National Influenza Surveillance Programme: Results of Influenza Activity in Portugal in the 2010/2011 Season

Pedro Pechirra, Paulo Gonçalves, Patrícia Conde, Baltazar Nunes, Raquel Guiomar


Introduction: Epidemiological surveillance of influenza, a disease associated with high mortality in the elderly and individuals’ belonging to risk groups, is essential for the characterization of influenza epidemics as well as for the monitoring of outbreaks and the emergence of viral strains resistant to antivirals.

Material and Methods: In the present study was analyzed the influenza surveillance data from 2010/2011 winter. The clinical, epidemiological and virological data related to cases of flu-like syndrome have been collected through the National Influenza Surveillance Programme, coordinated by the National Influenza Reference Laboratory (LNRVG) in collaboration with the Department of Epidemiology (DEP) of the National Institute of Health Doutor Ricardo Jorge (INSA) and the Directorate-General for Health (DGS).

Results: From the analysis of the data collected during the 2010/2011 winter season, influenza activity was moderate / high with an epidemic period of 8 weeks between week 50/2010 and 5 / 2011, with a peak of 121, 12 cases per 100 000 population in week 52/2010.

Discussion: The influenza B viruses (Victoria lineage) predominated in the early period to week 1 / 2011, when became to predominate influenza A(H1)pdm09 viruses. The largest proportion of cases of influenza was found in the age group of children between 5 and 14 years old. The viruses characterized antigenically and genetically were similar to strains included in the seasonal influenza vaccine 2010/2011, presenting some amino acid substitutions in antigenic sites. Most strains of A(H1)pdm09 influenza virus still remain sensitive to oseltamivir and zanamivir, although were detected sporadic cases of oseltamivir resistant viruses. To date, the LNRVG detected the presence of the H275Y substitution in the neuraminidase gene, associated with oseltamivir resistance, in three virus A(H1)pdm09. For one of them, oseltamivir resistance was confirmed by phenotypic assays.

Conclusion: Despite fears associated with the emergence of a new pandemic virus (rapid spread, high morbidity and mortality), the clinical and epidemiological characteristics of infection with influenza A(H1)pdm09, proved in the course of two seasons (2009/2010 and 2010/2011) to be very similar to seasonal flu. Is however noteworthy that during the pandemic season(2009/2010) the influenza virus A (H1N1) pdm09 were linked with particular characteristics with regard to their temporal distribution, age groups most affected, severe infections and death and associated risk factors.

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