Antibody responses against influenza A(H1N1)pdm09 virus after sequential vaccination with pandemic and seasonal influenza vaccines in Finnish healthcare professionals

Abstract

Background: Influenza A(H1N1)pdm09 virus has been circulating in human population for three epidemic seasons. During this time, monovalent pandemic and trivalent seasonal influenza vaccination against this virus have been offered to Finnish healthcare professionals. It is, however, unclear how well vaccine-induced antibodies recognize different strains of influenza A(H1N1)pdm09 circulating in the population and whether the booster vaccination with seasonal influenza vaccine would broaden the antibody cross-reactivity.

Objectives: Influenza vaccine-induced humoral immunity against several isolates of influenza A(H1N1)pdm09 virus was analyzed in healthcare professionals. Age-dependent responses were also analyzed.

Methods: Influenza viruses were selected to represent viruses that circulated in Finland during two consecutive influenza epidemic seasons 2009-2010 and 2010-2011. Serum samples from vaccinated volunteers, age 20-64 years, were collected before and after vaccination with AS03-adjuvanted pandemic and non-adjuvanted trivalent seasonal influenza vaccine that was given 1 year later.

Results: Single dose of pandemic vaccine induced a good albeit variable antibody response. On day 21 after vaccination, depending on the virus strain, 14-75% of vaccinated had reached antibody titers (≥1:40) considered seroprotective. The booster vaccination 1 year later with a seasonal vaccine elevated the seroprotection rate to 57-98%. After primary immunization, younger individuals (20-48 years) had significantly higher antibody titers against all tested viruses than older persons (49-64 years) but this difference disappeared after the seasonal booster vaccination.

Conclusions: Even a few amino acid changes in influenza A HA may compromise the vaccine-induced antibody recognition. Older adults (49 years and older) may benefit more from repeated influenza vaccinations.

Figure 1

A schematic presentation of study design. The study included 96 clinically healthy healthcare professionals aged 20–64 years. The subject received monovalent Pandemrix vaccination at day 0 and follow‐up samples were collected at days 21, 90, and 365. A subgroup of vaccinees (n = 13), who were low responders to the initial Pandemrix vaccination received a second dose at day 90 and an additional follow‐up sample was collected 21 days later. At day 365, all subjects received a second trivalent non‐adjuvanted seasonal influenza vaccine and follow‐up samples were collected at days 21 and 90 after the booster vaccination. The numbers at each time point indicate the number of subjects of whom serum samples were collected.

Figure 2

(A) Influenza A(H1N1)pdm09 strains isolated in Finland cluster in the same genetic groups with WHO reference strains in the phylogenetic tree of the HA. All sequences included in the phylogenetic tree constitute the entire 1698 nucleotide long coding region of HA. The horizontal lines are proportional to the number of nucleotide changes. (B) Schematic representation of amino acid differences in the HA molecule between the Finnish influenza A(H1N1)pdm09 viruses and the vaccine virus, A/California/07/2009. On the left, a side view of the monomeric structure of HA molecule of influenza A(H1N1(2009) (A/California/04/2009; RCSB Protein Bank accession number 3LZG) with previously identified H1 protein‐specific antigenic sites (Sa in red, Sb in blue, Ca1 in darker green, Ca2 in lighter green, and Cb in orange) of influenza A(H1N1) viruses and with the receptor binding pocket (RBP, purple) is shown. The amino acid changes of Finnish A(H1N1)pdm09 viruses compared with A/California/07/2009, the vaccine strain, are illustrated in the monomeric HA structure and colored as in A/California/07/2009 structure. Amino acid changes outside the antigenic sites are shown in yellow. Changes are illustrated by amino acid residue number and with serial number of virus where the respective amino acid change has been observed.

Figure 3

Antibody responses induced by vaccination with AS03‐adjuvanted pandemic influenza and non‐adjuvanted seasonal influenza vaccines. Antibody levels before and after vaccination with Pandemrix and seasonal influenza vaccines were analyzed by HI test using several influenza A(H1N1)pdm09 strains isolated during the 2009–2010 (A) or 2010–2011 (B) epidemic seasons. Serum samples were collected at six time points as indicated in the figure. Geometric mean titers were calculated for each viral strain and the significance of difference between post‐vaccination samples at day 21 and at day 365 were calculated using Student’s paired, two‐tailed t‐test. Statistically significant difference (P < 0·01) was observed to all viral strains.

Figure 4

Vaccination‐induced antibody immune response correlates negatively with age. Analysis of antibody titers was performed separately for two age groups: younger adults aged 20–48 years (black bars), and older adults aged 49–64 years (gray bars). The geometric mean titers and 95% confidence intervals for the different age groups were calculated for the 21 day samples after the first vaccination (A) and the 21 day samples after the second booster vaccination (B). Statistical significances of differences between the groups were calculated using Student’s two‐tailed t‐test, *P < 0·05. The correlation between the age of the vaccines and the antibody titers (natural logarithms, ln) against the A/California/7/2009 vaccine virus are represented with scatter plots and trend lines 21 days after the first vaccination (C) and 21 days after the second booster vaccination (D), and the significances (P values) of correlation coefficients are indicated in the figure.