Background: In the United States, more children are being vaccinated against influenza in August and September, months before peak influenza activity. Sustained vaccine efficacy through 12 months postvaccination has been demonstrated in children for live attenuated influenza vaccine (LAIV) but not trivalent inactivated influenza vaccine (TIV). Three large, randomized studies compared LAIV and TIV efficacy in children, providing the opportunity to examine the impact of time on the relative efficacy of the 2 vaccines.
Methods: For each study, the relative efficacy of LAIV versus TIV was analyzed by time interval (0–4 and >4–8 months postvaccination) for matched and mismatched strains.
Results: LAIV recipients had less influenza than TIV recipients during both intervals; the relative efficacy of LAIV versus TIV for matched strains in each study increased from 0 to 4 months (range, 25%–60%) to >4 to 8 months (range, 49%–89%). Analysis of the incidence of individual types/subtypes revealed the same pattern for the predominant matched strain in each study; no consistent pattern was seen for lower-incidence matched strains. For mismatched strains, similar relative efficacy was seen in each time interval.
Conclusions: For matched strains, data suggest that the relative efficacy of LAIV versus TIV in young children increases over time. Consistent with previous studies of TIV-induced immunity, this analysis suggests that the absolute efficacy of TIV against matched strains in children may be lower at >4 to 8 versus 0 to 4 months postvaccination. Relative efficacy against mismatched strains was similar over time, consistent with previous estimates of the absolute efficacies of the vaccines against mismatched strains. Further research is needed to confirm these findings and to characterize the duration of protection provided by TIV in children.