The role of live influenza vaccines in children

Live attenuated cold-adapted influenza vaccines (CAIVs) have been developed over the past two decades by taking advantage of the segmented RNA genome of influenza and creating attenuated reassortants containing contemporary hemagglutinin (HA) and neuraminidase (NA) genes. These vaccines have been shown to be easily administered, safe and immunogenic in adults and children. Recent trials of a trivalent live attenuated CAIV (CAIV-T, tradename FluMist, Aviron, Mt. View, CA) in children have demonstrated greater than 85% efficacy against culture positive H3N2 and B influenza illness and complications, such as otitis media. CAIV-T also prevented shedding of H1N1 virus in 83% of vaccinated subjects after a monovalent CAIV challenge. Nasal IgA and serum HA inhibition (HAI) antibody produced by these vaccines have been associated with protection against infection, but protection may exist even in the absence of identifiable antibody response. Work to date documenting phenotypic and genetic stability, low likelihood of reactogenicity, infrequent transmissibility and attenuating properties of reassortants heralds promise for the broad use of this vaccine. Targeting children to receive this vaccine may now prove practical and may serve to reduce overall influenza morbidity, given the significant contribution of the pediatric age group of children to influenza illness burden and community spread. Studies of vaccine use in community settings will aid in determining the public health future of this approach.