Superior Relative Efficacy of Live Attenuated Influenza Vaccine Compared With Inactivated Influenza Vaccine in Young Children With Recurrent Respiratory Tract Infections

Background: Young children have a high incidence of influenza and influenza-related complications. This study compared the efficacy and safety of cold-adapted influenza vaccine, trivalent (CAIV-T) with trivalent inactivated influenza vaccine (TIV) in young children with a history of recurrent respiratory tract infections (RTIs). Methods: Children 6 to 71 months of age were randomized to receive 2 doses of CAIV-T (n = 1101) or TIV (n = 1086), 35 ± 7 days apart before the start of the 2002–2003 influenza season and were followed up for culture-confirmed influenza, effectiveness outcomes, reactogenicity, and adverse events. Results: Overall, 52.7% (95% confidence interval [CI] = 21.6%–72.2%) fewer cases of influenza caused by virus strains antigenically similar to vaccine were observed in CAIV-T than in TIV recipients. Greater relative efficacy for CAIV-T was observed for the antigenically similar A/H1N1 (100.0%; 95% CI = 42.3%–100.0%) and B (68.0%; 95% CI = 37.3%–84.8%) strains but not for the antigenically similar A/H3N2 strains (−97.1%; 95% CI = −540.2% to 31.5%). Relative to TIV, CAIV-T reduced the number of RTI-related healthcare provider visits by 8.9% (90% CI = 1.5%–15.8%) and missed days of school, kindergarten, or day care by 16.2% (90% CI = 10.4%–21.6%). Rhinitis and rhinorrhea, otitis media, and decreased appetite were the only events that were reported more frequently in CAIV-T subjects. There was no difference between groups in the incidence of wheezing after vaccination. Conclusions: CAIV-T was well tolerated in these children with RTIs and demonstrated superior relative efficacy compared with TIV in preventing influenza illness.