Patterns of disparity: ethnic and socio-economic trends in breast cancer mortality in New Zealand

Objective To test whether trends in breast cancer mortality varied by ethnicity and socio-economic position during the 1980s and '90s in New Zealand. Methods Four cohorts of the entire New Zealand population for 1981 - 84; 86 - 89; 91 - 94 and 96 - 99 allowed direct determination of socio- economic trends in breast cancer mortality. For ethnicity, unlinked routine census and mortality data were used with adjustment factors for undercounting of Maori and Pacific deaths. Results Maori and non-Maori non-Pacific mortality rates changed little until mid-1990s with Maori experiencing 25% higher mortality. In 1996 - 99, Maori rates increased notably to become 68% higher than non-Maori non-Pacific (SRR 1.68; 95% CI: 1.49 - 1.90). Pacific women experienced an approximate three-fold increase in breast cancer mortality over time. There appeared to be reducing mortality among higher income and education groups but trends within socio- economic groups were not statistically significant. Nevertheless, by 1996 - 99, there was a significant 22% excess mortality ( SRR 1.22; 95% CI: 1.01 - 1.49) for low compared with high-income groups. Conclusions Widening ethnic, and probably, socio- economic disparities in breast cancer mortality are likely due to both underlying incidence and differential survival trends. Disparities are likely to increase once the full differential mortality benefits of screening impact on the population.