Background: The strength of the smoking-mortality association may vary over time and by ethnic group.
Methods: Cohort studies of 1.6 million (1981-84) and 1.9 million (1996-99) New Zealanders aged 25-74 years were formed by the linkage of census and mortality data. Comparing current smokers with never smokers, standardized rate ratios (RRs) and rate differences (RDs) were calculated for all-cause and ischaemic heart disease (IHD) mortality.
Results: Between 1981-84 and 1996-99 the all-cause mortality RR increased from 1.59 (95% CI 1.53-1.66) to 2.05 (1.97-2.14) for men and from 1.49 (1.42-1.56) to 2.01 (1.91-2.12) for women. All-cause RRs were significantly greater among non-Ma-ori non-Pacific than Ma-ori: 2.22 (2.12-2.33) compared with 1.51 (1.35-1.69) in men and 2.20 (2.09-2.33) compared with 1.45 in women (1.27-1.66), respectively, in 1996-99. This RR heterogeneity remained after adjusting for socio-economic factors and was similar for IHD. The RDs demonstrated less heterogeneity. For example, in 1996-99 the RDs were 627 per 100,000 (452-802) for Ma-ori compared with 464 (427-502) for non-Ma-ori non-Pacific among men, and 368 (228-509) compared with 340 (311-370) among women.
Conclusions: In New Zealand the relative effect of smoking on mortality differs over time and by ethnicity. We expect that such heterogeneity exists in other countries where the background mortality rates vary over time or between social groups. Information on this heterogeneity, including ethnicity-specific data, is needed to accurately determine the mortality burden owing to tobacco. The size of the RR estimates should be interpreted in the context of absolute mortality and effect measures.