Epidemiology and public health

What is epidemiology?

The word epidemiology is derived from the Greek ‘epi’, ‘demos’ and ‘logos’ and literally translated means the study (logos) of what is among (epi) the people (demos). It is the study of the occurrence and distribution of health-related states/events in specified populations. Its uses are listed in Box 2.1 .

The prevention of sudden infant death syndrome (SIDS) is one of the major success stories in epidemiology (see Box 1.1 ). Epidemiological studies, culminating in the New Zealand Cot Death Study, a three-year case-control study (1987–1990), identified three modifiable risk factors for SIDS, namely prone sleeping position, maternal smoking and lack of breastfeeding. A prevention programme (‘Back to Sleep’ campaign) was launched and resulted in a dramatic reduction in SIDS mortality. Australia and then the United Kingdom followed suit and the number of deaths in the UK fell from 1500 to 600 per year by the mid-1990s. The ‘Back to Sleep’ campaign has been implemented in many countries across the world with similar results.

Measuring health and disease of populations or groups

Certain terms are used when measuring health and disease of groups or populations. It is usually unhelpful to state numbers of events alone. In order to interpret the number, they need to be related to a denominator. For example stating 10 children were diagnosed with pertussis does not give as much information as the statement that 10 out of 20 children who had not been immunized were affected or that 10 children had pertussis out of a total population of 100,000 children.

• The numerator is the number of people known to have a specific disease or problem.

• The denominator is the total number of people at risk in the population.

If 100 children attended a Christmas party and 10 of them develop vomiting, the numerator is 10 and the denominator is 100.

Three terms are usually used to relate the number of cases of a disease or outcome to the size of the source population in which they occurred:

Infant mortality rate

Infant mortality rate is the number of children who die aged less than one year old per 1000 births. Some key facts are shown in Box 2.2 .

In 1911, 130 out of every 1000 children born in England and Wales would die before their first birthday. The decrease in infant deaths between 1911 and 2010 is because of considerable improvements in healthcare, including the control of infectious diseases and public health infrastructure over this time period. For the UK as a whole, infant mortality has been declining and is now only about a quarter of what it was in 1970.

Infant mortality rate is linked to several factors including access to healthcare services for mothers and infants, socio-economic status of the child's parents, the health of the mother, birth weight and the proportion of infants born preterm.

Terms used in child mortality statistics are:

• Stillbirths and perinatal mortality rates – reported per 1000 total births (live and stillbirths)

• Early neonatal, neonatal, postneonatal and infant mortality rates – reported per 1000 live births

• Childhood (1–15 years) mortality rates – reported per 100,000 population of the same age

The precise definitions of these terms relating to newborn infants are included in Chapter 10, Perinatal medicine . Analysis of the data around deaths identifies that the majority of deaths in childhood occur before one year of age; 70% of infant deaths in England and Wales in 2011 were neonatal deaths – deaths at less than 28 days. The most common cause of death, in children as a whole group, is now related to perinatal problems and congenital abnormalities.

Identification of cause of death

In England and Wales, stillbirths and neonatal deaths are registered using a special death certificate that enables reporting of relevant diseases or conditions in both the infant and the mother. The Office for National Statistics (ONS) has developed a hierarchical classification system (also referred to as the ONS cause groups), which allows the death to be assigned to a specific category, based on the likely timing of the damage leading to the death. This produces broad causation groups to enable direct comparison of neonatal and postneonatal deaths. The following are mutually exclusive categories:

Linkage of births and deaths

The linkage of birth and infant death records has been conducted since 1975 to obtain information on the social and biological factors relating to the baby and parents; it is collected at birth registration. Death registration gives only a limited amount of information about the parents of the deceased infant; for example, occupation of parent. However, a considerable amount of information is given at birth registration. This includes: age of each parent, number of previous children born (the mother's parity), country of birth of parents, place of birth and whether the baby was a singleton or multiple birth. Linking the infant death record to the birth record improves understanding of the key characteristics of the baby’s parents as further information is provided by the birth record. In 2010, 98% of infant deaths in England and Wales were successfully linked to their corresponding birth registration record. The potential use of such data is outlined in Box 2.3 .

