Changing ethnic inequalities in mortality in New Zealand over 30 years: linked cohort studies with 68.9 million person-years of follow-up

The decline in all-cause mortality rates for all ethnic groups in New Zealand is a favourable finding from a public health perspective. Even so, for males during the 1980s and 1990s there was actually a slight increase and stagnation in this pattern for Māori and Pacific. This development, largely driven by CVD, contributed to an increase in absolute inequalities, which are now falling in the three most recent cohorts.

Another main finding is the continuation of substantial relative and absolute inequalities in mortality for both Māori and Pacific in relation to European/Other. Pacific all-cause mortality has been falling at a slower rate than both Māori and European/Other across the whole study period, and particularly for females. This has resulted in increasing absolute inequalities for Pacific females while the opposite has been the case for Māori females across the whole study period.

All-cause relative inequalities are still high and increasing for Māori males and Pacific females. However, for Pacific males and Māori females, there are signs that relative inequality, whilst still high, has not been getting worse.

In terms of the main contributors to inequalities, there appears to be a transition, particularly amongst females, away from CVD and towards cancer, especially non-lung cancer. The steady decline in female European/Other non-lung cancer mortality rates compared to a steady rise in both Māori and Pacific has resulted in rises of both absolute and relative inequality. Furthermore, whilst there are examples of falling absolute inequalities (mainly respiratory disease and unintentional injury) relative inequalities are, on the whole, large (especially diabetes) and increasing.

The human cost associated with these inequalities is striking. Fortunately, many of the diseases driving this inequality can be addressed – as we detail further below.

The linking of individual census data to mortality records enabled us to analyze 30 years’ worth of mortality trends for the population of an entire country. Having six follow-up periods has now allowed us to have a greater detailed understanding of how trends in both the rate and inequalities are changing over time.

Furthermore, common deficiencies in ethnicity data have been circumvented by using the self-defined ethnicity variable from the Census, and numerator-denominator bias is avoided. The study size is also very large – with 68.9 million years of person time follow-up. However, there are still specific diseases (e.g., for female Pacific peoples for lung cancer and suicide for 1981–1984) where small numbers limited statistical power. Small numbers also limited the extent to which we could consider mortality-specific trends disaggregated by ethnicity and age. Where numbers permitted (for example, all-cause mortality), we found that trends in ethnic-specific standardized mortality rates are similar by age. Furthermore, there were people of a non-European ethnic background included in the European/Other group (i.e., people who do not self-identify as Māori, Pacific, Asian or European). However, these numbers are likely to be small and not have a significant effect on our findings. Future research could assess the sensitivity of our findings to this and build on previous work [12] to identify whether there is heterogeneity in mortality trends within ethnic groups.

The ethnic inequalities in mortality identified in this paper are so striking that, we argue, a consideration of how they can be reduced has to start with a critical assessment of how society is structured. Macroeconomic and social policy are one such structuring factor. Previous research for New Zealand has indicated that the widening in ethnic inequalities in 1980s and 1990s was contributed to by structural economic changes to liberalize the market economy [9].

Indeed, further risks for widening inequalities may persist if automation and other globalization forces result in increased unemployment and insecure employment among lower-skilled workers [22]. Nevertheless, there can be progress, as seen with more rapid increases in the percentage of Māori and Pacific school leavers with school qualifications [23]. However, faster progress is required if there is to be a significant erosion of the entrenched inequalities in health we identified. Measures such as additional education support, improvements in social housing provision, tax reform (e.g., re-introducing estate (inheritance) taxes, and reducing income taxes for low-income families) and a living wage should all be seriously considered.

Another structural driver of inequalities in mortality is the obesogenic environment [24]. In New Zealand, as with many other high-income countries, a high body mass index is now the largest single cause of health loss [25]. This is also likely to be a driver for inequalities from CVD, diabetes, and some types of cancer for both Māori and Pacific peoples (given much higher burdens of overweight and obesity [26]).

Detailed analysis of both cancer incidence and mortality by Teng et al. [27] has found that ethnic inequalities have actually widened most in the incidence of obesity-related cancers in New Zealand (breast and endometrial cancer in particular). While the New Zealand government has recently adopted an obesity control strategy, there is still scope for more substantive interventions to address obesogenic environments, e.g., Mexican-style taxes on sugary drinks [28], banning junk food marketing to children, and mandatory front of package health rating labeling of food. If such measures were well-designed, and appropriately implemented, they should contribute to reductions in inequalities (e.g., there is some evidence for greater response by lower socioeconomic groups in Mexico to reduced consumption of sugary drink tax after the new tax [28]).

The second most important cause of health loss in New Zealand is from smoking [25]. Furthermore, our results for ethnic inequalities in lung cancer (and to some extent for CVD, and other cancers) are likely to reflect the marked ethnic gradient in tobacco smoking in New Zealand; both currently and as projected into the future [29]. This highlights the potential for further advances in tobacco control; and, indeed the New Zealand government has a smoke-free nation goal for the year 2025.

Recent interventions include annual tobacco tax increases in the years 2010 to 2016, banning point-of-sales displays of tobacco, and a plan to finalize a law requiring the plain packaging for tobacco products. But interventions that may disproportionately benefit Māori may include tax increases (as per New Zealand modeling work [30]) and mass media campaigns designed for a Māori audience [31, 32]. More profound interventions to accelerate progress to the tobacco endgame have also been discussed for New Zealand and elsewhere: a sinking lid on sales [33], tobacco retail outlet reduction [34], and a smoke-free generation (i.e., no one born after the year 2000 is allowed to purchase tobacco [35]). The New Zealand public appear to be broadly supportive of such endgame strategies [36] and these are consistent with international calls to reduce the overall burden and inequalities from the global tobacco epidemic [37].

A further domain for inequality reduction interventions is in accelerating the ongoing declines in CVD risk. Modeling work for New Zealand has suggested that multiple population-wide dietary sodium reduction measures are likely to provide greater per capita health gain for Māori relative to non-Māori [38]. Health services also matter and there remain some gaps in terms of the provision of CVD risk screening in primary care for Māori vs. non-Māori (at 86% vs. 92% coverage respectively) [39]. However the target age-range for such screening for Māori starts at a young age (to assist with equity goals) and there have been successful campaigns to increase preventive CVD pharmacotherapy among Māori (e.g., for statin use [40]).

Finally, another area for health service improvements are addressing the inequalities, or inequities, in receipt of some cancer treatments between ethnic groups in New Zealand [41] and ethnic/racial groups internationally [42]. Some of these inequalities are challenging to address: e.g., higher comorbidities among ethnic groups, which may be perceived as limiting treatment options. But gains are likely to be possible, and will contribute somewhat to reducing (or at least slowing) ethnic inequalities in mortality. Improvements in diabetes care may also be a likely fertile area for policies and actions to reduce ethnic inequalities in mortality.