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The 2012 and 2013 annual influenza epidemics in Mexico were characterized by presenting different seasonal patterns. In 2012 the A(H1N1)pdm09 virus caused a high incidence of influenza infections after a two-year period of low circulation; whereas the 2013 epidemic presented circulation of the A(H1N1)pdm09 virus throughout the year. We have characterized the molecular composition of the Hemagglutinin (HA) and Neuraminidase (NA) genes of the A(H1N1)pdm09 virus from both epidemic seasons, emphasizing the genetic characteristics of viruses isolated from Yucatan in Southern Mexico. The molecular analysis of viruses from the 2012 revealed that all viruses from Mexico were predominantly grouped in clade 7. Strikingly, the molecular characterization of viruses from 2013 revealed that viruses circulating in Yucatan were genetically different to viruses from other regions of Mexico. In fact, we identified the occurrence of two genetic variants containing relevant mutations at both the HA and NA surface antigens. There was a difference on the temporal circulation of each genetic variant, viruses containing the mutations HA-A141T / NA-N341S were detected in May, June and July; whereas viruses containing the mutations HA-S162I / NA-L206S circulated in August and September. We discuss the significance of these novel genetic changes. [ABSTRACT FROM AUTHOR]

Influenza virus neuraminidase (NA) plays a pivotal role during viral growth since its sialidase activity allows the efficient release of nascent virions from infected cells. Consequently, mutations in the NA catalytic site affecting sialic acid (SA) cleavage may influence the biological properties of influenza viruses. This study reports two amino acid substitutions (N386K and P431S) in the NA of the influenza A(H1N1)pdm09 virus that emerged in 2009 in Mexico. The NA sialidase activity to cleave SA-like substrates, and viral growth were examined and the mutant viruses had various deficiencies. NA mutations N386K and P431S together or separately, and in the presence or absence of H275Y were further evaluated using recombinant influenza A/California/04/2009 (pH1N1) viruses containing single, double, or triple mutations. Viral growth was reduced in the presence of mutation P431S alone or combined with N386K and/or H275Y. Substrates hydrolysis was reduced when recombinant pH1N1 viruses were analyzed by NA inhibitory assays. Moreover, elution assays with guinea pig red blood cells indicated an unbalanced hemagglutinin (HA):NA functionality. Altogether, our data underline the functional significance of mutations at highly conserved sites in influenza virus NA glycoprotein and the occurrence of permissive mutations to compensate virus viability in vitro . [ABSTRACT FROM AUTHOR]