Influenza virus infection causes acute respiratory illness, which is accompanied by fever, general weakness, myalgia, and occasionally serious life-threatening illness or death. Major influenza viruses that cause seasonal epidemics worldwide are influenza A and influenza B. Influenza A viruses have specific subtypes, which depend upon the type of surface antigens as follows: hemagglutinins (H) and neuraminidases (N). In 2009, a novel influenza A H1N1 strain emerged and circulated throughout the world, which was named A(H1N1)pdm09. Currently, it is now one of the most important seasonal influenza A strains along with H3N2. Exact forecasting of dominant influenza type is very important for global vaccination strategies. However, it is unpredictable which influenza type will prevail year by year and varies from country to country. The difference in clinical manifestation and outcome between A(H1N1)pdm09 and A(H3N2) has not been clearly defined. Before 2009, several observational data analysis described higher influenza-associated hospitalization rate and mortality in A(H3N2) predominant seasons, but they are generally regarded as insignificant findings.

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