Epidemiology of tick-borne encephalitis in Sweden 1956-1989: a study of 1116 cases.

Tick-borne encephalitis (TBE) has occurred regularly in Sweden since it was first diagnosed in 1954. In Austria, where TBE is common, vaccination has been shown to reduce the incidence considerably. Since this vaccine is now available in Sweden we analysed 1116 cases of TBE from 1956 to October 1989 in order to define a strategy for vaccination. The cases were identified in the records from all infectious disease departments in Sweden complemented with data from the virological laboratories. All diagnoses were confirmed by a positive serological result. The analysis shows TBE to be concentrated to the archipelagos and coastline areas of the Baltic and the lake Mälar around Stockholm--about 85% of all cases came from the county of Stockholm. Conspicuous clustering occurred in some areas and the geographical distribution was remarkably constant over the time studied. As can be expected with a tick-borne infection, a strong seasonal variation was found, 85% of the cases occurring July to September. The age distribution showed the highest incidence from 20-70 years; there was a male majority in all age groups. Five patients died.