Module 2 Section 2

Question 1

Video - Incidence and Prevalence

Answer 1

• Incidence refers to the number of new cases of a disease in a specific time period, divided by the population at risk. It measures disease risk and helps assess the likelihood of a person developing the disease
• Prevalence is the number of existing cases of a disease in the population at a specific point in time. It measures the disease burden, taking into account both new and existing cases.
• Incidence is influenced by the number of new cases and is affected by risk factors
• Prevalence is influenced by both incidence and disease duration. If the disease lasts longer, prevalence will be higher, and if people die or are cured, prevalence will decrease.
• An example is given comparing Hollywood and the Bronx. Although Hollywood has a higher prevalence of coronary heart disease (CHD), the incidence is the same in both places; the difference is that the disease lasts longer in Hollywood due to better care.
• Incidence is a measure of risk, while prevalence is a measure of disease burden

Question 2

What is Prevalence?

Answer 2

• The prevalence of a disease tells us about the number of existing cases of that disease in a given population. This measure can be representative of a specific point in time, referred to as point prevalence, or indicative of a period of time, referred to as period prevalence

Prevalence = # of cases / total population

• A case refers to an individual who, at the specified time, has the disease

Question 3

Point Prevalence

Answer 3

• Point prevalence is a measure of the proportion of the population that has a certain disease at a specific point in time. To calculate point prevalence, the number of cases of a disease and the population count for a specific time are needed.
• In global health research, the use of point prevalence is rare. In practice, for example, it may require six months to survey a region of Somalia and determine the number of existing cases of tuberculosis, in which case period prevalence must be used

Point Prevalence = # of cases at a specific time / total population at that time

Question 4

Period Prevalence

Answer 4

• Period prevalence is a measure of the proportion of the population that has a certain disease over a defined period of time
• Period Prevalence = # of cases at a period of time / average population during that period of time

Question 5

At the beginning of September 2020, the population of Canada was approximately 37 971 020 and the number of active cases of COVID-19 was approximately 6393. What was the prevalence of COVID-19 in Canada at that time?

Answer 5

The prevalence of COVID-19 in Canada at the beginning of September 2020 was 0.0168% or 16.83 per 100 000

Question 6

The first known case of COVID-19 in Canada appeared on January 25th, 2020. From then to September 1st, 2020, the average population of Canada was 37 742 154. During that time period, the total number of COVID-19 cases in Canada rose to 128 948. What was the period prevalence of COVID-19 in Canada during this time period?

Answer 6

The period prevalence of COVID-19 between January and September 2020 was 0.341655% or 341.65 per 100 000

Question 7

What is Incidence?

Answer 7

• Incidence measures how quickly new cases of a disease arise in a population over a defined period of time. This is different from prevalence because the measure only considers new cases within the “at-risk” population, within the time period
• Incidence is also a measure of risk. Let’s say that there are two populations, where Population A has a higher incidence of a disease than Population B. Then you can say that individuals in Population A have a higher risk of developing the disease than those in Population B. There are two measures of incidence utilized in the field of global health, cumulative incidence and incidence rate.

Question 8

Cumulative Incidence (or Incidence Proportion)

Answer 8

• While prevalence is a measure of the proportion of the population who have the disease, cumulative incidence is a measure of the proportion of the population who develop the disease over a period of time. Cumulative incidence is a measure of risk

Cumulative Incidence = # of new cases of a disease over a time period / total population at risk

• It is not uncommon to see the numerator expressed based on the average population over the time period. This simplification may be made when it may be appropriate to assume most of the population is susceptible (e.g. chronic illnesses)
• Total population at risk excludes individuals who already have the disease or who are incapable of developing the disease. It is calculated by the equation:

(total population) - (# of existing cases)

Question 9

Incidence Density Rate (or Person-Time IR)

Answer 9

• Rather than measuring the number of people who develop a disease (which is essentially cumulative incidence), the incidence rate (IR) focuses on the length of time people were at risk of disease. Incidence rate is also sometimes referred to as incidence density

Incidence Rate = # of people who develop the disease / # of person-years at risk of disease
# of person-years at risk of disease = (# of people at risk) × (# of years at risk)

Question 10

Reporting Incidence and Prevalence

Answer 10

Sometimes the at-risk population is constant. However, it is not safe to assume the at-risk population is constant in certain situations

Question 11

Reasons for Increased At-Risk Populations

Answer 11

• Births
• Immigration

Question 12

Reasons for Decreased At-Risk Populations

Answer 12

• High incidence
• High death rate from other causes
• Emigration

Question 13

Key Characteristics of Incidence and Prevalence

Answer 13

Question 14

Incidence VS Prevalence

Answer 14

• Prevalence refers to the total number of existing cases of a disease in a population at a specific point in time.
• Incidence refers to the number of new cases of a disease that occur in a specific time period, within a population at risk. It measures the rate at which new individuals are developing the disease