Does mortality vary between pacific groups in New Zealand? Estimating Samoan, Cook Island Maori, Tongan, and Niuean mortality rates using hierarchical Bayesian modelling

Abstract

BACKGROUND Pacific mortality rates are traditionally presented for all Pacific people combined, yet there is likely heterogeneity between separate Pacific ethnic groups. We aimed to determine mortality rates for Samoan, Cook Island Māori, Tongan, and Niuean ethnic groups (living in New Zealand). METHODS We used New Zealand Census-Mortality Study (NZCMS) data for 2001-04, for 380,000 person years of follow-up of 0-74 year olds in the 2001-04 cohort for which there was complete data on sex, age, ethnicity (total counts), natality, and household income. Given sparse data, we used hierarchical Bayesian (HB) regression modelling, with: a prior covariate structure specified for sex, age, natality (New Zealand/Overseas born), and household income; and smoothing of rates using shrinkage. The posterior mortality rate estimates were then directly standardised.RESULTS Standardising for sex, age, income, and natality, all-cause mortality rate ratios compared to Samoan were: 1.21 (95% credibility interval 1.05 to 1.42) for Cook Island Māori; 0.93 (0.77 to 1.10) for Tongan; and 1.07 (0.88 to 1.29) for Niuean. Cardiovascular disease (CVD) mortality rate ratios showed greater heterogeneity: 1.66 (1.26 to 2.13) for Cook Island Māori; 1.11 (0.72 to 1.58) for Niuean; and 0.86 (0.58 to 1.20) for Tongan. Results were little different standardising for just sex and age. We conducted a range of sensitivity analyses about a plausible range of (differential) return migration by Pacific people when terminally ill, and a plausible range of census undercounting of Pacific people. Our findings, in particular the elevated CVD mortality among Cook Island Māori, appeared robust. CONCLUSIONS To our knowledge, this project is the first time in New Zealand that clear (and marked in the case of CVD) differences in mortality have been demonstrated between different Pacific ethnic groups. Future health research and policy should, wherever possible and practicable, evaluate and incorporate heterogeneity of health status among Pacific people.