The document discusses various measures of morbidity, including incidence, prevalence, attack rates, and disability rates, highlighting their importance in understanding disease within populations. It emphasizes that morbidity data is crucial for diseases with low case-fatality, providing definitions and calculations for understanding different related rates. The conclusion underscores that these morbidity measures are essential tools for epidemiologists in studying chronic diseases not accurately captured by mortality rates.

1. MEASURES OF
MORBIDITY
ALI, EMMANUEL ADAMU
DEPARTMENT OF COMMUNITY MEDICINE
COLLEGE OF MEDICAL SCIENCES
UNIVERSITY OF MAIDUGURI
June 2022
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2. Outline
 Introduction
 Uses of Morbidity data
 Sources of morbidity data
 Incidence
 Attack Rate & Secondary Attack Rate
 Cumulative Incidence
 Prevalence
 Factors influencing prevalence
 R/ship between Incidence &Prevalence
 Differences between Incidence &Prevalence
 Disabilty Rate
 Case Fatality Rate
 Conclusion
 References
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3. Introduction
 Death rates are particularly useful for
investigating diseases with a high case-fatality
 However, many diseases have low case-fatality,
for example, most mental illnesses like
schizophrenia, musculoskeletal diseases like
rheumatoid arthritis, and infections like
chickenpox and mumps.
 In this situation, data on morbidity (illness) are
more useful than mortality rates.
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4. Introduction
Definition
 Morbidity (from Latin morbidus: sick, unhealthy)
refers to having a disease or a symptom of disease,
or to the amount of disease within a population.
 Any departure, subjective or objective from a state
of physiological well being.
 Morbidity also refers to medical problems caused by
a treatment.
 It is usually represented or estimated using
prevalence or incidence.
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5. Introduction
Morbidity rate
 The morbidity rate measures the proportion of
people in a specified geographical location who
contracted a particular disease during a specified
period of time.
 It indicates the frequency of the disease appearing
in a population.
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6. Sources of Morbidity Data
 Population studies
 Hospital admissions and discharges
 Outpatient and primary health care consultations
 Specialist services (such as injury treatment)
 Registers of disease events (such as cancer and
congenital malformations)
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8. Measures of Morbidity
 Morbidity is measured in 3 units
 Frequency; Incidence & Prevalence
 Duration; Disability rate
 Severity; Case Fatality Rate
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9. Measures of Morbidity
 Incidence and Prevalence are two measures of morbidity
(illness).
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10. Incidence
 Incidence refers to the rate at which new events
occur in a population.
 It takes into account the variable time periods
during which individuals are disease-free and thus
“at risk” of developing the disease.
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11. Incidence
 The numerator strictly refers only to first events of
disease.
 The units of incidence rate must always include a
unit of time (cases per 10n and per day, week,
month, year, etc.)
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12. Incidence
 For example, studies have found that the 10-year
incidence of a major coronary event (such as heart
attack) among white men, ages 30–59, with diastolic
blood pressure 105 mmHg or above at the time they
were first seen, was found to be 183 per 1,000.
 This means that among 1,000 white men, ages 30–
59, who had diastolic blood pressure above 105
mmHg at the beginning of the 10-year period of
observation, 183 of them had a major coronary event
(heart attack or sudden death) during the next 10
years
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13. Incidence
Among white men with diastolic blood
pressure of <75 mmHg, the 10-year
incidence of a coronary event was
found to be 55/1,000
 Thus, comparison of these two incidence rates, 183/1,000
for those with high blood pressure versus 55/1,000 for
those with low blood pressure, may lead to the inference
that elevated blood pressure is a risk factor for coronary
disease.
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SPECIAL
INCIDENCE RATES

