SB218352_4

Influenza pandemics are caused by viruses that possess an Hemagglutinin molecule to which most of the population lacks immunity. If such virus is pathogenic to human and demonstrates the ability to transmit from person to person, the result is a global outbreak of disease that affects a high percentage of individuals in a short period of time and is likely to cause substantially increased mortality and morbidity in all countries of the world. Recently, purely avian influenza viruses, including the H5N1, H9N2 and H7N7 subtypes, have been directly transmitted to humans, raising concern over the possibility of a new influenza pandemic among the world's immunologically naive populations. In order to face this kind of situation, a pandemic influenza vaccine has to be developed.