Seasonal influenza

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Seasonal influenza tends to be about 75% influenza A and 25% influenza B with usually one or two strains causing most morbidity. It can be regarded to be consequential of population density and globalisation but probably these same two factors now ensure fair herd immunity to the virus. Influenza vaccination also helps considerably at a population level. As shown on the time graph seasonal influenza is winter associated in most of the world. Pandemic influenza is driven by a single strain. The severity varies from year to year. Since air travel became widely available there is often a correlation between Northern and Southern hemisphere winter outbreaks, so a bad seasonal influenza year in one hemisphere tends to be predictive of a bad year in the other hemisphere. Thus the 2017 Australasian season which was about half as bad as the 2009 seasonal pandemic was remarkably predictive of the strain in health services felt as far apart as California and UK. Further the 2009 Australasian severity was predicted from the 2008/9 season and it took another year in the Northern hemisphere for immunity to catch up. Incidence during year of influenza like illness in England and Wales 1967-2016