Increasing incidence of serious infectious diseases and inequalities in New Zealand: a national epidemiological study

Background: Although the burden of infectious diseases seems to be decreasing in developed countries, few national studies have measured the total incidence of these diseases. We aimed to develop and apply a robust systematic method for monitoring the epidemiology of serious infectious diseases.

Methods: We did a national epidemiological study with all hospital admissions for infectious and non-infectious diseases in New Zealand from 1989 to 2008, to investigate trends in incidence and distribution by ethnic group and socioeconomic status. We extended a recoding system based on the ninth revision of international classification of diseases (ICD-9) to the tenth revision (ICD-10), and applied this to data for hospital admissions from the New Zealand Ministry of Health, National Minimum Dataset. We filtered results to account for changes in health-care practices over time. Acute overnight admissions were the events of interest.

Findings: Infectious diseases made the largest contribution to hospital admissions of any cause. Their contribution increased from 20·5% of acute admissions in 1989-93, to 26·6% in 2004-08. We noted clear ethnic and social inequalities in infectious disease risk. In 2004-08, the age-standardised rate ratio was 2·15 (95% CI 2·14-2·16) for Māori (indigenous New Zealanders) and 2·35 (2·34-2·37) for Pacific peoples compared with the European and other group. The ratio was 2·81 (2·80-2·83) for the most socioeconomically deprived quintile compared with the least deprived quintile. These inequalities have increased substantially in the past 20 years, particularly for Māori and Pacific peoples in the most deprived quintile.

Interpretation: These findings support the need for stronger prevention efforts for infectious diseases, and reinforce the need to reduce ethnic and social inequalities and to address disparities in broad social determinants such as income levels, housing conditions, and access to health services. Our method could be adapted for infectious disease surveillance in other countries.

Funding: New Zealand Ministry of Health, New Zealand Health Research Council.