Background: Immunoglobulin A (IgA) is the predominant antibody produced in response to mucosal infections. The role of IgA in providing protection against influenza in children vaccinated with live attenuated influenza vaccine (LAIV) has not been well described.
Methods: Nasal IgA responses were assessed using data from 3 prospective, 2-year, randomized studies comparing LAIV with placebo in children 6-36 months of age. In each study, samples were collected in a subset of patients; a new cohort was enrolled each year. Ratios of strain-specific nasal IgA to total nasal IgA were calculated and prevaccination to postvaccination geometric mean fold-rises (GMFRs) were evaluated. Mean postvaccination IgA ratios were compared for subjects with and without confirmed influenza illness by study and in pooled analyses.
Results: Across studies, a higher percentage of children receiving LAIV had a ≥ 2-fold increase in strain-specific IgA ratio compared with placebo recipients. GMFRs after LAIV in years 1 and 2 ranged from 1.2 to 6.2, compared with 0.5-2.2 among placebo recipients. Similar responses were observed in subjects who were baseline seronegative and seropositive based on serum hemagglutination inhibition antibody titers. In years 1 and 2, the mean postvaccination strain-specific to total IgA ratio was 3.1-fold (P<0.01) and 2.0-fold (P<0.03) higher among LAIV recipients with no evidence of culture-confirmed influenza illness compared with LAIV recipients who developed culture-confirmed influenza illness; a similar and consistent trend was observed for each individual study and type/subtype.
Conclusions: The current analysis demonstrates that nasal IgA contributes to the efficacy of LAIV and can provide evidence of vaccine-induced immunity. However, the inherent heterogeneity in nasal antibody levels and variability in nasal specimen collection hinders the precise evaluation of mucosal antibody responses. Other studies have demonstrated that LAIV-induced immunity is also partially explained by T-cell immunity, serum antibody responses, and innate immunity, consistent with the multi-faceted nature of immunity induced by wild-type influenza infection and other live virus vaccines.
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