The authors have declared that no competing interests exist.
Conceived and designed the experiments: MR ML MS. Performed the experiments: ML MR KF. Analyzed the data: MR SR PS ML. Contributed reagents/materials/analysis tools: KF. Wrote the paper: SR MR RW KF PS ML MS. Interpretation of the data: SR MR KF RW PS ML MS. Critical revision of the manuscript for important intellectual content: SR MR KF RW PS ML MS. Obtained funding: ML MR KF RW MS. Administrative technical or material support: ML MR KF PS. Study supervision: ML MR KF.
To evaluate if, among children aged 3 to 15 years, influenza vaccination for multiple seasons affects the proportion sero-protected.
Participants were 131 healthy children aged 3–15 years. Participants were vaccinated with trivalent inactivated seasonal influenza vaccine (TIV) over the 2005–06, 2006–07 and 2007–8 seasons. Number of seasons vaccinated were categorized as one (2007–08); two (2007–08 and 2006–07 or 2007–08 and 2005–06) or three (2005–06, 2006–07, and 2007–08). Pre- and post-vaccination sera were collected four weeks apart. Antibody titres were determined by hemagglutination inhibition (HAI) assay using antigens to A/Solomon Islands/03/06 (H1N1), A/Wisconsin/67/05 (H3N2) and B/Malaysia/2506/04. The proportions sero-protected were compared by number of seasons vaccinated using cut-points for seroprotection of 1∶40 vs. 1∶320. The proportions of children sero-protected against H1N1 and H3N2 was high (>85%) regardless of number of seasons vaccinated and regardless of cut-point for seroprotection. For B Malaysia there was no change in proportions sero-protected by number of seasons vaccinated; however the proportions protected were lower than for H1N1 and H3N2, and there was a lower proportion sero-protected when the higher, compared to lower, cut-point was used for sero-protection.
The proportion of children sero-protected is not affected by number of seasons vaccinated.
Annual influenza vaccination for all children aged 6 to 59 months was recommended in 2004 for Canada,
In order to better define the effect of repeated vaccination with inactivated influenza vaccine on sero-protection in children, we conducted a prospective study. Our goal was to evaluate the effect of repeated vaccination (i.e., vaccination for more than one influenza season) with recommended vaccine antigens and to measure the effect on sero-protection against these antigens in children.
The study was approved by the Conjoint Health Research Ethics Board of the University of Calgary (Ethics ID 18970) and McMaster University HHS/FHS Research Ethics Board (REB project # 07-376). Informed written consent was obtained from the parents/guardians of the children. Written informed assent was additionally obtained from all children aged 7 years or older. The funding source had no role in study design; collection, analysis or interpretation of data; writing of the report; nor decision to submit the paper for publication.
Participants were healthy children (i.e., no underlying chronic medical conditions) aged 3–15 years residing on central Alberta Hutterite colonies who received study vaccine in 2007–08. Each year from the 2005/06 to 2007/08 influenza seasons, participating children were vaccinated (as part of a pilot study) with the trivalent inactivated seasonal influenza vaccine (TIV) of the year, according to the age-specific recommendations of the Canadian National Advisory Committee on Immunization
Children were classified as having been vaccinated for three seasons if they had been vaccinated by the study team in each of 2005–06, 2006–07, and 2007–08. Those vaccinated for two seasons were vaccinated in 2007–08 and 2006–07 or in 2007–08 and 2005–06. Those vaccinated for one season were vaccinated only in 2007–08.
Year | Vaccines used | Vaccine antigens |
2005–06 | Fluviral® GlaxoSmithKline Inc Lot 3FV27511 | A/New Caledonia/20/99 (H1N1) |
A/New York/55/04 (H3N2) | ||
B/Jiangsu/10/03 |
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2006–07 | Fluviral® GlaxoSmithKline Inc Lots C2753AA and C2755AA | A/New Caledonia/20/99 |
A/Wisconsin/67/05 |
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B/Malaysia/2506/04 |
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2007–08 | Vaxigrip® Sanofi Pasteur Lot C2984AA | A/Solomon Islands/3/06 (H1N1)-like strain (A/Solomon Islands/3/06 IVR-145) |
A/Wisconsin/67/05 (H3N2)-like strain (A/Wisconsin/67/05 NYMC X-161B) | ||
B/Malaysia/2506/04-like strain (B/Malaysia/2506/04) |
A/New York/55/04 is antigenically equivalent to the A/California/7/04 (H3N2) virus strain; B/Jiangsu/10/03 is antigenically equivalent to the B/Shanghai/361/02 virus strain
A/Hiroshima/52/05 is antigenically equivalent to the A/Wisconsin/67/05 virus strain
B/Malaysia/2506/04 belongs to the B/Victoria/02/87 lineage
Pre- and post-vaccination blood samples from participants were collected four weeks apart, centrifuged and the serum stored at −20°C before parallel testing against the influenza A and B vaccine components. Antibody titres to each influenza type and subtype were determined by the hemagglutination inhibition assay using antigens to A/Solomon Islands/03/06 (H1N1), A/Wisconsin/67/05 (H3N2) and B/Malaysia/2506/04. All subject and control sera were pre-treated with receptor-destroying enzyme prior to titration. Doubling dilutions from 1∶20 to 1∶2560 were performed for all subject and serum control samples. Acute and convalescent sera were tested in parallel for comparability of titres. The assay was performed using turkey red blood cells (Rockland Immunochemicals, Philadelphia, Pennsylvania) as the indicator together with 4HA units of the respective influenza antigen as described in the WHO method
For each age-group by antigen, we compared the proportions of children that sero-converted by season first using a cut-point of 40 and then of 320. For all analyses we used an unadjusted alpha of 0.05.
