Functional and Binding H1N1pdm09-Specific Antibody Responses in Occasionally and Repeatedly Vaccinated Healthcare Workers: A Five-Year Study (2009-2014)

Abstract

Background: In 2009, a novel influenza A/H1N1pdm09 emerged and caused a pandemic. This strain continued to circulate and was therefore included in the seasonal vaccines up to the 2016/2017-season. This provided a unique opportunity to study the long-term antibody responses to H1N1pdm09 in healthcare workers (HCW) with a different vaccination history.

Methods: HCW at Haukeland University Hospital, Bergen, Norway were immunized with the AS03-adjuvanted H1N1pdm09 vaccine in 2009 (N=55) and divided into groups according to their vaccination history; one vaccination (N=10), two vaccinations (N=15), three vaccinations (N=5), four vaccinations (N=15) and five vaccinations (N=10). HCW are recommended for influenza vaccination to protect both themselves and their patients, but it is voluntary in Norway. Blood samples were collected pre- and at 21 days, 3, 6, and 12 months after each vaccination, or annually from 2010 HCW without vaccination. ELISA, haemagglutination inhibition (HI) and microneutralization (MN) assays were used to determine the antibody response.

Results: Pandemic vaccination induced a significant increase in the H1N1-specific antibodies measured by ELISA, HI and MN. Seasonal vaccination boosted the antibody response, both in HCW with only the current vaccination and those with prior and current vaccination during 2010/11-2013/14. We observed a trend of increased antibody responses in HCW with only the current vaccination in 2013/14. A two- and three-year gap before vaccination in 2013/14 provided a more potent antibody response compared to annually vaccinated HCW.

Conclusions: Our long term follow up study elucidates the antibody response in HCW with different vaccination histories. Our findings contribute to our understanding of the impact of repeated vaccination upon antibody responses.

Trial registration: ClinicalTrials.gov NCT01003288.

Keywords: ELISA; H1N1pdm09; HI; MN; healthcare workers; influenza; pandemic vaccination; repeated vaccination.

Conflict of interest statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2021 Amdam, Madsen, Zhou, Bansal, Trieu and Cox.

Figures

Figure 1

The H1N1pdm09-specific ELISA IgG, HI, and MN antibody response after pandemic vaccination. The ELISA IgG (A), HI (C) and MN-titres (E) measured in the HCW during 2009/10 (2009pdm). Each symbol represents an individual ELISA, HI and MN titre, and the horizontal line representing the geometric mean titre (GMT) and 95% confidence interval. The GMT for each timepoint in each assay is shown over the graph. The dotted line at 40 and 80 represents the protective titre, in HI and MN respectively. The fold-changes from pre-vaccination ELISA IgG (B), HI (D) and MN titres (F) measured from pre-vaccination titres (D0) to post- pandemic vaccination (D21, 3M, 6M, 12M) are shown. Each symbol represents an individual ELISA IgG, HI and MN fold-change from pre-vaccination titres to day 21, 3, 6 and 12 months after vaccination. The horizontal line shows the geometric mean with 95% confidence interval. The dotted line at 4 indicates seroconversion, and the GMT is shown above the graph. Statistical differences were tested using the Friedman test, with Dunn ´s multiple comparison test. The stars indicate significant differences from pre-vaccination titres, using D0 as a reference. ***P < 0.001, *P < 0.05. Ns, not significant.

Figure 2

The H1N1pdm09-specific ELISA IgG, HI and MN antibody response in HCW with the prior and current vaccination, current vaccination only and unvaccinated HCW. The ELISA IgG (A), HI (B) and MN-titres (C) in HCW who received the prior & current vaccination in 2010/11 compared to unvaccinated HCW in 2010/11. The ELISA IgG (D, G, J), HI (B, E, H, K) and MN titres (C, F, I, L) in HCW who received the prior & current vaccination in 2011/12 – 2013/14 compared to HCW who received only the current vaccination, and unvaccinated HCW. The geometric mean titres with 95% confidence interval are presented. The dotted line at 40 and 80 represents the protective titre, in HI and MN respectively. The data was log-transformed and analysed with the two-way ANOVA with Dunnetts test for multiple comparisons, for detecting differences within a group. Statistical differences between the groups were tested with linear mixed-effect models, with Dunnett´s test for multiple comparisons. The stars indicate significant increases within a group, with pre-vaccination titres (D0) as the reference timepoint. ***P < 0.001, **P < 0.01, *P < 0.05. ### Indicates statistical differences (P<0.05) between groups.

Figure 3

The fold change after vaccination in 2010/11 – 2013/14 in HCW receiving the prior and current vaccination and the current vaccination only. The fold-change from pre-vaccination titres (D0) to post-vaccination titres (D21) measured in ELISA (A), HI (B) and MN titres (C) in the HCW with the current vaccination only and those with the prior and current vaccination. Each symbol represents an individual ELISA, HI and MN fold change. The horizontal line shows the geometric mean with 95% confidence interval, and the dotted line at 4 indicates seroconversion. Statistical differences between the groups were tested with the Kruskal-Wallis test, with Dunn´s multiple comparisons test. **P < 0.01, *P < 0.05.

Figure 4

The H1N1pdm09-specific antibody response over five years after pandemic and seasonal vaccinations. The geometric mean titre with 95% confidence interval measured in ELISA (A), HI (B), and MN (C) in repeatedly vaccinated HCW, and HCW with a one-, two- and three-year gap in vaccinations before vaccination in 2013/14. The titres at pre-vaccination (D0), and at 21 days (D21), 3, 6, and 12 months after vaccination are shown. The gap-years without vaccination is illustrated with a dotted line. The data was log-transformed and compared between groups with linear mixed-effect models, with Dunnett’s test for multiple comparisons. The repeatedly vaccinated HCW was used as the reference group. ***P < 0.001, **P < 0.01, *P < 0.05.

Figure 5

The fold change after vaccination from 2009/10 – 2013/14 in repeatedly vaccinated and HCW with a one-, two- and three-year gap in vaccinations before vaccination in 2013/14. The fold change from pre-vaccination titres (D0) to post-vaccination titres (D21) measured in ELISA (A), HI (B) and MN (C) in the HCW with repeated vaccinations, one-year gap (D–F) two-year gap (G–I) and a three-year gap (J–L) in vaccinations before getting vaccinated in 2013/14. Each symbol represents an individual ELISA, HI and MN fold change. The horizontal line shows the geometric mean with geometric standard deviation. The geometric mean is shown above the graph, and the number of vaccinations is shown below the graph. The dotted line at 4 indicates seroconversion. The Friedman test was used for detecting differences within the different subgroups, with Dunn´s test for multiple comparisons and the fold change in 2013/14 as the reference timepoint. ***P < 0.001, **P < 0.01, *P < 0.05.