Morbidity Measures3

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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


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


rate


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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Morbidity Measures3

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