Low birth weight

Birth weight is an important indicator of overall health and is influenced by a number of factors including smoking and drinking during pregnancy, low parental socio-economic status, education levels, low income and inadequate living conditions. In the UK nearly 8% of births are preterm.

In England and Wales, around 700,000 babies were born in 2013 of which approximately 50,000 were low birth weight (<2.5 kg). Of all those babies who were born with a low weight, 62% were preterm.

Under five year mortality

This is collected internationally and allows comparison between countries. In the UK in 2013, the under-five mortality rate was 4.9 per 1000 live births, the highest in western Europe; in Iceland it was 2.4, Sweden 2.7, Spain and Germany 3.6, France and Italy 3.7. It was worse in the UK than many eastern European countries, including Serbia, Estonia and Croatia. This means that 2000 more children die in the UK each year than if they lived in Sweden.

Deaths in later childhood

Data show that more children die in adolescence than in any period other than infancy. The World Health Organization (WHO) classifies deaths into communic­able and non-communicable disease (NCD). Deaths due to communicable disease are very low in the UK. However, for NCD deaths, for almost all ages the UK does worse than its comparators. Up to 74% of childhood deaths in the UK occur in children with co-morbidities, i.e. a long-term condition, of which the most common is a neurological or sensory condition.

Measures of disease frequency

The measures of disease frequency used most often in epidemiology are:

• Incidence : the number of new cases that occur during a specified period of time in a defined population

• Prevalence : the proportion of the population at risk that have the condition, where:

  $\text{Prevalence}=\frac{\text{number of cases}}{\text{population at risk}}$

• Population at risk : that part of the population which is susceptible to a disease.

For example, to calculate the prevalence of all childhood cancers in the age group 0–4 years in the UK:

• The numerator would be the number of children aged 0–4 years living in the UK at that time who were diagnosed with cancer

• The denominator would be the number of children aged 0–4 years living in the UK at that time (population at risk). The mid-year population is usually used.

Data on incidence and prevalence becomes more useful if converted into rates. The rates are usually expressed as per 1000, per 10,000, per 100,000, etc.

Example: As childhood cancers are rare (between 2008–2010 there was an average of 1603 new cases of childhood cancer each year in the UK), 883 (55%) in boys and 720 (45%) in girls, the rates are expressed per million – the crude incidence rate shows that there are 160 new cancer cases for every million boys in the UK, and 137 for every million girls.

Interrelationship between incidence and prevalence ( Fig. 2.1 )

Several factors can influence prevalence:

• The number of new cases (incidence) : if the number of new cases per year is high (high incidence) this will result in the prevalence being higher, e.g. up to 400 children are diagnosed with acute lymphoblastic leukaemia (ALL) every year compared to fewer than 15 children per year with chronic myeloid leukaemia (CML).

• The severity of the illness : if many children who develop the disease die, the prevalence rate is low, e.g. the survival rate for children with ALL is around 90% compared to 60% in children diagnosed with CML.

• The duration of the illness : if a disease lasts for a short time, its prevalence rate is lower than if it lasts for a long time, e.g. the duration of chickenpox compared with type I (insulin dependent) diabetes.

Risk

There are three common indices of risk: absolute risk, relative risk and attributable risk. The number needed to treat (NNT) is the inverse of attributable risk. The terms are defined in Box 2.5 and an example is shown in Question 2.2.

Chance – is the association a chance occurrence?

Chance is mainly determined by sample size – the larger the sample, the smaller the risk that the finding is due to chance alone. This is usually expressed as a ‘p-value’, usually with ‘confidence intervals’. Strictly speaking, the p-value is the likelihood of incorrectly rejecting the null hypothesis. Confidence Intervals (CIs) provide a range within which the true answer will lie at a population level (for more detail, see Chapter 38, Statistics ).

Bias – is the association due to a flaw in the methodology?

Bias is a systematic error in the methodology of the study that affects the results. There are several types of bias, such as selection bias, etc., which affect epi­demiological studies: