The development of a pandemic: how does flu spread?
One of the components of the definition of a pandemic virus is that it is a novel influenza virus; therefore many people, if not most people, have little or no immunity to it – less than to ordinary seasonal virus. We do not yet know what proportions of people will be in this situation. In a pandemic, some people will have no symptoms at all (asymptomatic infections) and many will have mild symptoms. However, a small proportion will have more severe symptoms and will benefit from hospitalisation and a very small proportion of the group will die prematurely, usually from complications of the influenza infection. The best way of estimating these proportions is to look back to the experience of previous pandemics: those of 1918, 1957 and 1968. These three pandemics differed in many of their characteristics, especially in their severity.
