In the present study, Ab responses to influenza virus were evaluated by HI assays. For the HI assay, 62 subjects were evaluated (28 subjects with a history of the seasonal influenza vaccination within 3 months [vaccine group] and 34 subjects without prior vaccination [nonvaccine group]). Table S1 in the supplemental material presents the raw data of HI assays of the 62 subjects. Between the vaccine and nonvaccine groups, there was no significant difference in Ab responses to 2009 H1N1 prior to 2009 H1N1 vaccination, presented by GMT (A), and the proportion of subjects with Ab titers of ≥1:40 (B). As expected, the vaccine group presented significantly higher GMTs than the nonvaccine group to the seasonal H1N1 contained in the seasonal influenza virus vaccine (P = 0.011) (C). We next investigated the Ab response to the 2009 H1N1 vaccine in the subjects with or without a recent history of the seasonal influenza vaccination. Importantly, the vaccine group experienced decreased Ab response to the 2009 H1N1 vaccine compared to that of the nonvaccine group, as determined by the proportion of subjects with seroconversion or a ≥4-fold increase in 2009 H1N1-specific Ab titer (P = 0.034) (D).
Fig. 1. Ab responses to the 2009 H1N1 influenza virus vaccine in groups with (vaccine group) or without (nonvaccine group) a recent history of the seasonal influenza virus vaccination. (A and B) Baseline GMTs to 2009 H1N1 (A) and proportions of subjects with (more ...)
As many subjects presented high GMTs to the seasonal H1N1 even without a recent seasonal influenza virus vaccination (C), we analyzed the relationship between preexisting Ab titer to the seasonal H1N1 virus and Ab response to the 2009 H1N1 vaccine. However, we found no difference in vaccine response to 2009 H1N1 between groups with high (≥1:40) and low (<1:40) Ab titers to the seasonal H1N1, as analyzed by the proportion of subjects with seroconversion or a ≥4-fold increase in Ab titer (E). Furthermore, no correlation was found during an analysis of preexisting Ab titer to the seasonal H1N1 virus versus the fold increase in the GMT to 2009 H1N1 (F). In summary, a recent vaccination history against the seasonal influenza virus reduced the vaccine response to the 2009 H1N1 vaccine, though a preexisting Ab response to seasonal H1N1 virus did not influence the 2009 H1N1 vaccine response.
Next, we explored if preexisting T cell immunity to the seasonal H1N1 virus could influence the immune response to the 2009 H1N1 vaccination. To assess preexisting T cell responses, PBMCs were stimulated with inactivated seasonal H1N1 particles, and an IFN-γ enzyme-linked immunospot (ELISpot) assay was performed. We found that the presence of preexisting T cell responses to seasonal H1N1 virus did not affect vaccine response to 2009 H1N1, as evaluated by the proportion of subjects with seroconversion or a ≥4-fold increase in Ab titer (A) and fold increases in GMT to 2009 H1N1 (B).
Fig. 2. Effect of preexisting T cell responses to seasonal H1N1 virus on the Ab responses to the 2009 H1N1 vaccine. (A and B) Proportions of subjects with seroconversion or a ≥4-fold increase in Ab titer to 2009 H1N1 (A) and a fold increase in Ab titer (more ...)
Finally, we performed multivariable logistic regression analysis to identify contributing factors affecting the vaccine response to 2009 H1N1. Our results showed that a recent history of seasonal influenza virus vaccination was an independent factor to make a significant impact on Ab response to the 2009 H1N1 vaccine (P = 0.045) (). As expected by the ceiling effect of Ab response (see Fig. S1 in the supplemental material), a baseline titer of ≥1:40 to 2009 H1N1 was also an independent factor to reduce Ab response to 2009 H1N1 (P = 0.021). A similar result was obtained in the analysis without exclusion of the subjects with a prevaccination titer of ≥1:320 to 2009 H1N1 (see Fig. S2 in the supplemental material).
Fig. 3. Multivariable logistic regression analysis. Multivariable logistic analysis was performed to identify contributing factors which affect the Ab response to the 2009 H1N1 vaccine. One subject was excluded from this analysis since T cell response data were (more ...)
Taken together, our findings demonstrate that a recent history of the seasonal influenza virus vaccination reduced the Ab response to the 2009 H1N1 vaccine. These data suggest that an original antigenic sin effect of the seasonal influenza virus vaccination may be diminishing the Ab responses to the 2009 H1N1 vaccine.