Morbidity Explained!

Ammar Suhail
Aug 31, 2021
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Thanks for reading the third edition of the newsletter. In the current edition, we will continue describing some common terminologies used in reporting diseases.  Last time we discussed Mortality and Morbidity. This time I will expand on the term morbidity and see how it is reported or what are the concepts used to describe morbidity.

Morbidity refers to having a disease or a symptom of a disease or the amount of disease within a population. In literature, morbidity is often described in two ways: incidence and prevalence. Let’s dive into these topics.

Incidence

Incidence measures the rate of occurrence of new cases of a disease or condition. Incidence is expressed as either incidence proportion or incidence rate.

Incidence Rate

The incident rate is the number of new cases of a disease within an at-risk population. This can be calculated by identifying the new number of cases during a specified time interval (numerator) divided by the number of people at risk for the disease (denominator). This measure is commonly used in epidemiology as a way to denote the occurrence of disease, illness, or accident. This helps determine how quickly a disease is spreading.

For example, you are studying a small town with a population of 800,000 at risk of developing type II diabetes. After 1 year, you find that 500 of the total population have tested positive for the disease. Therefore, to calculate the incident rate, you’ll divide the 500 cases by the population of 800,000. You can then say that the incident rate of diabetes in this population is 0.000625, or 62.5 cases per 100,000 people per year

Incidence proportion

An incidence proportion can be used to estimate the risk of developing a specific condition during a given time period. It’s calculated by dividing the number of new cases during a specific period by the population at the start of the period.

Incidence proportion is the proportion of an initially disease-free population that develops the disease, becomes injured, or dies during a specified (usually limited) period of time.

For example, in an outbreak of gastroenteritis among attendees of a picnic, 100 people ate Potato sandwiches, 30 of whom developed gastroenteritis. The Incidence proportion can be calculated as

Numerator = 30 persons who ate potato sandwich and developed gastroenteritis

Denominator = 100 persons who ate potato sandwich

Incidence proportion= (30/100) *100= 30%. Then the risk of gastroenteritis would be 30%.

Prevalence

It is the number of current cases (new and preexisting) at a specified point in time. Unlike incidence, it includes both new and existing cases. It can either be calculated at a specific point in time or over a specified period of time. The following are two types of prevalence:

‘Point’ prevalence refers to the prevalence measured at a particular point in time. It is the proportion of persons with a particular disease or attribute on a particular date.

‘Period’ prevalence refers to prevalence measured over an interval of time. It is the proportion of persons with a particular disease or attributes at any time during the interval.

Bottomline

Prevalence and incidence are frequently confusing. Prevalence refers to the proportion of persons who have a condition at or during a particular time period. In contrast, incidence refers to the proportion or rate of persons who develop a disease during a specific period of time—the image attached below highlights the overall concept.

Prevalence in this image is the total water in the tank (new and old cases), Incidence is the freshwater being added in, and death & recovery alter the disease's prevalence.

I hope you enjoyed reading it. Do subscribe and share it with others.

Till Next time,

Ammar Suhail

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