How did the 2009 pandemic compare with previous pandemics?

Abstract

The pandemic spread of the novel H1N1 virus in 2009 was unusual in that the attack rates and mortality rates were often lower than in earlier pandemics. However, most affected populations reported that morbidity and mortality were greater in younger adults than in children or older persons, reproducing the age distribution of earlier pandemics. A small proportion of affected persons became so ill that they required intensive care or died. Aboriginality, pregnancy and obesity were risk factors for severe disease, but many patients requiring intensive care had no identifiable risk factors. The pandemic features in 2009 can be explained, at least in part, in immunological terms. As with earlier pandemics, older people were probably protected by specific antibodies resulting from exposure to a similar virus which last circulated when they were children. In 2009, as in earlier pandemics, children were relatively protected against their first exposure to the new virus by the strength of their innate immune response, while many people of all ages were protected in whole or in part by short-lived, strain-transcending immunity resulting from their most recent exposure to seasonal influenza. On this view, the individuals developing severe disease were those who were too young to be protected by specific high-avidity antibodies, too old to be well-protected by innate immunity, and unlucky enough to have missed out on recent exposures to seasonal influenza.