Flu monitoring was carried out in children’s general hospital in St. Petersburg within three epidemic seasons (2010–2011, 2011–2012 and 2012–2013). 1916 patients under the age of 18 years were examined with the complex of virologic tests . The natural decreasing of flu incidence and predominant diagnosing in etiologic structure one of the virus serotypes [in the first A(H1N1)pdm09, in the second — A(H3N2)] have been observed during the first two years after a pandemic. In the third season restoration of the main characteristics of epidemic flu situation were detected: polietiology with annual change of serotypes proportions, late start (winter and spring), majority of younger children among hospitalaized patients and a mild course of disease. The most probable candidates [viruses of a subtype A(H3N2)] are revealed as a causal factor of significant increasing of disease incidence with the severe forms in the near future. Laboratory data were confirmed by increase in frequency of the complicated by pneumonia ARI at hospitalized patients during the periods of their maximum registration. All isolates received during the study were corresponded to the referens-strains included in vaccines. Thus, timely and appropriate vaccination during the studied period had to become an effective protection against a flu.

Flu monitoring was carried out in children’s general hospital in St. Petersburg within three epidemic seasons (2010–2011, 2011–2012 and 2012–2013). 1916 patients under the age of 18 years were examined with the complex of virologic tests . The natural decreasing of flu incidence and predominant diagnosing in etiologic structure one of the virus serotypes [in the first A(H1N1)pdm09, in the second — A(H3N2)] have been observed during the first two years after a pandemic. In the third season restoration of the main characteristics of epidemic flu situation were detected: polietiology with annual change of serotypes proportions, late start (winter and spring), majority of younger children among hospitalaized patients and a mild course of disease. The most probable candidates [viruses of a subtype A(H3N2)] are revealed as a causal factor of significant increasing of disease incidence with the severe forms in the near future. Laboratory data were confirmed by increase in frequency of the complicated by pneumonia ARI at hospitalized patients during the periods of their maximum registration. All isolates received during the study were corresponded to the referens-strains included in vaccines. Thus, timely and appropriate vaccination during the studied period had to become an effective protection against a flu.