Measures of Disease Frequency 0903_gaohongcai(1)
Transcript of Measures of Disease Frequency 0903_gaohongcai(1)
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Me asu res o fDise ase sFrequency(1)Gao Hongcai
Email: [email protected]
School of Public Health, Jining MedicalCollege
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Introduction Epidemiologists use a variety of
methods to summarize data. One
fundamental method is the frequencydistribution. The frequency distributionis a table which displays how manypeople fall into each category of a
variable such as age, income level, ordisease status.
In later lessons you will learn about
frequency distribution (time placepeople).
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Introduction cases:
people who have a disease or conditionbeing studiedare generally referred to as
cases.
People without the disease are called non-
cases.
non-cases :
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IntroductionThe most common form of
organizing epidemiologic informationis through collecting information
about cases and non-cases.
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Introduction counting cases of disease in a
population is the unique domain of
epidemiologyit is the corecomponent of disease surveillanceand a critical step in investigating an
outbreak.
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Introduction Case counts must be placed in proper
perspective, however, by using rates to
characterize the risk of disease for apopulation. Calculating rates for differentsubgroups of age, sex, exposure historyand other characteristics may identify high-
risk groups and causal factors. Such information is vital to the
development and targeting of effectivecontrol and prevention measures.
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Introduction The numbers are usually translated into
rates.
Rates indicate the risk of disease for apopulation.
This helps to identify high-risk groups and
causal factors. Data elements collected from cases and
non-cases are called variables (age, sex,race, weight, height, temperature, bloodpressure, disease, death etc)
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Why measure disease?To quantify or measure the
occurrence of disease is fundamental
in understanding: Description of health events in the
population
Impact of disease in the population Possible associations between factors
and disease
Possible mechanisms of spread
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Rates, Ratios and
Proportions Variables or data elements are summarized
to provide number of cases, proportions,rates and ratios = frequency
In epidemiology many variables have onlytwo possible categories these are called dichotomous variables(e.g. disease or no disease; alive or dead).
The frequency measures we use withdichotomous variables are ratios,proportions and rates.
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Rates, Ratios and
Proportions Before you learn about specific measures,
it is important to understand the
relationship between the three types ofmeasures and how they differ from eachother.
All three measures are based on the sameformula:
Ratio, proportion, rate = 10n
y
x
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Rates, Ratios and
Proportions
10n(10 to the nth power),is a constant that we
use to transform the result of the divisioninto a uniform quantity. The size of 10n
may equal 1, 10, 100,1000 and so on
depending upon the value ofn.
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Rates, Ratios and
Proportions For example,
100 = 1
101 = 10 102 = 10 10 = 100 103 = 10 10 10 = 1000
You will learn what value of 10n
touse when you learn about specificratios, proportions, and rates.
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= 5 /2 = 2.5 /1
Ratio
The quotient of 2 numbers Numerator NOT necessarily INCLUDED in the
denominator
Allows to compare quantities of different nature
Rates, Ratios and
Proportions
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For example, the sex of children attendingan immunization clinic could be compared
in either of the following ways:
In the first option,x(female) is completelyindependent ofy(male).
In the second,x(female) is included iny(all).Both examples are ratios.
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2--- = 0.5 100 = 50%4
Proportion
The quotient of 2 numbers Numerator NECESSARELY INCLUDED
in the denominator
Quantities have to be of same nature
Proportion always ranges between 0 and 1
Percentage = proportion 100
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Of the two ratios shown above,
the first is not a proportion, becausexis not a part ofy.
The second is a proportion, because
xis part ofy.
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Rate
2----- = 0.02 /year100
Observed in 1998
Numerator- number EVENTS observed for a given time
Denominator- population in which the events occur
(population at risk)- includes time
The quotient of 2 numbers
Speed of occurrence of an event over time
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rate, is often a proportion, with an addeddimension: it measures the occurrence of
an event in a population over time. The basic formula for a rate is as follows:
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Notice three important aspects of this
formula.
The persons in the denominator must reflect thepopulation from which the cases in the numerator
arose.
The counts in the numerator and denominatorshould cover the same time period.
In theory, the persons in the denominator must
be at risk for the event, that is, it should have
been possible for them to experience the event.
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Rates and Proportions Rates: tell us how fast the disease isoccurring in a population
Proportions: tell us what fraction of thepopulation is affected
Rates and Proportions can be used to estimate
the extent and significance of disease burden ina community or population
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As you can see from the above discussion,
ratios, proportions, and rates are NOT three
distinctly different kinds of frequency
measures. They are all ratios:
proportions are a particular type ratio, and
some rates are a particular type of
proportion.
