There are two kinds of influenza vaccine: a killed virus vaccine that is given by injection and a live virus vaccine that is given using a spray in the nose. The vaccine is recommended during flu season, which starts every fall in October in the United States. However, you can still receive the vaccination as late as early spring if the flu season lasts longer than usual.
The flu vaccine is recommended annually for all children ages six months to eighteen years, for all adults over age fifty, and for all other individuals at higher risk of developing complications from influenza. That includes people who have chronic heart, lung, or kidney disease, people with diabetes or immune deficiencies, and women who are pregnant. Note that the previous recommendation for healthy children was only for children ages six months to five years, but in 2008, the recommendation was changed to include all children. Children under nine years of age should receive two doses of the flu vaccine, one month apart, during the first flu season in which they receive the vaccine.
The live-virus nasal-spray vaccine is the more limited of the two types of influenza vaccines. It is only approved for people ages two to forty-nine and should not be given to people who are allergic to eggs, who are on aspirin therapy, or who have asthma. In addition, it should be given cautiously, if at all, to someone who has had Guillain-Barré syndrome (GBS) or to children who have recurrent wheezing. It is also not recommended for people with diseases such as diabetes or chronic illnesses of the heart, lung, or kidneys, or anyone with a weak immune system.
The killed-virus version of the flu vaccine comes in many different forms from many different manufacturers. Some are only for children over four years old; some are approved for children as young as six months old. Some versions contain thimerosal, a mercury derivative (see page 256 for more details on this ingredient), and some are thimerosal-free. In the end, however, there is an injectable version of the influenza vaccine for everyone over six months of age.