POPULATION MIXING AND SUDDEN-INFANT-DEATH-SYNDROME IN ENGLAND AND WALES

Background. 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. I t is therefore possible that the geography of SIDS will reflect that o f population migration. Methods. 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 abstract ed from published sources. Districts were classified by levels of in-m igration using data from the 1981 Census and observed and expected num bers of SIDS deaths were compared for areas experiencing different rat es of in-migration. Results. A significant association (P < 0.001) was found between SIDS deaths and rates of long distance migration into d istricts. In the group with the highest rate of long distance in-migra tion the ratio of observed to expected SIDS deaths was 62% greater tha n 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 shor ter distance in-migration. Conclusions. it is concluded that geographi cal variations in SIDS deaths in England and Wales are strongly associ ated with differences in rates of population mixing as measured by mig ration. 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.