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In epidemiology, however, we often
shorten the terms for these
measures in a way that makes it
sound as though they are completely
different. When we call a measure aratio, we usually mean a
nonproportionalratio;
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when we call a measure a
proportion, we usually mean a
proportional ratio that doesnt
measure an event over time, and
when we use the term rate, wefrequently refer to a proportional ratio
that does measure an event in a
population over time.
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We also use ratios, proportions, and,
most important rates to describethree aspects of the human
condition:
morbidity (disease),
mortality (death),
and natality (birth).
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Morbidity FrequencyMeasures
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To describe the presence ofdisease in a population, or the
probability (risk) of its occurrence,
we use one of the morbidity
frequency measures.
In public healthterms, disease
includes illness,injury, or disability.
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Table 1 shows several morbiditymeasures.
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All of these can be further elaborated intospecific measures for age, race, sex,
or some other characteristic of aparticular population being described.
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Table 2. shows a summary of the formulas forfrequently used morbidity measures.
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I will describe how you calculateeach of the morbidity measures
and when you would use it later.
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Incidence Rate
Definition:
Incidence refers to the occurrence of
new cases in a population during agiven period of time.
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Incidence Rate
Incidence rate is the most commonway of measuring and comparing the
frequency of disease in populations. We use incidence rates instead of raw
numbers for comparing disease
occurrence in different populationsbecause rates adjust for differences inpopulation sizes.
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Incidence Rate
Rate of development of disease during agiven period of time.
The incidence rate expresses theprobability or risk of illness in a populationover a period of time.
It is used to estimate the risk of developinga disease in a specified population during aspecified period of time.
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Incidence Rate
Since incidence is a measure of risk,when one population has a higher
incidence of disease than another, wesay that the first population is at ahigher risk of developing disease thanthe second, all other factors being
equal. We can also express this by saying that
the first population is a high-riskgrouprelative to the second population.
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Incidence Rate
An incidence rate (sometimesreferred to simply as incidence) is a
measure of the frequency with whichan event, such as a new case ofillness, occurs in a population over a
period of time.
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Incidence Rate
The formula for calculating anincidence rate as follows:
k=periodtimesametheduringriskatpopulation
periodgiven timeaduringocurringcasesnewrateIncidence
k=10n,
n=2,3,4,5,6
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Incidence Rate
Three key elements: Only new cases included in numerator
Total population at risk in thedenominator
Time element period over which new
cases developed
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numerator
The numerator of an incidence rate
should reflect newcases of disease
which occurred or were diagnosed
during the specified period.
The numerator should notinclude
cases which occurred or were
diagnosed earlier.
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denominator
Notice that the denominatoris thepopulation at risk. This means that persons
who are included in the denominator shouldbe able to develop the disease that is beingdescribed during the time period covered.
Unfortunately, unless we conduct a special
study, we usually cannot identify andeliminate persons who are not susceptibleto the disease from available populationdata.
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denominator
In practice, we usually use Census population
counts or estimates for the midpoint of the time
period under consideration.
If the population being studied is small and very
specific, howeversuch as a nursing home
populationwe can and should use exact
denominator data.
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denominator
The denominator should representthe population from which the
cases in the numerator arose. E.g. : If the numerator was limited
to civilian cases, it was necessary
for us to restrict the denominatorto civilians as well.
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denominator
Depending on the circumstances, the mostappropriate denominator will be one of the
following: average size of the population over the time
period.
size of the population (either total or at risk)
at the middle of the time period. size of the population at the start of the
time period.
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Incidence rate -10n For 10n, any value ofn can be used.
For most nationally notifiable
diseases, a value of 100,000 or 105 isused for 10n.
Otherwise, we usually select a value
for 10n so that the smallest ratecalculated in a series yields a smallwhole number (for example, 4.2/100,
not 0.42/1,000; 9.6/100,000, not
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Incidence rate -10n Since any value ofn is possible, the
investigator should clearly indicate
which value is being used. If weselected a value of 100,000, ourincidence rate is reported as 297.4
per 100,000.
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Incidence rate -time Rates imply a change over time. For disease incidence rates, the
change is from a healthy state todisease. The period of time mustbe specified.
For surveillance purposes, the periodof time most commonly used is thecalendar year, but any interval maybe used as long as the limits of the
interval are identified.
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Incidence rate -types When the denominator is the size of the
population at the start of the timeperiod, the measure is sometimes calledcumulative incidence.
This measure is a proportion, becauseall persons in the numerator are also inthe denominator. It is a measure of theprobability or riskof disease, i.e.,what proportion of the population willdevelop illness during the specified timeperiod.
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-types
In contrast, the incidence rate islike velocity or speed measured in
miles per hour. It indicates howquicklypeople become illmeasured in people per year.