Statistical analyses were done using STATA v10 (Statacorp, College Station, TX, U.S.A. and SAS v9 (SAS Institute, Cary NC).
Although consent was received for vaccination of 138 children in 2007/08, pre-post vaccination sera were available only for 131.
Age group |
Totals | ||
Number of seasons vaccinated | 3–5 years (%) | 6–15 years (%) | |
Vaccinated only one season (2007–08) | 11 (44) | 26 (24.5) | 37 |
Vaccinated two seasons OR (2007–08 and 2006–072007–08 and 2005–06) | 9 (36) | 9 (8.5) | 18 |
Vaccinated three seasons (2007–08 and 2006–07and 2005–06) | 5 (20) | 71 (67.0) | 76 |
Total | 25 | 106 | 131 |
P-Value (Fisher’s exact test) | <<0.001 |
Age-group as of November 1, 2007.
As can be seen in
Strain | Age group |
Vaccinated onlyone season(2007–08) | Vaccinated two seasons (2007–08and 2006–07 OR2007–08 and2005–06) | Vaccinated three seasons (2007–08 and 2006–07 and 2005–06) | P-Value |
A/Solomon Islands/3/2006 (H1N1) | 3–5 years | ||||
n(%) | 11 (100.0) | 9 (100.0) | 5 (100.0) | 1.00 | |
6–15 years | |||||
n (%) | 25 (96.2) | 9 (100.0) | 71 (100.0) | 0.21 | |
A/Wisconsin/67/05 (H3N2) | 3–5 years | ||||
n (%) | 11 (100.0) | 9 (100.0) | 5 (100.0) | 1.00 | |
6–15 years | |||||
n (%) | 26 (100.0) | 9 (100.0) | 71 (100.0) | 1.00 | |
B/Malaysia/2506/04 | 3–5 years | ||||
n (%) | 11 (100.0) | 9 (100.0) | 4 (80.0) | 0.13 | |
6–15 years | |||||
n (%) | 19 (73.1) | 7 (77.8) | 56 (78.9) | 0.83 |
Age-group as of November 1, 2007.
Strain | Age group |
Vaccinated only one season (2007–08) | Vaccinated two seasons (2007–08 and 2006–07 OR 2007–08 and 2005–06) | Vaccinated three seasons (2007–08 and 2006–07 and 2005–06) | P-Value |
A/Solomon Islands/3/2006 (H1N1) | 3–5 years | ||||
n (%) | 11 (100.0) | 9 (100.0) | 5 (100.0) | 1.00 | |
6–15 years | |||||
n (%) | 23 (88.5) | 9 (100.0) | 70 (98.6) | 0.06 | |
A/Wisconsin/67/05 (H3N2) | 3–5 years | ||||
n (%) | 10 (90.9) | 8 (88.9) | 5 (100.0) | 0.75 | |
6–15 years | |||||
n (%) | 24 (92.3) | 8 (88.9) | 69 (97.2) | 0.39 | |
B/Malaysia/2506/04 | 3–5 years | ||||
n (%) | 6 (54.5) | 3 (33.3) | 4 (80.0) | 0.24 | |
6–15 years | |||||
n (%) | 8 (30.8) | 2 (22.2) | 13 (18.3) | 0.41 |
Age-group as of November 1, 2007.
We found that most children were sero-protected against influenza regardless of number of seasons vaccinated; however, this proportion was less for influenza B than for influenza A. The impact of using a cut-point of 320 rather than the value of 40 that has been used in adult studies is important to understanding the serological response of children to vaccination; not surprisingly the proportion of children protected declines when the higher cut-point is used, particularly for influenza B. As sero-protection is the most relevant indicator from a public health perspective, future studies should use this higher cut-point in their analyses.
The strengths of this study include a larger sample and wider age-span than that of Zeman
There are limitations to our study: small sample sizes in some strata result in limited power, and we could not account for the effect of prior natural infection compared to immunization. Nevertheless, our data demonstrate that lack of priming did not have a detrimental effect on seroprotection, which is most relevant from a public health perspective. Finally, the changing status of the antigens in both vaccines and circulating strains makes interpretation of study findings challenging.
The proportion of children who are sero-protected against influenza B was less than for either influenza A (H1N1) or A (H3N2).
Alberta Health and Wellness donated seasonal influenza vaccine for 2005–06 and for 2006–07 (Fluviral® GlaxoSmithKline Inc) but provided no funding for the study. Seasonal vaccine for 2007–08 was purchased from Sanofi Pasteur (Vaxigrip ®). Serological analyses were done at the Alberta Provincial Laboratory for Public Health (Microbiology).