Prevalence is based on both incidence and duration of illness. **High prevalence of a disease within a population might reflect high incidence or prolonged survival without cure or both**. Conversely, low prevalence might indicate low incidence, a rapidly fatal process, or rapid recovery.

Period prevalence is the number of persons who have had the disease or attribute at any time over a specified period. Period prevalence is often expressed as below but it is not strictly a ratio it is a rate. Prevalence ratio is equal to the incidence rate multiplied by the duration of the event.

Prevalence and incidence measure different phenomena, but they are related. Prevalence is the proportion of a population that has a condition at a specific time, but the prevalence will be influenced by both the rate at which new cases are occurring (incidence) and the average duration of the disease.

If the average duration of disease remains constant, then preventive measures that reduce the incidence of disease would be expected to result in a decreased prevalence.

Incidence describes the current risk of getting a certain disease, while prevalence tells us how many people currently live with the condition, regardless of when (or even whether) they've been diagnosed with that particular disease.

For example, in a study of incident cases of diabetes with a one year time period, only those who developed diabetes over the course of the one year study period are considered incident cases. (denominator.) The total population at risk denominator includes the prevalent cases.

1 : the quality or state of being prevalent. 2 : the degree to which something is prevalent especially : the percentage of a population that is affected with a particular disease at a given time.

Therefore, incidence is the number of newly diagnosed cases of a disease. An incidence rate is the number of new cases of a disease divided by the number of persons at risk for the disease.

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

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.

The prevalence reflects the number of existing cases of a disease. In contrast to the prevalence, the incidence reflects the number of new cases of disease and can be reported as a risk or as an incidence rate. Prevalence and incidence are used for different purposes and to answer different research questions.

IRR Equal to 1: This indicates that the incident rate is equal among those in an exposed group and those in an unexposed group. For example, if smokers developed lung cancer at a rate of 7 per 100 person-years and non-smokers developed lung cancer at a rate of 7 per 100 person-years, then the IRR would be 7/7 = 1.

Incidences and prevalence are often reported with a population multiplier such “per m people” or “per m person-years.” To convert a rate or proportion to “per m people,” simply multiplying by m. For example, an incidence rate of 0.00877 per person-year = 0.008770 × 100,000 = 877 per 100,000 person-years.

A prevalence rate is the proportion of the population that has a health condition at a point in time.

However, prevalence is not a useful measure for establishing the determinants of disease in a population. Prevalence measures the frequency of existing cases of disease in a population.

The incidence rate is a measure of the frequency with which the event, in this case COVID-19, occurs over a specific period. Numerically, it is defined it as the number of new cases for the disease within a time frame, as a proportion of the number of people at risk for the disease.

Incidence contrasts with prevalence, which includes both new and existing cases. For example, a person who is newly diagnosed with diabetes is an incident case, whereas a person who has had diabetes for 10 years is a prevalent case.

Incidence. The incidence of a disease is the rate at which new cases occur in a population during a specified period. For example, the incidence of thyrotoxicosis during 1982 was 10/100 000/year in Barrow-in-Furness compared with 49/100 000/year in Chester.

Incidence rate measures are useful when describing how quickly disease occurs in a population. This information can be used to monitor the effectiveness of vaccination schemes or changes in the preventative measures being used to see if they cause a knock-on reduction in disease incidence rate.

Using Ratios to Compare 2 Populations

- Risk ratio (relative risk; e.g., incidence density ratio [IDR]) = Risk(Group A [Exposed])/Risk(Group B [Unexposed]) ...
- Odds ratio (OR) ...
- Rate and prevalence ratios = incidence rate or prevalence rate in the first population/ the respective rate in the second population.

the fact that something is very common or happens often: the prevalence of smoking among teenagers.

In current use, incidence usually means "rate of occurrence" and is often qualified in some way ("a high incidence of diabetes"). Incident usually refers to a particular event, often something unusual or unpleasant ("many such incidents go unreported").

Prevalence may be reported as a percentage (5%, or 5 people out of 100), or as the number of cases per 10,000 or 100,000 people. The way prevalence is reported depends on how common the characteristic is in the population.

Divide the population size by one thousand. In the example, 250,000 divided by 1,000 equals 250, which is called the quotient, the result of division. Divide the number of occurrences by the previous quotient. In the example, 10,000 divided by 250 equals 40.

Incidence risk is the total number of new cases divided by the population at risk at the beginning of the observation period. For example, if one hundred sow farms were followed for a year, and during this time 10 sow farms broke with a disease, then the incidence risk for that disease was 0.1 or 10%.