The use of multistate life-table models for improving population health.

We demonstrate how incidence, prevalence, remission, mortality (IPRM) models may be constructed on population life-tables, how the incidence of a condition may be calculated, and how the consequences of demographic changes and public health interventions may be predicted. We illustrate the methodology by applying it to the epidemiology of diabetes, physical inactivity and obesity in New Zealand.

Modelling physical activity: a multi-state life-table approach.

OBJECTIVE: To develop a consistent set of epidemiological estimates (incidence, prevalence, remission, mortality) for physical activity in New Zealand; project these estimates in the light of demographic trends; and predict the effectiveness of different health promotion strategies. METHOD: Multi-state life tables were constructed using physical inactivity prevalence data from the 1996/97 New Zealand Health Survey, and estimates of the relative risk of mortality, and of remission rates, from the literature. Statistics New Zealand population projections were used to forecast these multi-state life tables to 2021. Two physical activity health promotion strategies -uptake (remission enhancement) and maintenance (incidence or relapse reduction)--were simulated by changing the relevant epidemiological variables. RESULTS: The current fatal burden of physical inactivity in New Zealand is estimated to be 2,600 deaths per year (9% of all deaths). By 2021, the prevalence of physical inactivity will rise 4% as a result of demographic trends. Relapse reduction (enabling active people to remain active) is about 50% more effective than uptake enhancement (enabling inactive people to become active) as a physical activity health promotion strategy, but the two approaches are additive. Maximum realistic changes in relapse prevention and uptake enhancement could reduce the prevalence of physical inactivity by about 30%. CONCLUSIONS AND IMPLICATIONS: Multi-state life table methods can be used to model health risks (such as behaviours), as well as (chronic) diseases. The model has provided valuable insights for policy makers into the burden of physical inactivity in New Zealand, the impact of demographic trends, and the relative effectiveness of different health promotion strategies.

A challenge to the cross-cultural validity of the SF-36 health survey: factor structure in Maori, Pacific and New Zealand European ethnic groups.

This paper reports on a principal component factor analysis of the SF-36 health status questionnaire in the three major ethnic groups in New Zealand (New Zealand Europeans, Maori and Pacific). The SF-36 is hypothesised to have a two-dimensional structure with distinct (weakly correlated) mental and physical health components, and support for this structural model has generally been found cross-nationally. However, in Maori and Pacific models of health mental and physical dimensions are not generally seen as separable, or independently functioning. This raises the possibility that the questionnaire's hypothesised structural model would not be supported among Maori and Pacific ethnic groups. This study evaluated that possibility. The results of the analysis showed a similar factor structure among New Zealand Europeans, and younger Maori (<45 years) to that reported by Ware et al. for Western European countries. Among Pacific people and older Maori (45 years and over), however, the factor structure did not clearly differentiate physical and mental health components. Implications are discussed both specific to the SF-36 (and in particular the use of principal component summary scores), and more generally for the cross-cultural validity of self-reported health status measures.

Predicting and preventing measles epidemics in New Zealand: application of a mathematical model.

A mathematical model of the dynamics of measles in New Zealand was developed in 1996. The model successfully predicted an epidemic in 1997 and was instrumental in the decision to carry out an intensive MMR (measles-mumps rubella) immunization campaign in that year. While the epidemic began some months earlier than anticipated, it was rapidly brought under control, and its impact on the population was much reduced. In order to prevent the occurrence of further epidemics in New Zealand, an extended version of the model has since been developed and applied to the critical question of the optimal timing of MMR immunization.

SF-36 health survey reliability, validity and norms for New Zealand.

OBJECTIVE: To assess the acceptability, reliability and validity of the SF-36 health survey in the New Zealand population and provide key population norms. METHOD: The SF-36 questionnaire was part of the 1996/97 New Zealand health survey, a cross-sectional, nationally representative survey of 7,862 adults (15 years and over). RESULTS: Overall, in the New Zealand population the questionnaire performed as well as or better than in other national surveys, but there was variability in data completeness across subgroups, and responses were skewed towards the healthy end of the scales. Males scored higher than females on almost all scales; increasing age was associated with decreasing scores (with the exception of the mental health scale); and New Zealand Europeans tended to report better health than the other ethnic groups. CONCLUSIONS: Satisfactory psychometric performance was demonstrated for the SF-36 in the New Zealand population, but researchers need to find ways of increasing data completeness in population groups shown here to have lower completion rates. The questionnaire may be better at discriminating patient rather than population subgroups. The SF-36 normative data confirm in kind, if not in degree, population subgroup disparities in health status observed using objective measures. IMPLICATIONS: Overall, the SF-36 questionnaire appears to be a valid and reliable measure of health-related quality of life for the New Zealand population. However, this paper highlights issues for researchers using the SF-36, such as the skewed nature of responses obstained in a population sample.

The private costs of HIV/AIDS.

AIM: To identify and, where possible, measure and value the private costs related to HIV/AIDS, that is, those costs that fall on the person with HIV/AIDS and the family/household/informal caregivers. METHOD: Twenty-five people living with HIV - ranging from asymptomatic seropositive people, to people with AIDS - were followed prospectively to obtain information concerning the private costs (broadly defined) incurred. The participants resided in the Auckland, Hamilton, Wellington and Christchurch areas. RESULTS: Private direct costs rise steeply as the illness progresses, from around $100 per month for asymptomatic people to around $400 per month for people with AIDS. Both indirect costs (foregone income) and intangibles (measured by a range of indicators) were also considerable. CONCLUSION: The private costs of HIV/AIDS, defined in terms of direct, indirect and intangible costs, are significant and burdensome. Costing studies which ignore them will conceal, confuse and mislead.

Occupational risk of hepatitis B for police and customs personnel.

