Updated: 1/8/2021

Measures of Disease Frequency

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Introduction
  • Overview
    • incidence and prevalence are methods of measuring disease frequency in a population with respect to time
  • Incidence
    • describes the amount of new disease cases in at-risk people over a certain time period
    • at risk means those who are capable of developing the disease of interest 
      • do not already have the disease
      • are not vaccinated against the disease
      • have at-risk anatomy (patients without a prostate cannot get prostate cancer)
    • incidence can be specified 2 ways
      • # of new cases in a population/# of at-risk people in the population per unit time 
        • e.g., 9 cases of Kawasaki disease per 10,000 children per year
      • # of new cases in a population/time spent at risk (person-time) 
        • person-time = number of people at risk x time spent at risk
          • e.g., 9 cases of Kawasaki disease/10,000 child-years 
            • if we watched 10,000 at-risk people (children) for 1 year, 5,000 children for 2 years, or 1,000 children for 10 years, etc., we would see 9 new cases during that observation period
  • Prevalence
    • proportion of population that has a disease or risk factor at a specified point or period in time  
    • existing cases/total number of people in specific population at a particular time
      • e.g., percent of the U.S. population with diabetes in 2018
    • indicates overall disease burden of population
    • helpful for resource allocation
    • prevalence = incidence x duration 
    • factors that change prevalence 
      • factors that increase prevalence include 
        • increase in new cases (increased incidence)
        • improved quality of care → decreased mortality  longer duration
        • improved diagnostic ability → higher incidence
        • in-migration of cases or susceptible people
        • out-migration of healthy people
      • factors that decrease prevalence include 
        • high case-fatality rate → shorter duration (aggressive cancers)
        • decrease in new cases (decreased incidence)
          • could result from preventative efforts such as vaccination 
          • in-migration of healthy people
          • out-migration of cases
          • improved recovery/cure rate
 

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(M1.ST.14.81) A 4th grade class in Salem, Massachusetts has 20 students. Due to recent media coverage of the fallacious association between vaccines and autism, none of the students have been immunized against influenza this year. Fortunately, up to this point none of the students has come down with the flu. During the first week of flu season, however, 2 students contract influenza. In the second week, 3 more students contract influenza. And in the third week, 5 more students contract influenza. The other students remained healthy throughout the rest of the flu season. In this class, what was the risk of contracting influenza during the second week of the flu season? Tested Concept

QID: 103898
1

0.1

5%

(3/58)

2

0.15

19%

(11/58)

3

0.17

48%

(28/58)

4

0.25

19%

(11/58)

5

0.5

7%

(4/58)

M 1 D

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