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

Please cite this paper as: Khandaker et al. (2011) Systematic review of clinical and epidemiological features of the pandemic influenza A (H1N1) 2009. Influenza and Other Respiratory Viruses DOI: 10.1111/j.1750‐2659.2011.00199.x. 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 18·1 years, with the median ranging from 12 to 44 years. Cough (84·9%), fever (84·7%), headache (66·5%), runny nose (60·1%) and muscle pain (58·1%) were the most common symptoms of confirmed cases. One or more pre‐existing chronic medical conditions were found in 18·4% of cases. Almost two‐thirds (64%) of cases were aged between 10 and 29 years, 5·1% were aged over 50 years and only 1·1% were aged over 60 years. The confirmed case fatality ratio was 2·9% (95% CI 0·0–6·7%), 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 0·02%. 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.