Epidemiology and Infection

Original Papers

Influenza/vaccine

Estimating vaccine effectiveness against severe influenza in England and Scotland 2011/2012: applying the screening method to data from intensive care surveillance systems

H. L. THOMASa1a2a3 c1, N. ANDREWSa1, H. K. GREENa1, N. L. BODDINGTONa1, H. ZHAOa1, A. REYNOLDSa4, J. McMENAMINa4 and R. G. PEBODYa1

a1 Health Protection Services – Colindale, Health Protection Agency, England

a2 Field Epidemiology Training Programme (FETP), Health Protection Agency, England

a3 European Programme for Intervention Epidemiology Training (EPIET), European Centre for Disease Prevention and Control (ECDC), Stockholm, Sweden

a4 Health Protection Scotland, Scotland

SUMMARY

Methods for estimating vaccine effectiveness (VE) against severe influenza are not well established. We used the screening method to estimate VE against influenza resulting in intensive care unit (ICU) admission in England and Scotland in 2011/2012. We extracted data on confirmed influenza ICU cases from severe influenza surveillance systems, and obtained their 2011/2012 trivalent influenza vaccine (TIV) status from primary care. We compared case vaccine uptake with population vaccine uptake obtained from routine monitoring systems, adjusting for age group, specific risk group, region and week. Of 60 influenza ICU cases reported, vaccination status was available for 56 (93%). Adjusted VE against ICU admission for those aged ≥65 years was −10% [95% confidence interval (CI) −207 to 60], consistent with evidence of poor protection from the 2011/2012 TIV in 2011/2012. Adjusted VE for those aged <65 years in risk groups was −296% (95% CI −930 to −52), suggesting significant residual confounding using the screening method in those subject to selective vaccination.

(Received October 05 2012)

(Revised March 11 2013)

(Accepted March 13 2013)

(Online publication April 16 2013)

Key words

  • Immunization (vaccination);
  • influenza;
  • influenza vaccines

Correspondence

c1 Author for correspondence: Dr H. L. Thomas, Health Protection Agency, Health Protection Services Colindale, 61 Colindale Avenue, London NW9 5EQ, UK. (Email: lucy.thomas@hpa.org.uk)

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