15. Attack Rate
 When a population is at risk for a limited period only, the study
period can readily encompass the entire epidemic.
 When this is true, the incidence is generally referred to as an
attack rate.
 Attack Rate
No of cases of a specific disease x 100
Number of persons exposed
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16. Attack Rate
 For example, in studying an outbreak of food-
borne disease, the attack rate among those who
ate a certain food (i.e. population at risk) is
compared with the attack rate among those who
abstained from that food.
 Consider an outbreak of shigellosis in which 18
persons in 18 different households all became ill.
 If the population of the community was 1,000,
then the overall attack rate was 18 ⁄ 1,000 × 100%
= 1.8%.
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17. Secondary Attack Rate
 The secondary attack rate is defined as the probability
that infection occurs among susceptible persons
within a reasonable incubation period following
known contact with an infectious person (primary
case) or an infectious source.
 It is a key epidemiologic parameter in infectious
diseases that are transmitted by contact.
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18. Secondary Attack Rate
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19. EXAMPLE: Calculating Secondary
Attack Rates
 Consider an outbreak of shigellosis in which 18
persons in 18 different households all became ill. If
the population of the community was 1,000, then the
overall attack rate was 18 ⁄ 1,000 × 100% = 1.8%.
 One incubation period later, 17 persons in the same
households as these “primary” cases developed
shigellosis.
 If the 18 households included 86 persons, calculate
the secondary attack rate.
 Secondary attack rate = (17 ⁄ (86 − 18)) × 100% =
(17 ⁄ 68) × 100% = 25.0%
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20. Cumulative Incidence
 Cumulative incidence is a simpler measure of the
occurrence of a disease or health status.
 Unlike incidence, it measures the denominator only at the
beginning of a study.
 The cumulative incidence can be calculated as follows:
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21. Cumulative Incidence
 In a statistical sense, the cumulative incidence is the
probability that individuals in the population get the disease
during the specified period.
 Cumulative incidence is often presented as cases per 1000
population
 The cumulative incidence rate therefore is similar to the
“risk of death” concept used in actuarial and life-table
calculations.
 The simplicity of cumulative incidence rates makes them
useful when communicating health information to the
general public.
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22. Prevalence
Prevalence is defined as the number of existing cases of disease
at a given point of time
Prevalence of a disease is defined as:
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23. Prevalence
 Prevalence(P) is often expressed as cases per 100 (percentage),
or per 1000 population.
 In this case, P has to be multiplied by the appropriate factor:
10n.
 If the data have been collected for one point in time, P is the
“point prevalence rate.”
 It is sometimes more convenient to use the “period
prevalence rate,” calculated as the total number of cases at
any time during a specified period, divided by the population at
risk midway(average) through the period.
 Similarly, a “lifetime prevalence” is the total number of
persons known to have had the disease for at least some part
of their lives.
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25. Factors affecting prevalence
Increased by
 Longer duration of the disease
 Prolongation of life of patients
without cure
 Increase in new cases (increase in
incidence)
 In-migration of cases
 Out-migration of healthy people
 In-migration of susceptible people
 Improved diagnostic facilities
(better reporting) :
Decreased by
 Shorter duration of the disease
 High case-fatality rate from
disease
 Decrease in new cases
(decrease in incidence)
 In-migration of healthy people
 Out-migration of cases
 Improved cure rate of cases
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26. Epidemiologic Bathtub
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27. Interrelationship between
Incidence and Prevalence
 Prevalence is dependent on both incidence and disease
duration.
 Provided that the prevalence (P) is low and does not vary
significantly with time, it can be calculated approximately as:
P = incidence x average duration of disease
 Given the assumption that population is stable, and
Incidence and Duration are unchanging.
 Duration reflects the prognostic factors
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29. Differences between Incidence
& Prevalence
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30. Disabilty Rate
 A disability is any condition of the body or mind
(impairment) that makes it more difficult for the person with
the condition to do certain activities (activity limitation) and
interact with the world around them (participation
restrictions).
 According to the World Health Organization, disability has
three dimensions
 Impairment
 Activity Limitations
 Participation restrictions
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31. Case Fatality Rate
 Also called case fatality risk or case fatality ratio, is the
proportion of people who die from a specified disease
among all individuals diagnosed with the disease over a
certain period of time.
 Case fatality rate typically is used as a measure of disease
severity and is often used for prognosis where
comparatively high rates are indicative of relatively poor
outcomes.
 It also can be used to evaluate the effect of new treatments,
with measures decreasing as treatments improve.
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33. Conclusion
 The main function of epidemiology is to discover groups in
the population with high rates of disease and with low, so
that causes of diseases and the freedom from the disease
can be postulated.
 Measures of morbidity are important tools in the hands of
epidemiologists in studying chronic diseases whose
pictures are not accurately represented by measures of
mortality.
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Questions?
Comments?

35. References
 B. A Omotara; PRINCIPLES AND METHODS IN
EPIDEMIOLOGY
 R. Bonita, R. Beaglehole, T. Kjellström; Basic
epidemiology. 2nd edition.
 Sylvia Wassertheil-Smoller, Jordan Smoller; Biostatistics
and Epidemiology A Primer for Health and Biomedical
Professionals 4th Edition
 Principles of Epidemiology in Public Health Practice 3rd
Edition ;
 An Introduction to Applied Epidemiology and
Biostatistics October 2006 Updated May 2012 . Centers
for Disease Control and Prevention (CDC) Office of
Workforce and Career Development Atlanta,
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