Repeated immunization of children with inactivated and live attenuated influenza virus vaccines: safety, immunogenicity, and protective efficacy.

Annual immunization of healthy children with live attenuated or inactivated influenza virus vaccine currently is under consideration in an attempt to reduce influenza-associated morbidity and mortality in children and the impact of influenza in the community. However, few studies have assessed the effects of this practice. Available data from studies conducted in children and adults provide reassurance that repeated annual immunization is safe and effective. Although the frequencies of significant responses to immunization (such as significant rise in titer or evidence of vaccine virus infection) are lower among persons who were immunized previously when compared with persons immunized for the first time, no consistent increases or decreases in serum antibody levels achieved after immunization or in the level of protective efficacy have been noted. Additional data regarding the effects of annual immunization are needed, and continued efforts to develop improved vaccines for the prevention of influenza are indicated.