Flu shots are recommended annually for people who are 50 years of age or older, anyone with chronic heart, lung or kidney conditions, and those living in institutions. The vaccine has a 60% to 70% success rate in preventing infection among individuals with normal immune systems; efficacy is lower in individuals with weakened immune systems. The influenza vaccine should not be given to people who are allergic to eggs.

Approximately 8 million children and adolescents between 6 months and 17 years of age have one or more medical conditions that put them at increased risk of influenza-related complications and should be given the first vaccine available:

• Children with chronic disorders of the heart or lungs (such as asthma and cystic fibrosis)
• Children who have required regular medical follow-up or hospitalization during the preceding year because of conditions such as the following:
  • chronic metabolic diseases (including diabetes)
  • kidney dysfunction
  • sickle cell anemia
  • immunosuppression
• Children and teenagers receiving long-term aspirin therapy and therefore might be at risk for developing Reye's syndrome after influenza infection
• Adolescent girls who will be in the second or third trimester of pregnancy during the influenza season

For unvaccinated individuals who have been exposed to people with known influenza, especially if the exposed individual has risk factors, potential use of antiviral medication for more than 2 weeks and vaccination may help prevent illness.