Vaccine effectiveness of 2011/12 trivalent seasonal influenza vaccine in preventing laboratory-confirmed influenza in primary care in the United Kingdom : evidence of waning intra-seasonal protection

Pebody, R.G. and Andrews, Nick and McMenamin, J. and Durnall, H. and Ellis, J. and Thompson, C. and Robertson, Charles and Cottrell, S. and Smyth, B. and Zambon, M. and Moore, C. and Fleming, D.M. and Watson, J.M. (2013) Vaccine effectiveness of 2011/12 trivalent seasonal influenza vaccine in preventing laboratory-confirmed influenza in primary care in the United Kingdom : evidence of waning intra-seasonal protection. Eurosurveillance, 18 (5). 4. ISSN 1560-7917

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Abstract

The 2011/12 season was characterised by unusually late influenza A (H3N2) activity in the United Kingdom (UK). We measured vaccine effectiveness (VE) of the 2011/12 trivalent seasonal influenza vaccine (TIV) in a test-negative case–control study in primary care. Overall VE against confirmed influenza A (H3N2) infection, adjusted for age, surveillance scheme and month, was 23% (95% confidence interval (CI): -10 to 47). Stratified analysis by time period gave an adjusted VE of 43% (95% CI: -34 to 75) for October 2011 to January 2012 and 17% (95% CI: -24 to 45) for February 2012 to April 2012. Stratified analysis by time since vaccination gave an adjusted VE of 53% (95% CI: 0 to 78) for those vaccinated less than three months, and 12% (95% CI: -31 to 41) for those vaccinated three months or more before onset of symptoms (test for trend: p=0.02). For confirmed influenza B infection, adjusted VE was 92% (95% CI: 38 to 99). A proportion (20.6%) of UK influenza A(H3N2) viruses circulating in 2011/12 showed reduced reactivity (fourfold difference in haemagglutination inhibition assays) to the A/Perth/16/2009 2011/12 vaccine component, with no significant change in proportion over the season. Overall TIV protection against influenza A(H3N2) infection was low, with significant intraseasonal waning.