Abstract
The 2012/13 influenza season in Canada has been characterised to date by early and moderately severe activity, dominated (90%) by the A(H3N2) subtype. Vaccine effectiveness (VE) was assessed in January 2013 by Canada's sentinel surveillance network using a test-negative case-control design. Interim adjusted-VE against medically attended laboratory-confirmed influenza A(H3N2) infection was 45% (95% CI: 13-66). Influenza A(H3N2) viruses in Canada are similar to the vaccine, based on haemagglutination inhibition; however, antigenic site mutations are described in the haemagglutinin gene.
Highlights
The 2012/13 influenza season in North America has shown moderately severe activity, spiking over the December/January holiday period, with influenza A(H3N2) viruses predominating among typed/subtyped viruses to date in both Canada and the United States (US) [1,2].The updated 2012/13 A(H3N2) reference strain recommended by the World Health Organization as vaccine component for the northern hemisphere (A/ Victoria/361/2011-like) is antigenically distinct from that recommended for the previous season (A/ Perth/16/2009-like) [3], with 11 amino acid (AA) residue differences at antigenic sites of the haemagglutinin (HA) surface protein [4].Vaccine effectiveness (VE) in Canada was assessed by the country’s sentinel surveillance network in January2013
Of 999 nasal or nasopharyngeal specimens collected between 1 October 2012 and 23 January 2013, we excluded from the epidemic curve specimens from the following patients: those failing to meet the influenza-like illness (ILI) case definition or for whom it was unknown (n=24); those whose specimens were collected more than seven days after symptom onset or for whom the interval was unknown (n=132); those whose age was unknown (n=1) and those for whom influenza test results were unavailable or indeterminate (n=9)
Our interim 2012/13 VE results show that vaccination reduced the risk of medically attended laboratory-confirmed influenza due to the predominant A(H3N2) virus subtype by about half
Summary
The 2012/13 influenza season in North America has shown moderately severe activity, spiking over the December/January holiday period, with influenza A(H3N2) viruses predominating among typed/subtyped viruses to date in both Canada (about 90%) and the United States (US) (about 70%) [1,2]. Sequencing of the HA1 gene of a convenience sample (n=82) of available influenza A(H3N2) viruses, spanning the season so far but with emphasis on more recent activity, was undertaken for each province to identify AA substitutions within the 131 residues of antigenic sites A–E [11,12]. These were expressed as percentage identity and relatedness compared with the vaccine reference strain (A/Victoria/351/2011). HA sequences from reference strains used in the phylogenetic analysis were obtained from the EpiFlu database of the Global Initiative on Sharing Avian Influenza Data (GISAID) (Table 1)
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