[Influenza activity 2000/2001].
Influenza viral infections are an important cause of morbidity in all age groups and are associated with a high mortality rate amongst the elderly and in risk groups. The current study analyses data from the epidemiological surveillance of influenza during the 2000/2001-winter season. Clinical, epidemiological and virological data relative to cases of influenza syndrome were collected via the National Influenza Surveillance Scheme, in collaboration with the General Directory of Health and integrates the information obtained by the Network of Sentinel Medical Practitioners and Emergency Units. The data analysis hereby obtained shows a low influenza activity during the 2000/2001-winter season, with an epidemiological period characterized by small duration and intensity. The incidence rate of influenza-like illnesses increased above the base line during three weeks and didn't surpass 74 cases per 1,000,000 inhabitants. Influenza B viruses were predominant with the simultaneous presence of influenza A (H1 and H3). The antigenic and genetic characterisation of the isolates allowed the confirmation of the similarity between these viral strains and the vaccine strains as well as the extent of the antigenic drift. Despite the antigenic similarity between the majority of influenza B isolates and the vaccine strains it's noticeable that the genetic characteristics showed an evolution directed towards the B/Sichuan/379/99 strain, which would later be incorporated in the 2001/2002 influenza vaccine. The co-circulation of two different lineages was consequently observed by the phylogenetic analysis of the B strains isolated in our country.