Relative Efficacy of Live Attenuated and Inactivated Influenza Vaccines in Children as a Function of Time Postvaccination

Background: In the US, children routinely receive influenza vaccine in August and September, well before peak influenza activity. Vaccine efficacy in children ≥12 months postvaccination has been evaluated for live attenuated influenza vaccine (LAIV) but not trivalent inactivated influenza vaccine (TIV). Three randomized studies have compared the efficacy of LAIV and TIV in children (Study 1: ages 6-59 months, N=7852; Study 2: ages 6-71 months, N=2085; Study 3: ages 6-17 years, N=2211). LAIV is approved for use in eligible children ≥24 months of age.

Objectives: To examine the relative efficacy of LAIV and TIV postvaccination.

Methods: The primary endpoint was relative efficacy of LAIV vs TIV against culture-confirmed influenza by time interval (0-4 and >4-8 mo postvaccination) for matched strains. Match for B viruses in Study 1 was based on genetic sequence as serology was less reliable.

Results:  LAIV recipients had less influenza than TIV recipients in both time intervals. The relative efficacy (95% CI) of LAIV vs TIV increased from 0-4 months to 4-8 months: 60 (−10, 87) vs 89 (53, 99); 34 (−56, 74) vs 61 (25, 80); and 25 (−23, 54) vs 49 (1, 74) in Studies 1-3, respectively. In studies 1 and 2, there were more cases among TIV versus LAIV recipients in the final months of the influenza season (1 vs 17 after 4 mo in Study 1; 2 vs 11 after 5 mo in Study 2). By-strain analysis revealed the same pattern for the predominant matched strain; no consistent pattern was seen for other matched strains that circulated at lower levels.

Conclusions: For matched strains, the data suggest that the relative efficacy of LAIV vs TIV in children increased over time. Further research in children needs to confirm these findings and to characterize the duration of TIV protection. Sponsored by MedImmune.