Systematic review of clinical and epidemiological features of the pandemic influenza A (H1N1) 2009

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Abstract
The aim of this systematic review was to summarise the clinical and epidemiological features of the pandemic influenza A (H1N1) 2009. We did a systematic search of published literature reporting clinical features of laboratory-confirmed pandemic influenza A (H1N1) 2009 from 1 April 2009 to 31 January 2010. Forty-four articles met our inclusion criteria for the review. The calculated weighted mean age of confirmed cases was 18 center dot 1 years, with the median ranging from 12 to 44 years. Cough (84 center dot 9%), fever (84 center dot 7%), headache (66 center dot 5%), runny nose (60 center dot 1%) and muscle pain (58 center dot 1%) were the most common symptoms of confirmed cases. One or more pre-existing chronic medical conditions were found in 18 center dot 4% of cases. Almost two-thirds (64%) of cases were aged between 10 and 29 years, 5 center dot 1% were aged over 50 years and only 1 center dot 1% were aged over 60 years. The confirmed case fatality ratio was 2 center dot 9% (95% CI 0 center dot 0-6 center dot 7%), an extracted average from 12 of 42 studies reporting fatal cases (937 fatal cases among 31 980 confirmed cases), which gives an overall estimated infected case fatality ratio of 0 center dot 02%. Early in the pandemic, disease occurred overwhelmingly in children and younger adults, with cough and fever as the most prevalent clinical symptoms of the confirmed cases. A high infection rate in children and young adults, with sparing of the elderly population, has implications for pandemic influenza management and control policies.