A cross-sectional study was undertaken in 1987 to establish whether New Zealand police and customs officers are at excess risk of hepatitis B virus infection as a consequence of occupational exposure to human blood and penetrating injury. The study population comprised all full-time police (n = 5,193) and customs officers (n = 1,026) excluding only a small number on special duty who had already been immunized. The control group comprised the civilians employed by both organizations (n = 964). The prevalence of hepatitis B markers in the control group, when standardized for age, sex, and ethnic distribution, was 13.4%, which agrees well with New Zealand blood donor figures. The prevalence ratios for police officers and customs officers compared with the civilians (adjusted for age, sex, and ethnic distributions) were 0.82 (95% confidence interval (CI) 0.63-1.06) and 0.49 (95% CI 0.34-0.70), respectively. Multivariate analysis was used to further explore the differences in marker prevalence among the three groups, but failed to demonstrate any significant association between occupational variables and marker prevalence. There was an association between time spent living in high-risk areas of the country and marker prevalence. The authors conclude that the question as to whether police personnel should be immunized begs the wider issue of whether or not the whole New Zealand population should be so protected.

The 1985 national immunisation survey: hepatitis A.

In April 1985 a national immunisation survey was carried out, during which sera were collected from approximately 3000 randomly selected children throughout New Zealand. The sample comprised approximately equal numbers of new school entrants (mean age 5 years), standard 3 pupils (mean age 10 years) and form 4 students (mean age 15 years). This collection of sera was tested for antibody to hepatitis A virus, a marker of past infection with this virus, by means of a sensitive ELISA test. Prevalence of infection was found to be less than 1% in the 5 year olds, about 3% in the 10 year olds, and about 9% in the 15 year olds. Amongst the 10 and 15 year olds, but not the 5 year olds, Maori children were approximately three times more likely to have been infected than European children. Children resident in the eastern part of the North Island had a higher risk of infection than other children, even after controlling for ethnic distribution.

A survey of virus infections in New Zealand, 1982-6.

Between 1982 and 1986 virus infections were identified in 16,372 cases. These identifications were based on virus isolation and/or serological evidence of infection by the main virus diagnostic laboratories at Auckland, Waikato, Christchurch and Dunedin hospitals, and at the National Health Institute. The most frequent virus identifications reported were herpes simplex (46.7%), rotavirus (11.8%), respiratory syncytial virus (5.7%), and adenovirus (5.6%). During this period of surveillance, the most prominent feature has been the high incidence of herpes simplex which reached a peak in 1983 but which has abated only slightly since. Significant trends and virus outbreaks or epidemics were detected with the regular reporting of monthly virus identifications in the New Zealand Virus Report (NZVR); these included a measles epidemic in Auckland in 1984/85, major influenza A outbreaks in 1983, 1985 and 1986, the respiratory syncytial virus epidemic in the winter of 1986, the increased incidence of rotavirus predominantly in young infants and children during the winter months, outbreaks of enterovirus type 71 and parainfluenza type 3 infections in 1986, and rubella in 1984.

Hepatitis B in New Zealand children: the 1985 national immunisation survey.

In April 1985 a national immunisation survey was conducted, in the course of which blood samples were collected from 3000 randomly selected children throughout the country. There were 1000 new school entrants (mean age 5 years), 1000 standard 3 pupils (mean age 10 years), and 1000 form 4 students (mean age 15 years). The sera were tested for hepatitis B surface antigen, antibody to hepatitis B surface antigen, and antibody to hepatitis B core antigen, by ELISA. The prevalence of infection rose with age until by 15 years of age 13.1% of the study population (8.2% of the European and 42.0% of the Maori children) were marker positive. At all ages, Maori children were five times more likely to be positive for any marker, and approximately thirteen times more likely to be positive for antigen (actively infected), than the European children. Even when the data had been standardised for age and race, children resident in the eastern North Island were still almost three times more at risk than children in the South Island. Children in the remaining areas of the North Island were at approximately equal degrees of risk, intermediate between the high and low endemic areas mentioned. We conclude that universal childhood immunisation is necessary to control horizontal transmission of heptatis B virus in New Zealand.

Hepatitis A infection in New Zealand children.

Antibody to hepatitis A virus (anti HAV), a marker of past infection, was assayed in 2000 sera collected as part of a national survey in 1978-79. The sera were obtained from children and young people aged 0-21 years, resident in all health districts of New Zealand. Anti HAV was detected in 307 sera, giving an overall prevalence of 15.4%. Prevalence increased steadily throughout childhood but more slowly during adolescence. There was no sex differential, the age-standardised rate/100 being 15.5 (95% confidence interval 13.2, 17.8) for males and 16.6 (14.4, 18.9) for females. However, the age-standardised rate for Maoris was 39.5 (33.2, 45.8) compared to 16.1 (12.8, 19.4) for Europeans, giving a risk ratio of 2.1. In addition, a marked north-south gradient in prevalence was demonstrated: the rate for children in the northern half of the North Island, when standardised for age and ethnicity, was 19.8 (16.8, 22.7) compared to 5.2 (3.2, 7.1) for South Island children, giving a risk ratio of 3.8. The higher prevalence of hepatitis A infection in Maori and northern North Island children mirrors our previously reported findings regarding markers of hepatitis B infection in this serum collection.

Mononucleosis-like-syndrome associated with acute AIDS retrovirus infection.

A 47-year-old man developed an acute infectious mononucleosis-like illness with a moderate lymphocytosis and numerous atypical lymphocytes in the peripheral blood. Seroconversion to the AIDS-associated virus occurred between 14 and 20 days following the onset of the acute illness. He was found to have reduction of the T4:T8 ratio, low mitogenic response to PHA and cutaneous anergy when tested at 25 and 136 days. These tests had returned to normal by 262 days.