Estimating excess winter mortality in England and Wales

Johnson H and Griffiths C(2003) Estimating excess winter mortality in England and Wales. Health Statistics Quarterly 20: 19–24.

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Abstract: 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. It is therefore possible that the geography of SIDS will reflect that of 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 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. RESULTS: 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. CONCLUSIONS: 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. PIP: There has been some empirical support for the hypothesis that population mixing from internal migration may be related to increased infection or occurrence of excess of childhood leukemia. In this study, the incidence of sudden infant death syndrome (SIDS) was examined in relationship to geographic variation and degree of population mixing. It is known that SIDS is more common in the winter months, in lower social classes, in larger families, and is preceded by minor infections. The expectation is that SIDS will be higher in areas with greater numbers of long-distance migrants. Data were obtained on all 403 local authority districts from the 1979-83 Decennial Supplement on Area Mortality for England and Wales, and the 1981 Population Census. The observed and expected numbers of infant deaths from diseases of the respiratory system (ICD460-519) were collected along with SIDS (ICD 798.0) in order to examine whether there was underreporting or overreporting of SIDS in high migration areas. Data was controlled for confounding factors such as legitimacy and illegitimacy, and estimating, from legitimate birth data, social class data. The results showed that the ratio of observed to expected numbers of AIDS deaths increased with increased rate of migration from outside the region. Districts with 3 or more migrants per 100 residents had a 62% higher ratio than areas with 1 migrant per 100 residents. Differences were highly significant: x2 = 94.75, for 1 degree of difference, p 0.001. After adjustment for social class and illegitimacy, the results indicated a highly statistically significant trend that was stronger than the initial analysis: x2 = 101.99, 1 degree of difference, p 0.001. There was only a minor impact of confounding factors on the observed trend. There was a statistically significant negative trend for the relationship between infant respiratory deaths and inter-regional migration rates: x2 = 7.18, 1 degree of difference, p = 0.007. The opposite sign was interpreted as evidence of some diagnostic confusion in assignment of SIDS deaths, but the disproportion of numbers of respiratory and SIDS deaths in the highest and lowest migration groups was very large (159 deaths from SIDs and a deficit of 19.4 respiratory deaths in the highest group). The difference between the highest and lowest respiratory disease group was 7%, but 62% with SIDS. The provisional conclusion was that the extent of population mixing does affect SIDs incidence.

Context

• Informative report shows higher rates of death from many causes in winter months. The excess in deaths is considerable and appears to be increasing.