Sudden infant death syndrome, population mixing and oil-related development in western Norway

A number of features of sudden infant death syndrome (SIDS) point to an aetiological role for apparently minor infections. The spread of infections is influenced by the rate of contact between infectious cases and susceptibles in the population. This is likely to differ geographically depending on patterns of social contacts, with epidemics being particularly likely where an influx of migrants leads to high rates of mixing of populations lacking herd immunity. It has been shown that geographical variations in SIDS deaths in England and Wales are strongly associated with rates of long distance in-migration into districts. The large influx of population into parts of western Norway as a result of oil-related development provides an opportunity to examine this issue further. The most intensive development has been in the Stavanger area in the county of Rogaland. Published migration statistics show that there was a rapid build-up of long distance migration into this area reaching a peak in the late 1970s. Further north, the Bergen area in the county of Hordaland was little affected by oil-related activities in the 1970s. However, the more recent development of oil provinces further north has led to more activity in the Bergen area and an increase in in-migration from the mid 1980s onwards. Annual data from 1969 onwards on SIDS deaths in Rogaland and Hordaland counties and for Norway were used to assess whether trends in SIDS mortality and migration showed any associations. Before the main population influx Rogaland had a SIDS rate that was below the Norwegian average. However, since the mid 1970s SIDS rates have been significantly in excess of the national average. In Hordaland SIDS rates were low throughout the 1970s but increased substantially to be significantly in excess of the Norwegian average in the late 1980s. In both areas population influxes resulting from oil-related development were therefore followed by a significant increase in mortality rates from SIDS. There is no evidence that these trends can be accounted for by changing patterns of known risk factors for SIDS in these areas nor of artefacts in the registration of SIDS cases. The results of this study therefore support the British evidence that population mixing may be a significant factor in the aetiology of SIDS.