The highest attack rate for influenza virus infection occurs in school-age children. Live attenuated viruses administered by nasal spray promise to be the preferred route of vaccination for children; however, the age for first administration remains to be determined. Influenza vaccination of pregnant women who will be in the second or third trimester during influenza season not only protects the woman but should provide increased protection for the newborn by increased vertical transmission of antibody. Glezen and associates followed children from birth to one year to assess the risk of influenza during the first year of life and to determine the most effective time for initiation of active immunization.
A total of 209 infants were enrolled in the study at the time of birth. Only 26 infants were infected with influenza virus infection during the first six months of life (infection rate: 12.4 per 100 infants) compared with 43 infections occurring during the second six months of life (infection rate: 20.6 per 100 infants). About 45 percent of the infections were asymptomatic or manifested by afebrile upper respiratory illness only. Illnesses of greater severity were more likely to occur in the second six months of life. Infection rates varied significantly according to the number of older siblings, with the highest rate occurring among infants with three or more siblings.
The infection rate for influenza virus during the first year of life was less than one half the rate for respiratory syncytial virus or parainfluenza virus type 3. Lower respiratory tract illness was also less frequent in infants with influenza. The immunogenicity of influenza vaccines administered to infants at less than six months of age may not be immunologically efficient, probably because of the immaturity of the infant's immune system and the presence of maternal antibodies.
The authors conclude that routine immunization against influenza before six months of age is unnecessary for most infants. Annual immunization for infants more than six months of age offers the most promise of reducing the risk of infection in infants. Maximal protection is obtained by vaccinating older contacts until younger children receive the second annual dose. This is probably more efficient than the alternative schedule of two doses eight weeks apart for the first immunization of naive subjects.