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Cumulative Incidence
There are two types of Incidence rates :
1. Cumulative Incidence (CI): is the
proportion of people who become diseasedduring a specified period of time.
k
riskatpopulationTotal
periodspecificaduringdiseaseaofcasesnewofNumber
k=10n,
n=2,3,4,5,6
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Cumulative Incidence
CI provides an estimate of theprobability, or risk that an individual will
develop a disease during a specifiedperiod of time.
The time period must be clearly
specified when reporting the CI.
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Cumulative Incidence
The CI assumesThat the entire population is at risk at
the beginning of the study periodThat the populations has been followed
for the specified time interval
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Cumulative Incidence
In 2000: 733,151 new cases of gonorrhea were
reported among the United States civilianpopulation.
The mid-year civilian population wasestimated to be 246,552,000.
CI=[733,151/246,552,000] x 1,000=
2.97 per 1000 population.
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Incidence Density
2. Incidence density (ID) = measure of riskin a changing population where people are
free of disease at start and observed foronset of disease for different time periods
Shows how rapidly cases develop
In presenting an incidence density, it is
essential to specify the relevant time units.-i.e. number of cases per person-day,person-month, person-year, etc.
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Incidence Density
Person-time is the number of personsat risk for disease multiplied by the
length of time they are observed foronset of disease
Person years = # of person # of
years followed up
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Incidence Density Example: In a cohort study, 600 contraceptive
users were followed over the course of three yearsas follows:
100 women for 1year 200 women for 2 years
300 women for 3 years
The number of person-years of observation in this
study is: 100 women x 1 = 100 person-years
200 women x 2 = 400 person-years
300 women x 3 = 900 person-years
Total = 1400 person-years
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Incidence Density
Of these 600 people 10 developed thromboembolism
The incidence rate ofthromboembolism is calculated as: (10/1400)*k = 7.1 per 1000 person
years of follow up.
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Prevalence Rate
Prevalence, sometimes referred to as
prevalence rate, Is the proportion ofpersons in a population who have a
particular disease or attribute at a specified
point in time or over a specified period of
time.
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Prevalence Rate
The formula for presence of disease is:
k=
periodtimesametheduringpopulation
periodgiven timeaduringcasesexisting-preandnewallPrevalence
k=10n
The formula for prevalence of anattribute is:
k=periodtimesametheduringpopulation
periodgiven timeaduringattributeexisting-preandnewallPrevalence
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Prevalence Rate
Gives an indication of the burden ofdisease
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Prevalence Rate
The value of 10n is usually 1 or 100for common attributes.
The value of 10n may be1,000,100,000, or even 1,000,000for rare traits and for most
diseases.
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Prevalence Rate
There are two types of prevalence:
1. Point prevalence: is the presence
of a disease at a single point in time(snap shot).
kriskatpopulationpoint-midTotal
timeofpointspecificaatcasesexistingofNumber
k=10n,
n=2,3,4,5,6
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Prevalence Rate
The amount of disease present in a
population is constantly changing.
Sometimes, we want to know how much ofa particular disease is present in a
population at a single point in timeto get
a kind of stop action or snapshot lookat the population with regard to that
disease.
We use point prevalence for that
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Prevalence Rate
Point prevalence is not an incidence
rate, because the numeratorincludes pre-existing cases;
it is a proportion, because the
persons in the numerator are also in
the denominator.
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Prevalence Rate
At other times we want to know howmuch of a particular disease is present
in a population over a longer period.Then, we use period prevalence.
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Prevalence Rate
2. Period Prevalence: the presence of adisease at a particular time interval .
kriskatpopulationperiod-midTotal
timeofperiodspecificaatcasesexistingofNumber
Prevalence usually refers to pointrevalence
k=10n,
n=2,3,4,5,6
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Prevalence Rate
The numerator in period prevalence isthe number of persons who had a
particular disease or attribute at anytime during a particular interval.The interval can be a week, month, year,
decade, or any other specified timeperiod.
l i hi
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Relationship BetweenPrevalence and Incidence
The prevalence and incidence of disease are
frequently confused. They are similar, but
differ in what cases are included in thenumerator.
Numerator ofIncidence = new cases
occurring during a given time period
Numerator ofPrevalence = all cases
(existing and new)
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As you can see, the numerator of anincidence rate consists only of persons
whose illness began during a specifiedinterval.
The numerator for prevalence includesallpersons ill from a specified cause
during a specified interval (or at aspecified point in time) regardless ofwhen the illness began. It includesnot only new cases, but also old cases
during the specified interval.
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Relationship BetweenPrevalence and Incidence
Prevalence and Incidence are related
by average duration (D) of disease ina stable population
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High prevalence of a disease within apopulation may reflect high risk, or itmay reflect prolonged survival withoutcure.
