1. 2013 http://www.cdc.gov/flu/about/qa/disease.htm
2. 2013 http://www.cdc.gov/flu/about/disease/symptoms.htm
3. 2012 http://www.cdc.gov/flu/school/guidance.htm
4. 2011 http://www.cdc.gov/flu/school/slv/index.htm
5. Prevention and control of influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP)-United States, 2012-2013 influenza season;US Centers for Disease Control and Prevention (CDC);MMWR Morb Mortal Wkly Rep,2012