Immunity to influenza A virus infection in young children: a comparison of natural infection, live cold-adapted vaccine, and inactivated vaccine.

Live attenuated, cold-adapted (ca) influenza A vaccines administered intranasally have been well characterized as safe and immunogenic, but comparative data on protective efficacy are required for further development. In this study, 59 young children were divided into the following four groups based on prior exposure to influenza A (H3N2) virus: natural infection, live ca vaccine given intranasally, inactivated vaccine given im, and no previous exposure. Virus challenge with homologous live ca vaccine occurred 12 months after vaccination or natural infection. Prior natural infection and live ca vaccine significantly reduced ca virus shedding after challenge compared with inactivated vaccine or no prior exposure to influenza A virus. Prechallenge nasal IgA, detected almost exclusively in subjects naturally infected or vaccinated with live ca virus, was associated with protection. Although inactivated vaccine failed to produce significant local IgA during the primary response, it seemed to prime for secondary local antibody responses after challenge with live ca virus.