Abstract
In Denmark, the 2012/13 influenza season has been dominated by influenza A(H3N2). We estimated the vaccine effectiveness (VE) of the trivalent influenza vaccine by linking national registers in a test-negative case-control study of patients tested for influenza aged ≥65 years. The adjusted VE against laboratory-confirmed influenza A and B was -11% (95% CI: -41 to 14) and 69% (95% CI: 26 to 87), respectively. Genetic characterisation of the influenza A(H3N2) viruses indicated genetic drift, with seven substitutions at key antigenic sites.
Highlights
In Denmark, consultation rates for influenza-like illness and the number of patients testing positive for influenza increased in week 51 and peaked in week 52 2012
We describe the vaccination coverage among influenza A patients in intensive care units (ICUs), and characterised circulating influenza A(H3N2) viruses genetically
In the current season, which to date has been dominated in Denmark by influenza A(H3N2) virus, we calculated an adjusted vaccine effectiveness (VE) point estimate of −11% against laboratory-confirmed influenza A in patients aged ≥65 years tested for influenza
Summary
In Denmark, consultation rates for influenza-like illness and the number of patients testing positive for influenza increased in week 51 (starting 17 December) and peaked in week 52 (starting 24 December) 2012. Information on patients aged ≥65 years tested for influenza A and B virus by PCR was obtained from the Danish Microbiology Database; information on administered TIV from weeks 39 to 48 was obtained from the Danish vaccination register [2,3]. Vaccination coverage among influenza A patients aged ≥65 years in all Danish ICUs was estimated by linking data from the national influenza ICU surveillance system [4] with that in the vaccination register. A total of 1,443 patients aged ≥65 years were tested during the study period for influenza: 364 and 35 tested positive for influenza A and B, respectively (Table 1) Those who tested negative were considered as controls. We included reference HA sequences from influenza A(H3) viruses, obtained from the EpiFlu database of the Global Initiative on Sharing Avian Influenza Data (GISAID) (Table 3)
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