Unusual patterns of IgG avidity in some young children following two doses of the adjuvanted pandemic H1N1 (2009) influenza virus vaccine

Abstract

During the 2009-2010 H1N1 influenza pandemic, an adjuvanted monovalent vaccine containing ∼25% of the normal antigen dose and AS03 adjuvant was widely used in Canada. This vaccine was found to be well-tolerated and immunogenic in young children (D. W. Scheifele et al., Pediatr. Infect. Dis. J. 30:402-407, 2011). We report here additional analyses to further characterize the humoral response to this vaccine. We measured standard hemagglutination inhibition (HAI) and microneutralization (MN) titers, as well as influenza virus-specific IgG avidity and subclass distribution by enzyme-linked immunosorbent assay in 73 subjects. Sera were collected before (day 0) and 3 weeks after each dose of vaccine (days 21 and 42). Most children (55/73) had undetectable HAI and MN titers at day 0 (presumed to be antigen naive) and mounted good responses at days 21 and 42. The majority of these children (43/55) had the expected pattern of an increasing IgG avidity index (AI) after each dose of vaccine (not detected [ND], 0.30, and 2.97 at days 0, 21, and 42, respectively). The avidity responses in the remaining children (12/55) were quite different, with AIs increasing abruptly after the first dose and then declining after the second dose of vaccine (ND, 8.83, and 7.15, respectively). These children also had higher concentrations of influenza virus-specific IgG1 and IgG3 antibodies at day 21. Although the antibody titers were similar, some antigen-naive children demonstrated an unusual pattern of avidity maturation after two immunizations with AS03-adjuvanted, low-dose influenza virus vaccine. These data suggest the presence of subtle differences in the quality of the antibodies produced by some subjects in response to this vaccine.

Fig 1

Humoral responses in young children immunized with two doses of AS03-adjuvanted pdmH1N1 split vaccine. Serum samples were collected before and 3 weeks after each immunization, which were administered at visit 1 and visit 2. The HAI titers in panels A, D, G, and J of the serum samples were previously determined and are adapted from Scheifele et al. (8) with the permission of the publisher. MN titer (B, E, H, and K) and AI by avidity ELISA (C, F, I, and L) of serum samples were determined in the present study. (A, B, and C) All subjects analyzed; (D, E, and F) subjects divided into primed or naive subgroups based on HAI titer at visit 1; (G, H, and I) primed subjects further subgrouped based on increasing (primed AI^incr) or stable (Prime AI^flat) avidity; (J, K, and L) naive subjects further subgrouped based on increasing (naive AI^incr) or biphasic (naive AI^biphasic) avidity. Geometric means and 95% confidence intervals are shown. Numbers in brackets beside group names denote the numbers of subjects in each subgroup. Asterisks on connecting lines indicate significant changes between time points of the same group. Asterisks on data points indicate significant differences between two subgroups. ND, not detected; *, P < 0.05; **, P ≤ 0.01; ***, P ≤ 0.001.

Fig 2

Influenza virus-specific IgG subclass analysis in young children immunized with two doses of AS03-adjuvanted pdmH1N1 split vaccine. Serum samples were collected before and 3 weeks after each immunization, which were given at visit 1 and visit 2. The concentrations of influenza virus HA-specific IgG subclass antibodies were determined by ELISA and the percent contribution of subclass antibodies was calculated individually for each subject at each time point and then averaged for each group. Subjects were divided into subgroups as described in Fig. 1. All subjects (A and B) were divided into primed and naive subgroups (C and D) and then further divided into primed groups with increasing or stable AI (primed AI^incr and primed AI^flat) (E and F) or naive groups with increasing or biphasic AI (naive AI^incr or naive AI^biphasic) (G and H). Geometric means and 95% confidence intervals are shown. For subclass concentrations, the IgG1 concentration is graphed on the left y axis, while the IgG2, IgG3, and IgG4 concentrations are graphed on the right y axis. Delta symbols (Δ) at the tops of bars indicate a significant increase (P < 0.05) of the same subclass antibody from the previous time point. Significant differences of the same subclass antibody between subgroups are indicated by brackets. ND, not detected; *, P < 0.05; **, P ≤ 0.01; ***, P ≤ 0.001.