Comm Eye Health Vol. 10 No. 24 1997 pp 60-62. Published online 01 December 1997.

Disease incidence

Jennifer Evans BA MSc

Epidemiologist, 'Glaxo' Department of Ophthalmic Epidemiology, Moorfields Eye Hospital and, Institute of Ophthalmology, City Road, London, EC1V 2PD, United Kingdom

Related content

In everyday language, ‘incidence’ is commonly taken to refer to the occurrence of an event or disease. Most people use the word interchangeably with ‘prevalence’. In epidemiology, however, incidence has a very specific meaning that is different to prevalence, which is the number of cases in a defined population at a particular point in time (see previous article in this series).

Definition: The incidence of a disease is the number of new cases arising in a given period of time in a specified group of people (population).

Although prevalence and incidence are related, they measure different aspects of disease burden in a population.

The incidence of a disease will depend on its aetiology, i.e., why it occurs. For example, there may be an infectious agent which requires certain conditions for transmission, or it may be that the disease occurs due to some genetic factor, with or without certain predisposing environmental conditions. The prevalence of a disease depends not only on the incidence (how often new cases occur in a particular group of people), but also on the course of the disease, how long it lasts, whether it can be treated, and whether people die as a result of it.

Once an individual is affected by an eye disease, with visual impairment, they often have the disease for the rest of their life and it does not often directly cause their death (although it might increase their risk of death). Table 1 shows the prevalence and incidence of age-related macular degeneration which is the most frequent cause of blindness in western industrialised economies.

Table 1. Prevalence and incidence of age-related maculopathy in Beaver Dam, Wisconsin, USA

* No.at risk % with disease: ‘prevalence’ ** No.at risk at baseline % developing disease over 5 years: ‘incidence’
Early age-related maculopathy (ARM) 4771 15.6 2834 8.2
Late ARM – age-related macular degeneration 4771 1.6 3502 0.9

From Beaver Dam Eve Study 1, 2

* Number examined

** Number without disease at start of five-year period

1 Klein R, Klein BEK, Linton KL. Prevalence of age-related maculopathy. The Beaver Dam Eye Study. Ophthalmology 1992; 99: 933-43.

2 Klein R, Klein BEK, Jensen SC, Meuer SM. The five-year incidence and progression of age-related maculopathy: The Beaver Dam Eye Study. Ophthalmology 1997; 104: 7-21.


The ‘numbers at risk’ for incidence vary for the two stages of the disease because it is defined as people without disease at the start of the study period. As early stages of the disease are more common, there are less people at risk at the beginning than for late stages of the disease.

In general, we have much more information about the prevalence of common eye diseases than we have about incidence This is because it is easier to go out and count the number of people affected now (cross-sectional or prevalence survey), than it is to collect together a group of people (cohort) without the disease and follow them up over time (cohort or longitudinal study) As the Beaver Dam example shows, there arc normally many more prevalent than incident cases of disease (depending on the time period studied). This means prevalence surveys can, in general, be smaller in size than cohort studies (see previous article on sample size) making them easier and more practical to do

There are two measures of incidence the incidence risk and the incidence rate (see box Measures of Incidence) The incidence risk is also known as the cumulative incidence It provides an estimate of the probability that an individual will develop a disease during a specified period of time. Using Table 1, we can see that an individual (aged 43-86) living in Beaver Dam, has an 8% risk of developing the early signs of the disease over a five-year period but a much lower risk (0.9%) of getting late stage disease.

The incidence rate is also known as the force of morbidity or incidence density. It is considered to be a measure of the instantaneous rate of development of disease in a population. The incidence rate is likely to be a more accurate measure of disease incidence than the incidence risk because it takes into account the fact that, in most studies, not everyone is followed-up for all of the time. The two measures are related, however, as incidence rate can be used to estimate an individual’s risk of developing disease over a given period of time.


