Efficacy of vaccination with live attenuated, cold-adapted, trivalent, intranasal influenza virus vaccine against a variant (A/Sydney) not contained in the vaccine

Objective: To determine the safety, immunogenicity, and efficacy of revacci nation of children with live attenuated influenza vaccine. Study design: A 2-year multicenter, double-blind, placebo-controlled, effic acy field trial of live attenuated, cold-adapted trivalent influenza vaccin e administered by nasal spray to children. This report summarizes year 2 re sults, a year in which the epidemic strain of influenza A/Sydney was not we ll matched to the vaccine strains, Each year, vaccine strains were antigeni cally equivalent to the contemporary inactivated influenza vaccine. In year 2, a single intranasal revaccination was administered. Active surveillance for influenza was conducted during the influenza season by means of viral cultures. Influenza cases were defined as illnesses with wild-type influenz a virus isolated from respiratory secretions. Results: In year 2, 1358 (85%) children, 26 to 85 months of age, returned f or revaccination. The intranasal vaccine was easily accepted, well tolerate d, and immunogenic. Revaccination resulted in 82% to 100% of the vaccinated children in a subset studied for immunogenicity being seropositive as comp ared with 26% to 65% of placebo recipients, depending on the influenza stra in tested. No serious adverse events were associated with the vaccine. In a ddition to the strains in the vaccine, antibody was induced to the variant strain A/Sydney/H3N2. In year 2, influenza A/Sydney/H3N2, a variant not con tained in the vaccine, caused 66 of 70 cases of influenza A; nonetheless, i ntranasal vaccine was 86% efficacious in preventing A/Sydney influenza. Eig ht cases of lower respiratory tract disease were associated with A/Sydney i nfluenza; all cases were in the placebo group. Conclusions: This live attenuated, cold-adapted influenza vaccine was safe, immunogenic, and efficacious against influenza A/H3N2 (including a variant , A/Sydney, not contained in the vaccine) and influenza B. The characterist ics of this vaccine make it suitable for routine use in children to prevent influenza.