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Systematic review and meta-analysis
For some groups traditionally considered to be ‘high risk’, the evidence of an increased risk of severe influenza-associated illness is poor quality
  1. Peter Horby
  1. Centre for Tropical Medicine, Oxford University, Oxford, UK
  1. Correspondence to : Dr Peter Horby, Centre for Tropical Medicine, Oxford University, Roosevelt Drive, Headington, Oxford OX3 7FZ, UK; peter.horby{at}

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International and national influenza immunisation policies recommend that influenza vaccination is targeted at population subgroups thought to be at high risk of severe or complicated influenza illness. Mertz and colleagues set out to assess the quality of evidence for considering such groups to be at increased risk of severe or complicated influenza.


The authors conducted a systematic literature review of studies reporting associations between at least one putative risk factor and various severe illness outcomes (pneumonia, hospitalisation, intensive care admission, ventilator support and death) in people with evidence of seasonal or pandemic influenza infection. All of the 239 studies included in the final analysis were observational in nature (97% were cohort studies) and most (92%) included only participants with laboratory-confirmed influenza infection. The quality of evidence was assessed using the Grading of Recommendations Assessment, …

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  • Competing interests None.