Measures of incidence

Risk: number of new cases in cohort during defined time period / total number ‘at risk’ at beginning of time period

Rate: number of new cases in cohort during defined time period / total ‘person time at risk’ during time period


Measures of effect

Ratios

Relative risk or risk ratio = Incidence risk in people exposed / Incidence risk in people not exposed
Relative rate or risk ratio = Incidence rate in people exposed / Incidence rate in people not exposed

Differences

Risk difference = (Incidence risk in exposed – incidence risk in not exposed)

Rate difference = (Incidence rate in exposed – incidence rate in not exposed)


Health service planning

Data on incidence are useful in health service planning. It is important to know the number of new cases of disease which will arise in a population in the future to be able to plan services required and how to deal with them. For example, in India, approximately 2.6 million cataract operations are done each year. In spite of this huge effort, the number of people blind with cataract in the country is increasing. Table 2 shows the results of projections from a population-based study of incidence of blinding cataract in Central India.

This study is very important because it shows that in order for eye health care services to have a realistic chance of dealing with cataract blindness in India, there should be a minimum of at least 4 million cataract operations done each year. This has implications, not only for allocation of health care resources, but also for the realisation that more research needs to be done to find ways of preventing or slowing down the progression of cataract.

Who is at risk?

As well as providing information on disease burden in the community, epidemiologists are also interested in whether different groups have different risks of developing disease. This can be useful for health service planning and can also help in the process of finding out the causes of disease or aetiology. We define groups of people according to whether they have been exposed to a particular factor. We can then measure the effect of that factor (see box: Measures of Effect).

Table 2. Incidence of blindness from cataract in India

Age Population of India Population at risk Incidence rate per person year Projected number of new cases arising every year
0-34 579810000 576197477 0 0
35-39 50376000 50205811 0.0019 95486
40-44 41665000 41301114 0.00253 104486
45-49 35783000 35203518 0.00595 208610
50-54 31654000 29867081 0.01336 399157
55-59 27369000 25138933 0.02388 600218
60-64 22088000 18182851 0.03734 678482
65 plus 38407000 29660851 0.0581 1723399
Total 827152000 805756980   3811185

From: Minassian DC, Mehra V. 3.8 million blinded by cataract each year: projections from the first epidemiological study of incidence of cataract blindness in India. Br J Ophthalmol 1990; 74: 341-3.

Table 2 clearly shows there is an increasing risk of developing blinding cataract with increasing age. The incidence rate in people aged 65 and over is 0.0581 per person year compared to 0.0019 in the age-group 35-39. We can calculate the rate ratio as 0.0581/0.0019 = 30.6. That is, in India, people aged 65 years and above are over 30 times as likely to develop blinding cataract compared to people aged 35-39.

Calculating rate difference

We can also calculate the rate difference. For example, the incidence rate at ages 60-64 is 0.03734 per person year (Table 2) which means that we could predict that if we followed up 100 people in that age-group in India for one year, approximately 4 will develop blinding cataract during that year. At ages 65 plus, we could predict that approximately 6 out of 100 people per year would develop blinding cataract (incidence rate 0.0581 per person year). That means that there are an extra 2 cases of blinding cataract attributable to the effect of increasing age: the rate difference is 0.0581-0.03734 = 0.02076 per person year (i.e., 2 people per 100 person years).

It is always important to consider both the risk (or rate) ratio and risk (or rate) difference when investigating the effect of a particular factor. If a disease is rare, even if an exposure increases the risk by many times, an individual exposed to that factor may still have a low absolute risk of developing disease.

Summary

  • ‘Incidence’ is a specific measure of disease burden in the population and must be distinguished from the prevalence of disease, although the two measures are related.
  • Comparing incidence in different groups of people, either by division (ratios) or subtraction (differences), gives us an idea of the effect of (possibly causal) factors on the development of disease.
  • Knowing the incidence of a disease is extremely useful for health service planning.
  • Measures of incidence are not available for many diseases because the studies required to collect the information (cohort or longitudinal studies) are expensive and difficult to conduct.

References

1 Klein R, Klein BEK, Linton KL. Prevalence of age-related maculopathy. The Beaver Dam Eye Study. Ophthalmology 1992; 99: 933-43.

2 Klein R, Klein BEK, Jensen SC, Meuer SM. The five-year incidence and progression of age-related maculopathy: The Beaver Dam Eye Study. Ophthalmology 1997; 104: 7-21.

3 Minassian DC, Mehra V. 3.8 million blinded by cataract each year: projections from the first epidemiological study of incidence of cataract blindness in India. Br J Ophthalmol 1990; 74: 341-3.