Map created by CUHK SpatioEpi Group
Influenza is caused by the influenza virus. The influenza virus is not a single virus but rather the name for a range of virus subtypes. Subtypes may undergo mutations and could take on new forms. Currently the 3 main subtypes in circulation around the world are Influenza A H3N2, H1N1 (previously "swine flu") and Influenza B. These viruses take turn to cause influenza seasons. In the United States the influenza season has started early this season, with some places like the New York State declaring public health emergency in January 2013. H3N2 is the main cause of infections in the current season, which seems to have peaked recently. The maps show the distribution of the three viruses between the last 2 weeks of December 2012 and the first two weeks of January 2013, expressed as the number of positive cases for the specific virus in processed specimens. The higher rates of H3N2 remain localized to US and Canada. Elsewhere a mixed pattern involving H1N1, H3N2 and B in varying proportion is seen. Data for the map were obtained from World Health Organization's FluNet (http://apps.who.int/globalatlas/dataQuery/default.asp). In Hong Kong, the influenza season has started in January 2013 before the Chinese New Year. Interestingly respiratory symptoms have become more common since December 2012, as revealed in the community surveillance project EcSS (www.EcSS.hk).
Influenza is a respiratory infection caused by the influenza virus, different forms of which are normally circulating in not just human beings but pigs and birds. All influenza viruses are divided into A, B and C, and further distinguished by their H and N antigens. There are currently 18 H (standing for haemagglutinin) and 11 N (standing for neuraminidase) antigens. Epidemics occurred when novel viruses are introduced to the human population. Pandemic influenza A (H1N1), previously referred as Swine flu, was an example causing worldwide outbreaks in 2009. The genetic structure of this influenza virus contains segments from pig, bird and human, reflecting the occurrence of reassortment. Studies suggested a case fatality rate of 0.4% for the novel infection, and a higher tendency for causing disease in young people compared to other forms of seasonal flu. Since 2009, the same virus has continued to cause outbreaks in different populations at different timepoints.
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