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H1N1 pandemic flu symptoms are generally mild, brief

March 22 2010

Clinical question What are the clinical features of the H1N1 pandemic influenza?

Bottom line Symptoms of the 2009 H1N1 pandemic influenza were generally mild and lasted only a few days in most patients. Viral shedding occurs for a mean of 6 days. (Level of evidence = 2b)

Synopsis The government of China attempted to identify all cases of pandemic H1N1 influenza within their borders. Patients with confirmed cases were quarantined and closely observed. This provided an opportunity to systematically observe patients with pandemic influenza and determine the typical features and course of the infection. The average age of patients was 23 years, and cases were fairly evenly divided between men and women. Only 6.1% of patients were older than 50 years, and only 3.5% were younger than 5 years; most had recently traveled to North America or Australia. Approximately two-thirds of patients had fever and cough, the most common symptoms, with a duration of fever of 3 days (range = 1-11 days), and viral shedding for a mean of 6 days (range = 1-17 days). Hypokalemia (25%) and lymphopenia were common; the latter was observed in 68% of adults and 92% of children. Less than one third had an elevated C-reactive protein or sedimentation rate, and abnormalities on chest radiography were rare. Chest pain, conjunctival congestion, nausea, vomiting, and diarrhea were also rare (<5% of patients). Chills and myalgias were uncommon, occurring in approximately 7% to 10% of patients. The median incubation period was 2 days, with a range of 1 to 7 days.

Cao B, Li X-W, Mao Y, et al; National Influenza A Pandemic (H1N1) 2009 Clinical Investigation Group of China. Clinical features of the initial cases of 2009 pandemic influenza A (H1N1) virus infection in China. N Engl J Med. 2009;361(26):2507-2517.