Influenza A (H1N1) 2009: A perspective at the end of the pandemic

Background. Influenza is an important illness from a public health perspective, with high morbidity and health costs. From the start of the XXth century there have been four influenza pandemics, the last one being in 2009, caused by a new strain of influenza A (H1N1) virus. Objective. To present the most relevant epidemiological data concerning influenza A (H1N1) 2009 pandemic. Method. Review of official sources and the main scientific publications about this subject. Results. Two pandemic waves occurred in 2009; the first one started in April and ended in August, and the second one started in September and ended in December in the majority of the countries. Children and young adults were the most affected groups and people older than 65 years were the least affected. The mortality rate was lower than seasonal influenza, with the highest rates occurring in people older than 50, although the highest number of deaths was reported among adults aged 20-59. Vaccination is the most important preventive measure. Post commercialization surveillance studies demonstrate that pandemic vaccines are safe. Conclusions. Pandemic influenza has different epidemiologic characteristics than seasonal influenza, with differences in the age groups most frequently affected and those with higher risk of complications.