The vaccine is specifically recommended for people who are at high risk for developing serious complications as a result of influenza infection. These high-risk groups include all people aged 65 years and older, people with chronic pulmonary and cardiovascular conditions, diabetes, immunosuppression and severe forms of anemia. The vaccine is also recommended for residents of nursing homes and health care workers. As influenza viruses continue to evolve over time, the vaccine is updated each year to include the most current influenza virus strains. Influenza viruses undergo two types of changes.
Antigenic drift occurs with influenza A and B viruses.
The fact that influenza viruses continually change is the major reason that vaccine must be taken every year. Although vaccination is the primary method for control of influenza, antiviral agents also have a role in the prevention and treatment of influenza type A infection. Type C influenza viruses associated with sporadic cases and mild outbreaks. A and B influenza viruses, influenza C is not considered a public health threat. Up to 40,000 excess deaths and 150,000 excess hospitalizations occur during such epidemics.