Epidemiological aspects of seasonal and pandemic influenza in recent years and emerging viruses

Influenza is a major public health problem worldwide. Epidemiological and virological surveillance has been demonstrated to be crucial for prevention of both seasonal and pandemic influenza. Recent influenza seasons have been characterized by the circulation of the strain A(H1N1)pdm09. It was firstly identified in 2009 and accounted for the most recent influenza pandemic. In spite of moderate mortality rates observed during 2009 influenza pandemic, quite interestingly severe cases of influenza by the new pandemic strain A(H1N1)pdm09 were more frequent in young adults, previously healthy and without risk factors. Of note, circulation of influenza A(H3N2) virus with antigenic and genetic drifts was predominant in 2014-2015 influenza season. The variant A/Switzerland/9715293/2013 replaced A/Texas/50/2012 in vaccine formulation of season 2015-2016. Recent influenza seasons in South Italy (Apulia region) were characterized by lower and later incidence peaks respect to those of Northern Italy probably due to differences in climate. Influenza vaccine coverage rates in Apulia steadly declined in general population, especially in individuals ≥ 65 years, creating concern in health authorities. Avian influenza viruses (AIVs) such as H5N1, H7N9, H9N2 and H10N8 are capable to spread to domestic poultry and then to humans from natural reservoir. Therefore, these viruses are considered as emerging with a special concern related to their potential to cause next influenza pandemic.