2.3 - Disease Occurrence

While there are many measures of disease frequency, epidemiologists most often estimate the occurrence of disease in a population in terms of incidence or prevalence of the disease. The key difference between these two measures is the time of disease onset—are existing cases counted or only new cases?

  • Incidence: counts new cases of the disease (or outcome)
  • Prevalence: counts new and existing cases of the disease (or outcome)

Incidence (new cases)

Incidence quantifies development of disease. Incidence can be estimated using data from a disease registry data or a cohort trial. There is an implicit assumption of a period of time, such as new cases within a month (or a year).

A summary incidence rate can estimate risk (e.g., probability of disease in an individual) if risk is constant across the summarized groups.

As defined, incidence is a count of new cases. However it is often expressed as a proportion of those at risk. The denominator includes all persons at-risk for the disease or condition, i.e. disease-free or condition-free individuals in the population at the start of the time period. Persons in the denominator, those at-risk, should be able to appear in the numerator. Obviously, the denominator would not include persons who already have the disease or condition. Incidence can also be expressed in terms of person-time at risk.

Rates are usually expressed per 100, 1,000, or 100,000 persons. In a strict application, "rate" should only be used when the denominator is an estimate of the total person-time at risk. (You will find the term "rate" used inconsistently in epidemiologic reports.. It is better to seek the source of the numbers than to rely on the nomenclature.)

Two Common Measures of Incidence Section

Cumulative Incidence Rate
The cumulative incidence rate (also known as incidence rate, especially for public health surveillance) consists of the number of persons who newly experience the disease or studied outcome during a specified period of time divided by the (average) total population at risk. This calculation assumes all persons in denominator contribute an equal amount of time to the measure. It is usually presented with an upper and lower bound.
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What is the lowest that a cumulative incidence rate could be? What is the highest?

  • What is the lowest that a cumulative incidence rate could be?

    • 0

  • What is the highest?

    • 1 or 100%, depending on how you want to measure it. With incidence of 100%, every person at risk has become a case.

Incidence Density Rate
Incidence density rate (also known as incidence rate; person-time rate, force of morbidity/mortality, hazard rate,disease intensity) is the number of persons who newly experience the outcome during a specified period of time divided by the sum of the time that each member of the population is at-risk This is also presented with upper and lower bounds.
Think about it!
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What is the lowest that an incidence density rate could be? What is the highest?

  • What is the lowest that an incidence density rate could be?

    • 0

  • What is the highest?

    • ... it depends on how the denominator is defined. Person-years is used most frequently. Persons/person-years must be less than or equal to 1.

Issues Related to Prevalence (new and old cases) Section

Since prevalence counts both new and existing cases, the duration of the disease affects the prevalence. Diseases with a long duration will be more prevalent than those with shorter duration. Chronic, non-fatal conditions are more prevalent than conditions with high mortality. The prevalence of disease is directly related to the duration of the disease:

Prevalence = Incidence × Average Duration

Prevalence is not a an apt descriptor of an acute condition.

Similar to incidence, persons included in the denominator must have the potential for being in the numerator, i.e. at-risk for the disease or condition. Prevalence is often expressed after multiplication by 100 (%), 1000 or 100,000.

The term rate may be inappropriately applied to prevalence. Prevalence is a proportion, usually reflecting the proportion with a disease at a particular time. (point prevalence)

The prevalence pool is the subset of the population with the condition of interest. The prevalence pool is not generally useful for hypothesis-driven epidemiologic research because these are not new cases, but can be useful in tracking the natural history of the disease, evaluating effects of treatments or disease burden.

For most etiologic research, incidence is the more appropriate measure. Studying the incidence of a rare condition however, poses a challenge. Given a small number of new cases, it can be preferable to estimate prevalence instead of incidence in these situations. For example, birth defect rates, reported as the numbers of cases/live births is a prevalent measure.Similarly, an autopsy rate is a prevalent measure.

Two common measures are point prevalence and period prevalence. The difference is whether the estimate is made over a period of time or at one specific time as illustrated below:

Point prevalence: prevalence of condition of interest at a specific time.

Number of existing cases on a specific date/ Number in the defined population on this date

Point prevalence ranges from 0 to 100. (%)

Point prevalence can be estimated from a cross-sectional survey or disease registry data by calculating the percentage with a particular disease or condition on a particular date. E.g. what percentage had a particular type of flu on 1/17/2009?

