To determine the safety, immunogenicity, and efficacy of revaccination of children
with live attenuated influenza vaccine.
A 2-year multicenter, double-blind, placebo-controlled, efficacy field trial of live
attenuated, cold-adapted trivalent influenza vaccine administered by nasal spray to
children. This report summarizes year 2 results, a year in which the epidemic strain
of influenza A/Sydney was not well matched to the vaccine strains. Each year, vaccine
strains were antigenically 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 influenza virus isolated
from respiratory secretions.
In year 2, 1358 (85%) children, 26 to 85 months of age, returned for revaccination.
The intranasal vaccine was easily accepted, well tolerated, and immunogenic. Revaccination
resulted in 82% to 100% of the vaccinated children in a subset studied for immunogenicity
being seropositive as compared with 26% to 65% of placebo recipients, depending on
the influenza strain tested. No serious adverse events were associated with the vaccine.
In addition 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 contained in the
vaccine, caused 66 of 70 cases of influenza A; nonetheless, intranasal vaccine was
86% efficacious in preventing A/Sydney influenza. Eight cases of lower respiratory
tract disease were associated with A/Sydney influenza; all cases were in the placebo
group.
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 characteristics of this vaccine make it suitable for routine
use in children to prevent influenza.