Conversely, low prevalence mayindicate low incidence, a rapidly fatalprocess, or rapid recovery.
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We often use prevalence ratherthan incidence to measure the
occurrence of chronic diseasessuch as osteoarthritis which havelong duration and dates of onset
which are difficult to pinpoint.
R l ti hi B t
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Relationship BetweenPrevalence and Incidence
Deaths,
Cured,
Lost...
DurationPrevalence
Incidence
P=ID
R l ti hi B t
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Relationship BetweenPrevalence and Incidence
If a disease lasts a long time, there will behigher prevalence in the population
If incidence is low but those affected havethe condition for long = the prevalence willbe high relative to the incidence
If disease is short-lived, there will be lowprevalence in the population
If incidence is high but the disease durationis short then the prevalence will be lowrelative to the incidence
R l ti hi B t
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Relationship BetweenPrevalence and Incidence
Example: a follow up study of theFramingham data showed males and
females had same IR for heart diseasebut females had a higher prevalenceof disease
WHY?
R l ti hi B t
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Relationship BetweenPrevalence and Incidence
PR = What proportion of thepopulation has this condition?
IR = At what rate do new casesarise over time in the population?
R l ti hi B t
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Relationship BetweenPrevalence and Incidence
Example: in Richmond City, 2006, there
were a total of 5,000 hepatitis B cases of
which 1,000 were newly diagnosed. The total population of Richmond City is
200,000.
Prevalence in 2006:(5,000/200,000)*1000=25 per 1,000
population
Incidence in 2006: (1,000/200,000)*
U f P l d
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Uses of Prevalence andIncidence Measures
Prevalence is mostly used for
planning, evaluating
Incidence is used to identify causal
relationships
Factors which increase or decrease prevalence rate
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Factors which increase or decrease prevalence rate
Rates Are Increased by Rates Are Decreased by
Immigration of ill cases Immigrationofhealthypersons
Emigration of healthy persons Emigration of ill cases
Immigration of susceptible cases orthose with potential of becomingcases
Improved cure rate of cases
Increaseddeath rates from the diseases
Prolongation of life of caseswithout cure (increase of durationof disease)
Decrease in occurrence of new cases
Shorter duration of disease
Increase in occurrence of new cases(increase in incidence)
Death
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Attack Rate
Attack Rate: Is a variant of anincidence rate, applied to a narrowly
defined population observed for alimited time, such as during anepidemic or outbreak of a disease.
It is usually expressed as a percent, so10n equals 100.
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Attack Rate
For a definedpopulation (the population at risk),during a limited time period,
100periodtheofbeginningat theriskatPopulation
periodtheduringpopulationtheamongcasesnewofNo.rateAttack =
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Attack Rate
Example: Of 75 persons who attended achurch picnic, 46 subsequently developed
gastroenteritis. To calculate the attack rate ofgastroenteritis we first define the numeratorand denominator:
x= Cases of gastroenteritis occurringwithin the incubation period forgastroenteritis among persons who
attended the picnic = 46
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Attack Rate
The attack rate of gastroenteritis is:
The risk of developing gastroenteritis in this
population was 61%.
%611007546 =
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Attack Rate Notice that the attack rate is a proportion
the persons in the numerator are also inthe denominator.
This proportion is a measure of theprobabilityor riskof becoming a case.
In the example above, we could say that,
among persons who attended the picnic,the probability of developinggastroenteritis was 61%, or the risk ofdeveloping gastroenteritis was 61%.
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Secondary Attack Rate
Secondary attack rate:is a measure ofthe frequency of new cases of a diseaseamong the contacts of known cases.
The formula is as follows:
k=contactsofnumberTotal
periodtheduringcasesprimaryofcontactsamongcaseofNo.rateattackSecondary
k=10n,
n=2,3,4,5,6
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To calculate the total number of householdcontacts, we usually subtract the number ofprimary cases from the total number ofpeople residing in those households.
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Secondary Attack Rate
Example:
Seven cases of hepatitis A occurred
among 70 children attending a child carecenter.
Each infected child came from a differentfamily.
The total number of persons in the 7affected families were 32
One incubation period later, 5 family
members of the 7 infected children also
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Calculate the attack rate in thechild care center and the
secondary attack rate amongfamily contacts of those cases.
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1. Attack rate in child care center:
x= cases of hepatitis A among
children in child care center = 7
y= number of children enrolled in
the child care center = 70
Attack rate= 100 = 100 =
10%y
x
70
7
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2. Secondary attack rate:
x= cases of hepatitis A among family contacts of
children with hepatitis A = 5 y= number of persons at risk in the families (total
number of family memberschildren alreadyinfected) = 32 7 = 25
Secondary attack rate
= 100 = 100 = 20%y
x
25
5