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Introduction
In Canada, public health authorities sought to immunise the population against pandemic influenza A/H1N1 during the autumn of 2009. They selected a monovalent AS03 adjuvanted product as the primary formulation for distribution in Canada (about 23 million doses; 95%). A second unadjuvanted product made up most of the remaining vaccine distributed (about 1.1 million doses; <5%) and was targeted primarily at women in the first half of pregnancy. Both products were domestically manufactured in Laval, Quebec, Canada and distributed by GlaxoSmithKline. A small quantity of unadjuvanted vaccine was also supplied by CSL Biotherapies from Australia (about 200000 doses; <1%). Canada procured enough pandemic vaccine to immunise the entire population free of charge. Supply was initially limited, however, requiring sequencing of roll-out of vaccine (web appendix A). In participating provinces, the first target groups-typically consisting of very young children, pregnant women, and people aged over 65 with comorbidity-began receiving vaccine on 26 October 2009 (week 43). Vaccine was made available to sequential groups over the ensuing four weeks and finally to all members of the population starting from 19-25 November 2009 (weeks 46-47). Some people who did not belong to sequenced groups may have received vaccine before this date, at the discretion of immunisation providers.
Pre-marketing immunogenicity trials in Europe, Japan, and North America showed that a single dose of the AS03 adjuvanted vaccine produced in Canada was highly immunogenic. 1 Fold rises in geometric mean titres after a single dose were substantially above thresholds specified by the European Agency for the Evaluation of Medicinal Products (EMEA) for approval of seasonal vaccines. 2 Results from these trials among population groups including children, young adults, and adults aged over 60 showed seroprotection rates exceeding 90%. 1 3 4 A randomised trial of children aged 6 months to 12 years (n=937) showed that AS03 adjuvanted pandemic H1N1 vaccine was more immunogenic than a non-adjuvanted whole virus formulation, especially among children under 3 years of age. 5
One the basis of these findings, health authorities anticipated that a single dose of the AS03 adjuvanted pandemic H1N1 vaccine administered in Canada would be highly protective in all age groups. Several caveats to the interpretation of immunogenicity data exist, however. Firstly, quantification of immunogenicity is typically based on the haemagglutination...