## Incidence

### Definition

**Incidence** refers to the number of new cases of a certain disease or event in a population over a specified period of time. There are 2 ways of measuring incidence: **cumulative incidence** (CI) and **incidence rate** (IR). Both are measures of the risk of contracting a disease or condition.

**Cumulative incidence**per 1,000 = (Number of*new*cases of disease occurring in a population over a*specified period of time*) / (Number of total individuals in the population who are*at risk*for developing the disease during the*specified**period of time*) x 1,000:- CI is also known as cumulative incidence proportion, incidence proportion, risk, attack rate, or probability of developing the disease.
- CI is not a true rate but a proportion, as the people included in the numerator are
**also**included in the denominator (total population) - The denominator should include only “at-risk
*“*individuals. Individuals who already have the disease or are incapable of contracting the disease must be excluded from the population.

- The number used to multiply depends on the frequency of the disease. For example, 1,000 may be sufficient to use for a common disease, but 100,000 or 1,000,000 would be required for a rare disease. Using 100 as the multiplier makes the CI proportion a percentage.
- CI does NOT take into account:
- People who became
**lost to follow-up** - People who die
**from another cause**(a “competing risk”) during the observation period **When**people developed the disease (i.e., at the beginning, middle, or late in the observation period)

- People who became
- The synonym
**attack rate**is commonly used during outbreaks of disease and in hypothetical predictions.**Food-specific attack rate per 100**= (Number of susceptible or at-risk individuals who were exposed to a pathogen or suspect food who became ill) / (Number of total individuals who were exposed to that pathogen or food)- Example: If 30 people developed gastroenteritis after eating potato salad, and 100 people in total ate the salad, then the AR = 30 / 100 x 100 = 30%, which is the risk of becoming ill after eating the salad.

- CI is used mostly used in situations where the follow-up time is relatively short and there is relatively little loss to follow-up. Otherwise, epidemiologists generally use the
**incidence rate.**

**Incidence rate**per 1,000 = (Number of*new*cases*specified period of time*) / (Total person-time, or the sum of the time periods of observation of each person who was observed) x 1,000- IR is a true rate and is also known as person-time rate or incidence density.
**Total person-time**: based on the eligibility criteria for the study population that an individual must be at risk (disease-free) and actively followed. Once a person develops the disease or is lost to follow-up, their contribution of time at risk ends. If their contribution ends after 1 year, then that person has contributed 1 person-year (py) to the study. For example:- 1 individual at risk who is in a 1-year study = 1 py
- 5 individuals at risk who are in a 5-year study = 25 py
- 5 individuals at risk who are in a 5-year study, with 2 being lost to follow-up or disease after 1 year while the remaining 3 continue to be at risk for the full 5 years = 2 py + (3 x 5) = 17 py

### Related videos

### Examples

**Cumulative incidence**

Over the past year at a particular hospital, 500,000 patients were admitted for hip or knee replacements. Five thousand of those patients over that year had a venous thromboembolism (VTE) during their admission. What is the risk of developing VTE over the past year at this hospital?

- There are 5,000 new cases of VTE over the year in a population at risk of 500,000.

- Therefore,
**1%**of those admitted, or**1 out of 100 patients**, experienced VTE during their hospitalization.

**Incidence rate**

Over the time period from 2000 to 2009, in Quebec, Canada, there were 91,000 cases of VTE over a population, with a disease-free time over that period that totals 74,200,000 py. What is the IR of VTE in Quebec over this time period?

Therefore, the IR is **0.0012 cases per py. **By convention, this is multiplied by 10,000 to get **12 cases per 10,000 py **of observation.

## Prevalence

### Definition

**Prevalence**is the number of affected individuals present in a population at a particular point in time divided by the number of total individuals in the population at that point in time (point prevalence) or over a period of time (period prevalence). Prevalence is typically expressed as a**proportion**or**percentage**.

**Point prevalence**is like a “snapshot”**Does not measure risk**, but provides an**important measure of the burden of disease**in a community, which is necessary for resource allocation.- Prevalence cannot provide a risk estimation, as it does not give information about when the disease developed. Some individuals may have developed the disease a week ago, some last year, and some 20 years ago.
- There is a relationship between prevalence and incidence (
**Prevalence = Incidence × Duration of disease**) as long as:- The population is in a steady state.
- The disease rate is constant.
- The prevalence is not too high.

### Example

**Prevalence versus incidence**

- In 2 hypothetical cities, with 1,000 inhabitants in each, there is a contagious disease with a prevalence of 100 / 1,000 in City A and a prevalence of 60 / 1,000 in City B.
- There is not enough information to determine which city would be “less risky” to live in without considering incidence.
- However, if City B has an incidence of 20 cases / year, while City A has only 4 cases / year, then City B is riskier.
- The prevalence in City B is lower because the average survival of a diseased person in City B is only 3 years, while a diseased person in City A lives an average of 25 years.
- For City A: 4 / year x 25 years = 100 / 1,000
- For City B: 20 / year x 3 years = 60 / 1,000

**Period prevalence**

Over the past year, there were 12 million patients in the Medicare database, and 200,000 Medicare patients suffered a VTE. What is the period prevalence of VTE in Medicare patients in the United States over the past year?

- The period prevalence for the year is a measure of the proportion of Medicare patients who had VTE. There were 12 million patients in the Medicare database; 200,000 of these patients had VTE.
- Therefore, the
**period prevalence**of VTE in Medicare patients for the year was**1.6%**.

**References**

- Greenberg, R. S. (2014). Epidemiologic measures.
*R. S. Greenberg (Ed.), Medical Epidemiology: Population health and effective health care, 5e ().*New York, NY: McGraw-Hill Education. accessmedicine.mhmedical.com/content.aspx?aid=1108589766 - Thacker, S. B. et al. (2006). Measuring the public’s health.
*Public health reports (Washington, D.C.: 1974), 121(1), 14–22.*DOI:10.1177/003335490612100107 - Celentano, David D., ScD., M.H.S., & Szklo, Moyses, MD, M.P.H., DrP.H. (2019). The occurrence of disease: I. disease surveillance and measures of morbidity.
*Celentano, David D., ScD, MHS, & Szklo, Moyses, MD, MPH,DrPH (Eds.), Gordis epidemiology(pp. 41-64)*https://www.clinicalkey.es/#!/content/3-s2.0-B9780323552295000036 - Celentano, David D., ScD., M.H.S., & Szklo, Moyses, MD, M.P.H., DrP.H. (2019). The occurrence of disease: II. mortality and other measures of disease impact.
*Celentano, David D., ScD, MHS, & Szklo, Moyses, MD, MPH,DrPH (Eds.), Gordis epidemiology (pp. 65-93)*from https://www.clinicalkey.es/#!/content/3-s2.0-B9780323552295000048 - CDC. Section 1: Frequency Measures, Lesson 3: Measures of Risk.
*Principles of Epidemiology in Public Health Practice, Third Edition. CDC.*Retrieved August 1, 2020, from https://www.cdc.gov/csels/dsepd/ss1978/lesson3/section1.html - Boston University School of Public Health. Measures of Disease Frequency. Retrieved on August 1, 2020, from https://sphweb.bumc.bu.edu/otlt/MPH-Modules/EP/EP713_DiseaseFrequency/index.html