Effect of yearly vaccinations with live, attenuated, cold-adapted, trivalent, intranasal influenza vaccines on antibody responses in children

Background. The cold-adapted, trivalent influenza vaccine (CAIV-T) may become an option for annual vaccination. However, there is little information regarding the immune response to repeated immunization with CAIV-T. Objective. To determine the antibody response to repeated immunization with CAIV-T and to compare this with the response after the first CAIV-T immunization. Design and methods. Healthy children were offered CAIV-T immunization for 4 consecutive years, and blood samples were taken from a subset in Years 1, 2 and 4. In Year 4,156 similarly aged children who had not received influenza vaccine previously were immunized with the same CAIV-T. Results. The H3N2 and B components of the CAIV-T induced high antibody titers in Year 1 that were maintained during 4 years. The HIM titers were lower than the H3N2 or B titers. Comparison of the group immunized for 4 consecutive years with the group immunized for the first time revealed the following: (1) before immunization yearly immunized subjects were more likely to be seropositive to each of the three vaccine strains than those immunized for the first time (P < 0.05 for each); (2) after immunization the percentage of seropositive subjects to each of the strains was similar; (3) after immunization titers were higher in the subjects immunized for the first time than those immunized yearly (P < 0.05 for H3N2 and B). Conclusion. Yearly vaccination with CAIV-T induced high antibody titers, especially to the H3N2 and B strains in the vaccines. The titers in those immunized with CAIV-T for the first time were higher than in those immunized for 4 consecutive years.