GMS-CDC

Influenza A( H1N1)v has spread rapidly in all parts of the globe in 2009 as a true pandemic, although fortunately a clinically mild one. The relevant evolutionary steps for the new virus to adapt to human populations occurred very early during the pandemic, before the end of April.

Of the several resulting clades or clusters, clade 7 appeared later and proved more successful, substituting all other early clades before the bulk of the worldwide infections occurred.

Introduction
2009 influenza  A(H1N1)v pandemic virus has emerged following a recent reassortment event between swine strains [1][2]. Its jump in the human population has been tentatively dated back to the beginning of the year [3][4], and very early in its history the new virus could be differentiated in clades or, as later defined, clusters .[5] The significance of these findings is not clear, both in terms of a possible evolutionary pathway of the pandemic virus and in terms of pathogenicity. The early data showed that  clade 7 (as in ref. 4, or cluster 2 in ref. 5) appeared in New York a few weeks after clades 1 and 2 were isolated in Mexico and California, and originated late in March, but all clades were reported to co-circulate in all continents thereafter. After September, a second more intense peak has involved most temperate countries in  the Northern Hemisphere. However, viral sequence information on this second outbreak is relatively scant, and no clear trend in viral evolution has been outlined yet. In Italy most clades were circulating in the first months of the pandemic, when the great majority of the infections were imported by travellers (mostly from North and South America) who had become infected abroad. As in most European countries, a second, more intense wave of infections occurred in Italy during the period October-November 2009. Unlike the first epidemic peak of imported infections, this peak was powered by the rapid local spread of the virus (which had been circulating at low intensity during the whole summer) in children and adolescents (and their contacts) due to the opening of schools, kindergartens and other communities after the summer holidays.

Source: Plos Current