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Effectiveness of vaccine against pandemic influenza A/H1N1 among people with underlying chronic diseases: cohort study, Denmark, 2009-10

BMJ 2012; 344 doi: (Published 25 January 2012) Cite this as: BMJ 2012;344:d7901
  1. Hanne-Dorthe Emborg, epidemiologist,
  2. Tyra Grove Krause, senior consultant,
  3. Anders Hviid, senior investigator,
  4. Jacob Simonsen, statistician,
  5. Kåre Mølbak, director, state epidemiologist
  1. 1Division of Epidemiology, Statens Serum Institut, 2300 Copenhagen, Denmark
  1. Correspondence to: H-D Emborg hde{at}
  • Accepted 5 October 2011


Objective To determine the effectiveness of an adjuvanted monovalent vaccine against pandemic influenza A/H1N1 among people with underlying chronic diseases.

Design Historical cohort study.

Setting Mandatory national reporting systems, 2 November 2009 to 31 January 2010, Denmark.

Participants 388 069 people under 65 years of age with a diagnosis in the past five years of at least one underlying disease expected to increase the risk of severe illness after influenza.

Main outcome measures Laboratory confirmed H1N1 infection and influenza related hospital admission with laboratory confirmed H1N1 infection. Estimates of vaccine effectiveness were adjusted for age and underlying disease.

Results The effectiveness of pandemic vaccine against confirmed H1N1 infection 14 days after one dose of vaccine was 49% (95% confidence interval 10% to 71%). The effectiveness of vaccine against admission to hospital for confirmed H1N1 infection was 44% (−19% to 73%).

Conclusions The adjuvanted monovalent vaccine against pandemic influenza A/H1N1 was offered late in the 2009-10 influenza season. Among chronically ill people, this vaccine offered protection against laboratory confirmed H1N1 infection but only offered non-significant protection against influenza related hospital admissions confirmed as H1N1 infection. This finding is of public health relevance because the population of chronically ill people is a major target group for pandemic vaccinations and because of the delayed availability of pandemic vaccines in a forthcoming pandemic.


  • Contributors: KM and TGK designed the study. H-DE, KM, and TGK drafted the manuscript. H-DE managed and analysed the data. JS helped analyse the data. AH contributed to data analysis, interpretation of the results, and writing of the manuscript. All authors reviewed and approved the final manuscript. H-DE is the guarantor.

  • Funding: This study was supported by European Vaccine Manufacture through a grant managed by EpiConcept. The funding source did not influence or participate in the design and conduct of the study; the collection, management, analysis, and interpretation of the data; writing the manuscript; or the decision to submit the article for publication. The researchers were independent of the funder.

  • Competing interests: All authors have completed the ICMJE uniform disclosure form at (available on request from the corresponding author) and declare: support from European Vaccine Manufacture through a grant managed by EpiConcept; no financial relationships with any organisations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work

  • Ethical approval: Not required.

  • Data sharing: No additional data available.

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