The smoking-mortality association varies over time and by ethnicity in New Zealand

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-Maori non-Pacific than Maori: 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 ( .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 Maori compared with 464 (427 - 502) for non-Maori 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.