Influenza virus infections in infants.

Background: Universal immunization of children with live attenuated cold recombinant vaccine has been proposed. The renewed recommendation for maternal immunization with influenza vaccine should increase the amount of antibody transmitted to the infant and postpone the need for active immunization. This study examines the risk of influenza during the first year of life to provide information about the time to initiate active immunization.

Methods: Infants followed from birth to 1 year of age in the Houston Family Study were monitored weekly for influenza virus infection. Serum specimens were tested for evidence of infection at 4-month intervals.

Results: One-third of 209 infants were infected during the first year; most of the infections occurred during the second 6 months of life. Only 26 of 69 infections were detected before 6 months of age compared with 43 afterward. More striking was the concentration of serious illnesses in the latter half of the first year; 8 of 9 otitis media episodes and 9 of 11 lower respiratory tract illnesses occurred in the older infants.

Conclusions: The combination of increased maternal antibody titers that should result from influenza immunization and the lesser risk of influenza in the first 6 months of life allows initiation of active immunization of children after 6 months of age.