In the past 3 years, the 2009 pandemic influenza virus H1N1 (pH1N1) has led to many severe or fatal cases. The virus-related factors that cause severe or fatal disease are not clear. The clinical and molecular characteristics of pH1N1 infections with severe or fatal disease were examined to understand the correlation between pH1N1 infection and disease severity. Since 2009, three pH1N1 influenza epidemic outbreaks have occurred in Shenzhen, China. One hundred forty-six severe cases were confirmed in the first wave in 2009. In severe cases, a high proportion (49.3%) of patents displayed high fever (>39.0°C), and 73.2% of patients had pneumonia and tracheobronchitis. Seven fatal cases were recorded: three with viral encephalitis and four with respiratory failure. The results of sequencing and phylogenetic analysis showed that the viruses from fatal or severe cases were scattered throughout the phylogenetic tree. Four substitutions (D222G, D222N, D222E, and Q223R) were observed on the 220-loop of the receptor-binding sites of the HA gene. Both D222G and D222N were associated statistically with severe disease. The 2011 viruses had evolved into two distinct branches. Ten specific point mutations occurred in the 2011 virus. In summary, high fever, lower respiratory tract infections and serious complications were the main features of severe cases. Gene variation seemed not to be the main reason for severe disease. Vaccination is the effective mean to prevent infection and severe disease.
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