

The vaccine cannot be given before a child is 6 months old. Children under 9 years who are getting the flu shot for the first time need two doses at least four weeks apart. In subsequent years when the shot is given (or the first time in children over age 10 and in adults), only one dose is necessary.

Influenza is the virus that causes the flu. This virus changes slightly each year, so that every winter different strains are spread throughout the world.

Influenza causes high fever, upper respiratory symptoms (such as cough and runny nose), and muscle aches that can be so pro­found they make it difficult to stand or walk. Flu causes more symptoms in older children and adults than in infants, but it is far more dangerous for the very young and the very old. In infants, influenza is one of the most common causes (if not the most common cause some winters) of respiratory distress and hospital­ization. Historically, flu epidemics have killed millions of people. Flu is spread easily from person to person, especially from chil­dren to older adults.

Children who are at higher risk for respiratory failure, hospital­ization, or even death from influenza include children born pre­maturely and children with a history of asthma, cystic fibrosis, chronic pulmonary or cardiac disease, sickle-cell anemia, chronic renal disease, or immune system diseases such as HIV. Women in their second and third trimesters of pregnancy are often en­couraged to get a flu shot at the beginning of the flu season. This vaccine will protect the mothers from febrile illness during preg­nancy and protect their fetuses by providing antibodies that get passed through the placenta.

Each year, as the strains of influenza evolve, the shot changes. Therefore, the vaccine is useful for only one year at a time. Because its effectiveness depends on how well the scientists who design the vaccine can predict the upcoming winter flu strains, some years it is more effective than others.

The flu shot is a killed vaccine. A nasal spray called FluMist is a live-attenuated vaccine similar to MMR and varicella vaccine. FluMist is currently only available to healthy people ages 5 to 49. Side effects occur within 12 hours of receiving the shot. With the flu shot, redness and swelling can occur at the site of the injec­tion. One batch of flu vaccine used in 1976 was associated with the development of temporary paralysis of the nerves called Guittain-Barre’ syndrome; this has not been reported since. FluMist has been reported to cause fever and muscle aches in many recipients.

Because flu vaccine is prepared in embryonated eggs, it should not be given to anyone with a severe egg allergy. If it is, a serious allergic reaction, including anaphylaxis, can occur. Most children with a mild or moderate egg sensitivity can be safely immunized with flu vaccine.

Many formulations of flu vaccine contain thimerosal as a pre­servative, but mercury-free forms also are available. FluMist is thimerosal-free, but it has not yet been approved for use in chil­dren under five years of age.