For those who can't get to the link. This is excerpted from the CDC bulletin for 2011-2012:
Recommendations for Vaccination
Routine annual influenza vaccination is recommended for all persons aged ?6 months (1). To permit time for production of protective antibody levels (4,5), vaccination should optimally occur before onset of influenza activity in the community, and providers should offer vaccination as soon as vaccine is available. Vaccination also should continue to be offered throughout the influenza season.
Although influenza vaccine strains for the 2011--12 season are unchanged from those of 2010--11, annual vaccination is recommended even for those who received the vaccine for the previous season. Although in one study of children vaccinated against A/Hong Kong/68 (H3N2) virus, vaccine efficacy remained high against this strain 3 years later, the estimated efficacy of vaccine decreased over the seasons studied (6). Moreover, several studies have demonstrated that postvaccination antibody titers decline over the course of a year (7--10). Thus, annual vaccination is recommended for optimal protection against influenza.
Vaccine Doses for Children Aged 6 Months Through 8 Years
Children aged 6 months through 8 years require 2 doses of influenza vaccine (administered a minimum of 4 weeks apart) during their first season of vaccination to optimize immune response. In a study of children aged 5 through 8 years who received trivalent inactivated vaccine (TIV) for the first time, the proportion of children with protective antibody responses was significantly higher after 2 doses than after 1 dose (11).