Influenza, also known as the flu, is a contagious disease that is caused by the influenza virus. It attacks the respiratory tract in humans (nose, throat, and lungs). The flu is different from a cold. Influenza usually comes on suddenly and may include these symptoms: fever, headache, tiredness (can be extreme), dry cough, sore throat, nasal congestion and body aches. These symptoms are usually referred to as "flu-like symptoms.
According to the ACIP and the American Academy of Pediatrics (AAP):
Children aged 6 months to 8 years who are receiving the influenza vaccine for the first time or whose previous vaccination status is unknown should receive two (2) doses of vaccine separated by at least four weeks.
Children aged 6 months through 8 years who have NOT received two (2) or more total doses of seasonal influenza vaccine since July 2010 should receive two (2) doses of vaccine separated by at least four weeks.
Children aged 6 months through 8 years who received two (2) or more total doses of seasonal influenza vaccine since July 2010 and all children 9 and older should receive one dose of seasonal influenza vaccine.
The National Institute of Allergy and Infectious Diseases explains the emergence and potential spread of new influenza viruses.