Genetic drift influenza A(H3N2) virus hemagglutinin (HA) variants originated during the last pandemic turn out to be predominant in the 2011-2012 season in Northern Italy

► Influenza A(H3N2) virus hemagglutinin (HA) variants circulated in the 2011-2012 season in Northern Italy. ► Influenza A(H3N2) virus HA variants had a considerable impact particularly among the elderly. ► These influenza A(H3N2) virus HA variants had a common origin dating back to A(H1N1)2009 pandemic. ► Time scale phylogeny is important to give early information on newly emerging strains. ► Bayesian methods should be implemented within influenza virological surveillance.