Introduction

Incidence is the rate of new cases or events over a specified period for the population at risk for the event. In medicine, the incidence is commonly the newly identified cases of a disease or condition per population at risk over a specified timeframe.[1] An example of incidence would be 795,000 new strokes in the United States, annually. Here the incidence is 795,000 new strokes, the population in the United States, and the timeframe is one year.  Alternatively, incidence can be specified as person-years. For example, there may be 324 million people in the United States for the measured year, so strokes could be specified as having an incidence of 2.5 strokes per 1,000 person-years. This means there will be on average 2.5 strokes if we watch 1,000 people in the United States for one year. To calculate the person-years incidence of strokes in the United States we perform the following: (795,000 strokes)/(324,000,000 people in the United States during the year) = 2.5 strokes / 1,000 person-years.

Function

Incidence = (New Cases) / (Population x Timeframe)

An example will help demonstrate this equation and is provided below.

You watch a group of the 5,000 people in your town. During a five-year period, 25 individuals are newly diagnosed with diabetes mellitus. What is the annual incidence of diabetes mellitus for your town?

• (25 new cases diabetes mellitus)/(5,000 people x 5 years) =
• (25 new cases) / (25,000 people-year) =
• 0.001 cases/people-year =
• 1 case / 1000 people-year

The above can be interpreted as "If we watch 1,000 people in the town for one year we would expect one person, on average, to be newly diagnosed with diabetes mellitus during the year of observation."

Sometimes the period of observation may be given as fractions of a year.  If the incidence can be assumed to be stable over the short term, then we can use multiplication to calculate the person-year incidence. Below is a calculation using fractionals of a year for the time period and multiplication to calculate the annual incidence as person-years:

A health worker finds that over the past three months there have been four new diagnoses of lead poisoning in children in her community.  She estimates there are 60,000 children in her community at risk for lead poisoning. If we assume a stable incidence of lead poisoning in children what is the annual person-year incidence of lead poisoning for the children at risk?

• (4 new cases/(60,000 people x 3 months)

There are two ways to solve this problem.  One method is to solve for people-months then convert the final answer into people-years.  Alternatively, we can first convert to people-years before solving.  Either method should provide the same result.  We will perform both calculations to show we get the same answer.  We will first do the approach of calculating people-months then convert to people-years.

• (4 new cases)/(60,000 people x 3 months) =
• (4 new cases)/(180,000 people-months) =
• 0.00002 cases/people-months

Now to convert from cases/people-months to cases/people-years

• (0.000022 cases/people-months) x (12 months / year) =
• 0.00026 cases/people-year =
• 0.26 cases/1,000 people-year

Let us now try the other approach of first converting people-months to people-years and calculating the incidence to show we arrive at the same answer as above.  First, we start with what we know:

• (4 new cases)/(60,000 people x 3 months)

Now we convert from people-months to people-years

• [(4 new cases)/(60,000 people x 3 months)] x (12 months / year) =
• [(4 new cases)/(60,000 people)] x ((1 / 3 months) x (12 months / year)) =
• [(4 new cases)/(60,000 people)] x (4 / year) =
• (16 new cases/(60,000 people-year) =
• 0.00026 cases/people-year =
• 0.26 cases/1000 people-year

We can see then that with either method we arrive at the same answer of 0.26 cases/1,000 people-year.  The important thing to keep in mind is that the units for all parts have the correct units.

Issues of Concern

 Incidence is commonly confused with prevalence. Incidence is the rate of new cases or events during a specified period; whereas, prevalence is the total cases present at one specific time, both new and old cases. Incidence occurs when the new case is diagnosed, and each new case diagnosed increases the prevalence. Prevalence decreases when the disease is cured, or the patient dies. The cure for a disease or death of a patient does not affect the incidence of the disease. In the image below, the incidence is the new additions to the reservoir, the prevalence is the total number in the reservoir, and cure/death decreases the reservoir. The incidence is a measure of the risk of getting the disease during a specified period; whereas, prevalence is a measure of how much burden of the disease there is in the population at one specific moment in time.[2][1][3][4][5]

A second common error with incidence is not adequately defining the population at risk. Incidence specifies the number of new diagnoses for the at-risk population of a disease. Changing the specified population will also change the incidence. For example, the incidence of stroke is approximately 250/100,000 people-year for all individuals in the United States. The incidence decreases to 24/100,000 people-year if we only look at those ages 15 to 54. The incidence increases to 391/100,000 people-year if we only look at those older than 54 years.  Thus the incidence differs between different populations (age groups in this example).  Let us look at breast cancer as a second example where incidence is different between groups.  The incidence of invasive breast cancer in females in the United States is about 161/100,000 people-year versus the incidence of 1.6/100,000 people-year for males in the United States.  We can see a 100 fold difference in the incidence of invasive breast cancer between females and males in the United States.[6]

Additionally, we must be careful to ensure all of the individuals included in the population used to calculate the incidence are truly at risk. For example, each year approximately 61,380 individuals are diagnosed with uterine cancer.  If we used the total United States population of 324 million, we would calculate an incidence of 18.9/100,000 people-year. In reality, only females are at risk for uterine cancer, and thus only females should be used as the population at risk. Assuming there are approximately 157 million females in the United States, we would calculate a more accurate incidence of uterine cancer of 39/100,000 people-year in the United States. Thus the use of the wrong population at risk can have significant effects when calculating the incidence.[7][8]

Clinical Significance

Incidence is a measure of how commonly or frequently a disease occurs in a specified population over a period by providing a quick measurement of new disease diagnoses. Incidence is thus a measure of a risk of the disease for a specified population during a specified period.

Other Issues

Incidence is not a static number but changes over time. As an example, the incidence of chickenpox (varicella zoster virus) was historically similar to the birth rate of the population as almost all individuals contracted chicken pox during childhood. After the introduction of vaccination for varicella zoster virus, the incidence of chickenpox decreased by 90%. Incidence also can increase over time. Before the widespread use of cellular telephones and other handheld electronic devices, the traffic death incidence was decreasing secondary to improved vehicle safety. After cell phones and other electronic devices had become widespread, the incidence of traffic deaths increased as more individuals engaged in distracted driving.

Enhancing Healthcare Team Outcomes

Incidence is key to understand for the health care team.  Many aspects of medicine depend on allocating resources based on either the incidence or prevalence of a disease process.  The health care team as a whole thus must concurrently decide if the case is a new case (incident case).  Additionally incidence has significant implications for public health and epidemiology as many outbreaks are identified then tracked through incidence to identify and hopefully quell the spread of the disease.