Explorative analysis of spatial patterns of influenza incidences in Sweden 1999—2008

Information about the spatial spread of epidemics can be useful for many purposes. In this paper, the spatial aspect of Swedish influenza data is analyzed with the main aim of finding patterns that could be useful for statistical surveillance of the outbreak, i.e. for detecting an increase in incidence as soon as possible. In Sweden, two types of data are collected during the influenza season: laboratory diagnosed cases (LDI), collected by a number of laboratories, and cases of influenza-like illness (ILI), collected by a number of selected physicians. Quality problems were found for both types of data but were most severe for ILI. No geographical pattern was found. Instead, it was found that the influenza outbreak starts at about the same time in the major cities and then occurs in the rest of the country. The data were divided into two groups, a metropolitan group representing the major cities and a locality group representing the rest of the country. The properties of the metropolitan group and the locality group were studied and it was found that the time difference in the onset of the outbreak was about two weeks. This justifies a different spatial model than the one usually used for infectious diseases.