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The clinical picture of influenza may vary from mild respiratory infection to pneumonia requiring intensive care. Annual epidemics are most commonly caused by H3N2 or H1N1 type influenza A or influenza B viruses. The population's immune protection against a new virus type is low, whereupon morbidity and mortality may be high. Vaccinations are the most important means to decrease influenza morbidity. Annual variation and quick intercontinental migration of influenza viruses, combined with the possibility of the creation of reassortant viruses, are significant challenges for the development of influenza vaccines.