1. Incidence and Prevalence
Manish Chaudhary
MPH (BPKIHS)

2. Incidence
a “motion picture” – describes what is happening in a population. Incidence is defined as the number of new cases divided by the population at risk over time.
Incidence therefore includes three components:
1. New cases
2. Population at risk.
3. Interval of time.
Note that:
• Incidence involves the passage of time.

3. Prevalence
a kind of “still life” picture – is the most basic of epidemiologic measures.
It is defined as the number of cases divided by the population-at-risk.
Prevalence has three components:
1. Existing cases
2. Population “at risk” to have the condition
3. Point (or sometimes a period) in time to which the prevalence applies

5. Relationship of incidence and prevalence

6. Relationship between Prevalence and Incidence
In a stationary population, in which there is no migration of cases or non cases, if the incidence, prevalence, and duration of a condition remain constant then the number of new cases that occur must be balanced by the number of exiting cases that leave the population through death or cure.
In such a situation, the prevalence is a function of incidence and the average duration of being a case.
For a rare disease, prevalence ≈ incidence × duration
P= I X D

7. Cumulative incidence (CI), a.k.a. Incidence proportion (IP)
The definition of CI is based on the following “ideal” scenario: 1. A population known to be free of the outcome is identified at a point in time (a cohort); 2. All members of the cohort are at risk of experiencing the event or outcome (at least once) for the entire period of time; 3. All first events or outcomes for each person are detected.

8. Calculating cumulative incidence

9. Calculating cumulative incidence

10. Some things to note about CI: 1. The period of time must be stated (e
Some things to note about CI: 1. The period of time must be stated (e.g., “5-year CI”) or be clear from the context (e.g., acute illness following exposure to contaminated food source); 2. Since CI is a proportion, logically each person can be counted as a case only once, even if she or he experiences more than one event; 3. As a proportion, CI can range only between 0 and 1 (inclusive), which is one reason it can be used to directly estimate risk (the probability of an event).

11. Sample calculation:
200 people free of chronic disease X observed over 3 years, 10 cases of X develop
3-year CI = 10 cases / 200 people = 10/200 = .05
Thus, the 3-year risk for one of the 200 people to develop disease X, conditional on not dying from another cause, is estimated as 0.05 or 5%.

12. Commonly used CI indicator…..
Attack Rate •In the outbreak setting, the term attack rate is often used as a synonym for risk. •Risk of getting the disease during a specified period, such as the duration of an outbreak. •AR does not explicitly specify the time interval because for many food-borne disease outbreaks we know that most cases occur within a few hours or a few days after the exposure

13. Overall attack rate –Total number of new cases divided by the total population Food-specific attack rate –number of persons who ate a specified food and became ill divided by the total number of persons who ate that food

14. Secondary attack rate

15. Uses of SAR
attempts to measure the degree of spread of a disease within a group that has been exposed to an agent by contact with a case • Used in evaluating the efficacy of a prophylactic agent • Also used to determine whether a disease of unknown etiology is communicable and thus may indicate the possible etiological role of a transmissible agent

16. Incidence density (ID)
Note that:
• ID is a relative rate, not a proportion.
The units of time must be stated, since otherwise the numeric value is ambiguous (e.g., 15 cases/100,000 person-years = 15 cases/1,200,000 person-months).*

19. Two complementary measures of incidence: CI and ID
Cumulative incidence (CI) 1. increases with period of observation (i.e., it is “cumulative”) 2. has problems with: - multiple events in one subject - differing follow-up times for subjects But 3. it is not necessary to know exact time of onset of the disease 4. directly estimates risk

20. Incidence density (ID) 1
Incidence density (ID) 1. suggests ability to extrapolate over time - “duration free”; 2. accommodates: - multiple events in one subject - different follow-up times for subjects 3. does not require a cohort to estimate or interpret 4. may be more appropriate for etiologic inference

21. Prevalence
In the medical and public health literature, the word prevalence is often used in two ways:
–Point prevalence
–Period prevalence

22. Point prevalence
•Prevalence measured at a particular point in time. •It is the proportion of persons with a particular disease or attribute on a particular date

23. Period prevalence
•Prevalence measured over an interval of time. •It is the proportion of persons with a particular disease or attribute at any time during the interval.

27. the incidence of the disease during the seven-year period is the number of new events (3) divided by the sum of the lengths of time at risk of getting the disease for the population (33 person-years), i.e. 9.1 cases per 100 person years;
the cumulative incidence is the number of new events in the population at risk (3) divided by the number of people in the same population free of the disease at the beginning of the period (7), i.e. 43 cases per 100 persons during seven years
the average duration of disease is the total number of years of disease divided by the number of cases, i.e. 10/3 = 3.3 years;
the prevalence depends on the point in time at which the study takes place; at the start of year 4, for example, it is the ratio of the number of people with the disease (2) to the number of people in the population observed at that time (6), i.e. 33 cases per 100 persons. The formula P= I X D for prevalence would give an estimated average prevalence of 30 cases per 100 population (9.1 × 3.3);
case fatality is 33% representing 1 death out of 3 diagnosed cases.

28. Thank You