Population mixing and sudden infant death syndrome in England and Wales

Infections may be involved in some cases of sudden infant death syndrome (SIDS). Exposure to such infections is likely to differ considerably between areas depending on rates of population mixing. It is therefore possible that the geography of SIDS will reflect that of population migration.

For 403 local authority districts in England and Wales the number of SIDS deaths in the first year of life and the number of livebirths during the period 1979-1983 were abstracted from published sources. Districts were classified by levels of in-migration using data from the 1981 Census and observed and expected numbers of SIDS deaths were compared for areas experiencing different rates of in-migration.

A significant association (P < 0.001) was found between SIDS deaths and rates of long distance migration into districts. In the group with the highest rate of long distance in-migration the ratio of observed to expected SIDS deaths was 62% greater than in the group with the lowest rate of in-migration. This association was strengthened after adjustment for social class and illegitimacy. A weaker, but statistically significant, association was found for shorter distance in-migration.

It is concluded that geographical variations in SIDS deaths in England and Wales are strongly associated with differences in rates of population mixing as measured by migration. The findings support the hypothesis that population mixing, by its influence on exposure to infections, may be a significant factor in the aetiology of SIDS.