Period prevalence: prevalence of outcome of interest during a specified period of time. Less frequently used.

Number of cases that occurred in a specified period of time/ Number in the defined population during this period

Period prevalence generally ranges from 0 to 100 %. (Theoretically, period prevalence can exceed 100% if you allow individuals who had the disease more than once to be counted for each case of the disease within the reporting period.)

E.g. what percentage of the population had an episode of flu between October and May in the most recent flu season?

Example 2-1: Cancer Rates Section

news

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What is the lowest that an incidence density rate could be? What is the highest?

  • The title of the article implies a rate; a number with a numerator and a denominator within a time period. Yet, what does the first sentence of the article cite? ... "The number" ... Is the number the same as the rate? What are the data?

  • Also look at the first sentence of the second paragraph. "... death rates from cancer continue to fall...", but the headline states that cancer rates will double! What is the true story here?

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If the number or proportion diagnosed is truly increasing and the death rate decreasing, what will happen to the prevalence?

Increase—cancer is a chronic condition, so an increase in the number diagnosed with a decrease in the death rate will lead to increased prevalence.

Other Measures of Disease Frequency Section

Many summary measures of health status and risk can be derived from basic counts of deaths, persons with disease or risk factors and population estimates. The method of presenting the data, including the choice of presenting absolute frequency data or data relative to a population can dramatically alter perceptions of risk.

These last two have been used by economists:

  • Mortality Rate = number of deaths in a year divided by the midyear population (cumulative incidence).
  • Cancer Incidence Rate = new cancer cases in a year divided by the midyear population (cumulative incidence).
  • Case-Fatality Rate = deaths due to the specific cause divided by the number of cases from that specific cause (this isn't a rate at all - it is a percentage).
  • Attack Rate = cases due to a specific cause in a short period of time divided by the population at risk, often associated with an epidemic situation (cumulative incidence).
  • Autopsy Rate = cases per number of autopsies (point prevalence)
  • Birth Defect Rate = cases of birth defect divided by number of live births (or live births plus still births) (prevalence).
  • Proportionate Mortality = death from specific cause per number of deaths from other causes (proportion).
  • Years of Potential Life Lost = the sum of the number of years over the population that deaths occur before a specific age (i.e., death at 58 years of age would contribute 7 years to YPLL 65) for a specific population.
  • Years of Productive Life Lost = the sum of the number of productive years over the population that are not available due to premature death.

Example 2-2: Incidence / Prevalence Section

Consider a population of 20 individuals. Each horizontal line in the figure below represents a person in that population who became a case of a disease for which there is no immunity, i.e., an individual can get this disease multiple times. The length of the line indicates the duration of the disease for each case. Each person became a case on the first day of the month and ended after the last day of the month. The case number is in the left-hand column. For instance, case 5 became a case of the disease for one month from March 1, 1999 until April 1, 1999.

Excercise graph

Calculate:

  1. What was the prevalence on May 1, 1999?
  2. What was the prevalence for all of 1999?
  3. What was the cumulative incidence during 1999?
  4. What was the incidence density for the entire period July 1, 1998 through June 30, 2000?

Try this exercise on your own and then play the video below to see if our results are the same!

Discuss!

Now, let's expand our thinking and think about these questions:

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Come up with an answer to this question by yourself and then click on the button below to reveal the solution.
  • How would the chart be different if this condition could only occur once for each person?
  • What are examples of diseases that a person has only once in their lifetime?
  • How would these measures of frequency be different?
  • Which measures would be different?
  • Would they increase or decrease?

Answers:

If the condition can only occur once, once the subject has had the disease, they are no longer available for the numerator or denominator. Examples of conditions for which there is no second case for an individual: smallpox, diphtheria, polio, death.

It is difficult to predict what would happen in general. In this specific example, persons 1,3, 4, 5 and 6 would have contributed fewer months to the denominator for incidence density which becomes 4 persons/374 person- months, a small decrease in incidence density. Cumulative incidence would decrease with one less person in the numerator and two less in the denominator. (1/16 vs 2/18). The point prevalence will not change, but the period prevalence may change, depending on the definitions used. Be careful when using these measures to understand the counting rules!