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Measures of Morbidity
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James Maxwell, physicist (18311879)
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We owe all the great advances in knowledge
to those who endeavour to find out how
much there is of anything.
James Maxwell, physicist (18311879)
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William Thomson,
engineer, mathematician, and physicist (18241907)
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If you can measure that of which you speak,
and can express it by a number, you know
something of your subject, but if you cannot
measure it, your knowledge is meager and
unsatisfactory.
William Thomson, engineer, mathematician,
and physicist (18241907)
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Sources of data for illness
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Measures of Morbidity
Incidence
Prevalence
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Incidence
Measured by incidence rate:
the number of new cases of a disease thatoccur during a specified period of time in a
population at risk for developing the disease.
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Incidence rate
Cumulative Incidence
Incidence density
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Cumulative Incidence
(Number ofnewcases during a given period of
time/Population at risk during the same time
period) x 1000
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Example
Among 60 people attending a 12-month
residential detoxification program in Mysore,
50 tested HIV negative at the start of the
program in January 1998. At the end of the
program in December 1998, 3 of the 50
participants tested positive for HIV.
Calculate the cumulative incidence.
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Incidence Density
Number of new cases during a given period
Total person-time of observation
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Uses of Incidence rates
Measures the risk of acquiring the disease
To identify the cause or etiology of disease
To explore the relationship between anexposure and the risk of disease
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Prevalence
number of affected persons present inthe population at a specific time
divided by the number of personsin the population at that time
what proportion of the population isaffected by the disease at that
time?
???
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Prevalence
Measured by Prevalence Rate
Divided into two types:
1. Point prevalence rate
2. Period prevalence rate
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Proportion of individuals in aspecified population at risk whohavethe disease of interest at a
given point in time.
POINT PREVALENCE RATE
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Proportion of individuals in a
specified population at risk whohavethe disease of interest over aspecified period of time.
Annual prevalence rate
(When the type of prevalence rate is notspecified it is usually point prevalence)
PERIOD PREVALENCE RATE
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Relation between Incidence and
Prevalence
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When Insulin was introduced for the first time,
what happened to the prevalence of diabetes?
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Relation between Incidence and
Prevalence
Prevalence = Incidence Duration of disease
P= I x D
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City Population
screened
Positive CXR for TB
Mysore 1000 100
Mandya 1000 60
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City Population
screened
Positive CXR
for TB
Prevalence Incidence
Mysore 1000 100 100 4
Mandya 1000 60 60 20
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City Prevalence Incidence Duration
Mysore 100 4 25 years
Mandya 60 20 3 years
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Uses of Prevalence rates
Measures -burden of disease in a community.
Eg:
How many people in the community have arthritis?
To determine
-how many clinics are needed
-what types of rehabilitation services are needed
-how many and what types of health professionals areneeded.
Needed for planning health services.
To make future projections and
To anticipate the changes
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Exercises
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Exercises-1
In 2005 all the children in Government schools
of Nanjangud were examined for evidence of
leprosy. The procedure was repeated again in
2006. The following were the results:
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Exercise 1 contd
2005
a. No. of children on the rolls 52,600
b. No. of children examined 48,000
c. No. of children found to have active leprosy 288
2006
d. No. of children on rolls 54,000
e. No. of children examined for the first time 6,000
f. No. of active cases among the above 46
g. No. of children re-examined 40,000
h. No. of old cases among them (i) Active
(ii) Inactive
40
200
i. No. of new cases among the re-examined 80
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Exercise 1 contd
Questions:
1. What proportion of children on the rollswere examined?
a. in 2005b. in 2006
2. What was the prevalence of leprosy? a. in 2005
b. in 2006
3. What was the incidence of leprosy during 2005-2006?
4. Can the estimated prevalence and incidence rate be said
to be applicable to:
a. The entire group of school children
b. All children in the area belonging to 5-14 yrs age group
5. Had the second survey been conducted in 2007 (instead of
2006) and X no. of new cases been detected from N no. of
re-examined children, what would be the formula for
annual incidence rate?
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Exercise 2
200 new born children were followed up till
their 2nd birthday to study the incidence of
diarrhoea. The following table shows the
distribution of the children according to thenumber of diarrhoeal episodes.
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No. ofepisodes
Number of children
1st year 2nd year
0 0 40
1 40 80
2 120 60
3 20 15
4 15 3
5 5 2
Total 200 200
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Exercise 2 contd
Calculate the incidence of diarrhoea
a. During the first year
b. During the second year
c. Overall during the first 2 years of life.
