Pandemic influenza A (H1N1) virus emerged in Mexico during the spring of 2009 and spread rapidly worldwide. Cases of infection occurred mostly in children and young adults. Infected patients have minor symptoms to severe. However, some patients developed severe illness and some died most of these were adults between the ages of 20-50 years, with or without underlying medical conditions. Importantly, most patients had respiratory failure due mainly to primary influenza pneumonia. In addition, in experimentally infected animals, the level of pulmonary replication of the influenza A (H1N1) 2009 virus was higher than that of seasonal influenza viruses.
In August 2010, the WHO declared the pandemic (H1N1) 2009 to be over. However, the pandemic influenza A (H1N1) 2009 expected as a seasonal virus and its behaviour cannot be predicted. The 1918–1919 pandemic occurred in three season: a mild first time in spring and summer 1918, an extremely lethal second time in autumn 1918, followed by a less severe third time in winter 1919. Furthermore, the pattern of successive season of varying severity also occurred in the 1957 and 1968 pandemic. In addition, no study has specifically analysed influenza A (H1N1) 2009 only. But also study pneumonia during the 2010–2011 influenza season.
The aim of this study was to ascertain whether there have been changes in the epidemiology of influenza A (H1N1) 2009 infection. we compared hospitalized adults with confirmed influenza A (H1N1) 2009 pneumonia during the pandemic period with those hospitalized during the first post-pandemic influenza season.