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 is once again the predominant strain after the 2009 pandemic. Its molecular epidemiology and phylogeny were investigated during the 2011-2012 season in Northern Italy. The epidemiological and virological influenza surveillance was carried out within the framework of the Italian Influenza Surveillance Network. The hemagglutinin (HA) gene of the A(H3N2) viruses detected was analyzed by means of a time-scaled phylogenetic approach. In Northern Italy, the 2011-2012 epidemic wave was sustained almost exclusively by influenza A(H3N2) viruses (87.2% of total influenza virus detections). The consultation rates for influenza-like illness (ILI) in the age group ≥65 years were 1.5 to 6-fold higher than those registered during the previous eight epidemics: A(H3N2) was the only virus identified in this group. The phylogenetic analysis of A(H3N2) viruses showed viruses belonging to the A/Victoria/208/2009 genetic clade, characterized by substitutions in HA antigenic sites with respect to the A/Perth/16/2009-like 2011-2012 vaccine strain. About one-third of analyzed sequences fell into group 6 and two thirds into group 3 (subdivided into 3A, 3B, and 3C). The time scale reconstruction of the phylogeny showed several independent introductions of A(H3N2) groups between summer and winter of 2011. However, the common origin of all the circulating A(H3N2) strains dated back to the 2009 pandemic period (November 2009). The time scale phylogenetic approach is of particular importance for the evaluation of the introduction and circulation of new variants in the area. Therefore, it should be implemented within the framework of influenza virological surveillance.