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Attack rate
Expressed as percentage
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Specific attack rate
Food specific attack rate
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Exercise
After a dinner attended by 100 people, 12
individuals become ill. All 100 people are
interviewed about their food consumption at
the dinner. The interviews show that 8 of the12 people who are ill and 25 of the 88 who are
healthy ate fish.
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ILL WELL Total Attack rate
Ate fish 8 25 33Did not eat fish 4 63 67
12 88 100
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Eaten Not eaten
ILL Not Ill Total Attack
rate
ILL Not Ill Total Attack
rate
Breakfast 9 20 29 24 45 69
Lunch 18 42 60 15 23 38
Dinner 11 19 30 22 46 68
Sandwich
es
25 24 49 8 41 49
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Eaten Not eaten
ILL Not Ill Total Attack
rate
ILL Not Ill Total Attack
rate
Breakfast 9 20 29 31 24 45 69 35
Lunch 18 42 60 30 15 23 38 39
Dinner 11 19 30 37 22 46 68 32
Sandwich
es
25 24 49 51 8 41 49 16
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Secondary Attack Rate (SAR)
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Index Case
First Person who comes to the attention ofpublic health authorities
Primary Case
First Person/s who acquires the disease froman exposure
Attack rate
Secondary Case
Person who acquires the disease from anexposure to the primary case
Secondary attack rate
Some definitions
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Secondary Attack Rate (SAR)
Definition:
No. of exposed (& susceptible) persons
developing the disease within the rangeof the incubation period, following
exposure to the primary case.
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Secondary Attack Rate
Used to estimate the spread of disease in a family,
household, dorm or other group environment.
Measures the infectivity of the agent and the effects of
prophylactic agents (e.g. vaccine)
Secondaryattack rate
(%)
No. of exposed persons developing the diseasewithin the range of the incubation period
Total no. of persons (susceptible &)exposed to the primary case
= x 100
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Example:
Family of 6 consisting of 2 parents (already
immune) and 4 children who are susceptible
to measles. There occurs a primary case and
within a short period of time 2 secondarycases among the remaining children. Calculate
the SAR.
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Mumps experience of 390 families exposed
to a primary case within the family
15258415210-19
872044204505-9
501002503002-4
SecondaryPrimaryNo. susceptible before
primary cases occurredTotal
Age in
years
CasesPopulation
Secondary attack rate 2-4 years old
(50)/(250-100) x 100 = 33%
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Exercise
Seven cases of hepatitis A occurred among 70children attending a childcare center. Eachinfected child came from a different family. The
total number of persons in the 7 affected familieswas 32. One incubation period later, 5 familymembers of the 7 infected children alsodeveloped hepatitis A. Calculate the attack rate in
the child care center and the secondary attackrate among family contacts of those cases.
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Attack rate in childcare center:
Cases of hepatitis A among children in
childcare center
Number of children enrolled in the
childcare center
7
70
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Secondary Attack rate among house hold
contacts:
Cases of hepatitis A among family contacts of
children with hepatitis A
Number of persons at risk in the families
5
25
Age (yrs) No. of persons exposed to a case Cases of Hepatitis
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g (y ) p p p
No. Secondary Attack rate (%)
Did not receive vaccine
0-4 42 2
5-9 45 5
10-14 32 6
15-19 26 3
20+ 83 4
Received vaccine
0-4 17 1
5-9 21 0
10-14 13 0
15-19 3 0
20+ 17 1
Age (yrs) No. of persons exposed to a case Cases of Hepatitis
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g (y ) p
No. Secondary Attack rate (%)
Did not receive vaccine
0-4 42 2 4.8
5-9 45 5 11.1
10-14 32 6 18.8
15-19 26 3 11.5
20+ 83 4 4.8
Received vaccine
0-4 17 1 6.0
5-9 21 0 0
10-14 13 0 0
15-19 3 0 0
20+ 17 1 1.4
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Herd Immunity
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Herd Immunity
Resistance of a group of people to an attack by
a disease to which a large proportion of the
members of the group are immune.
h d h d i i ?
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Why does herd immunity occur?
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Conditions to be met
The disease agent must be restricted to a
single host species
Transmission direct from one member of the
host species to another
Infections must induce solid immunity
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Herd immunity operates if the probability of
an infected person encountering every other
individualin the population (random mixing)
is the same.
Wh t t f l ti t
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What percentage of a population must
be immune for herd immunity to
operate?
Varies
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Example
Poliomyelitis
Two types of vaccines
OPV
IPV
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Why is the concept of herd immunity
so important?
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