* Children younger than 5 years old. The risk for severe complications from seasonal influenza is highest among children younger than 2 years old.
* Adults 65 years of age and older.
* Persons with the following conditions:
o Chronic pulmonary (including asthma), cardiovascular (except hypertension), renal, hepatic, hematological (including sickle cell disease), neurologic, neuromuscular, or metabolic disorders (including diabetes mellitus);
o Immunosuppression, including that caused by medications or by HIV;
o Pregnant women;
o Persons younger than 19 years of age who are receiving long-term aspirin therapy;
o Residents of nursing homes and other chronic-care facilities.

(taken from CDC website)