Mpundu MKC MSc Epidemiology and Biostatistics, BSc Nursing, RM, RN

Mpundu MKC MSc Epidemiology and Biostatistics, BSc Nursing, RM, RN
Epidemiology BSPH-122 Measures of Disease Frequency (Measures of Disease Occurrence) Mpundu MKC MSc Epidemiology and Biostatistics, BSc Nursing, RM, RN

Importance of measures of Disease Frequency
Provides a picture information about a disease, framing public health questions and guiding resource allocation Describes the absolute risk of a disease in a defined population Can be categorised or stratified to gain insight into the pathogenesis of disease

a) Morbidity Frequency Measures
These measures are called “measures of disease frequency.” Incidence prevalence Used to describe the presence of disease in a population or probability (risk) of its occurrence Describes the rates of occurrence of disease in a population In Public Health terms, disease includes: illness, injury and disability disea

1. Incidence Rates (Incidence)
The most common way of measuring and comparing the frequency of disease in the populations IR are used instead of raw numbers for comparing disease occurrence in different populations because rates adjust for differences in population sizes IR expresses the probability or risk of illness in a population over a period of time Since incidence is a measure of risk, when one population has a higher incidence of disease than another, we say that the first population is at greater risk of developing disease than the second, all other factors being equal or the first population is a high risk group relative to the other 4 4

1. Incidence Rates (Incidence)
This is the number of new cases occurring in a defined population during a specified period of time”. A measure of frequency with which an event such as illness ,injury or disability occurs in a population over a period of time IR= 𝑿 𝒚 x10n IR= 𝑵𝒆𝒘 𝒄𝒂𝒔𝒆𝒔 𝒐𝒇 𝒂 𝒔𝒑𝒆𝒄𝒊𝒇𝒊𝒆𝒅 𝒅𝒊𝒔𝒆𝒂𝒔𝒆 𝒅𝒖𝒓𝒊𝒏𝒈 𝒂 𝒔𝒑𝒆𝒄𝒊𝒇𝒊𝒆𝒅 𝒕𝒊𝒎𝒆 𝒑𝒆𝒓𝒊𝒐𝒅 𝑷𝒐𝒑𝒖𝒑𝒖𝒍𝒂𝒕𝒊𝒐𝒏 𝒂𝒕 𝒓𝒔𝒊𝒌 𝒅𝒖𝒓𝒊𝒏𝒈 𝒕𝒉𝒆 𝒔𝒂𝒎𝒆 𝒕𝒊𝒎𝒆 𝒑𝒆𝒓𝒊𝒐𝒏 x 1000

e.g. incidence rates If there are 500 new cases of malaria in a population of 30,000 people in a year I R= X 1000 = 16.7 per 1000 population per year Incidence rates: Measures the rate at which new cases are occurring in a population. Not influenced by duration of disease. 6

1. Incidence Rate IR use are restricted to acute/outbreaks conditions
IR is like velocity or speed measured in kilometres per hour It indicates how quickly people become ill measured in people per year

1. Incidence Rates The numerator of an incidence should reflect New cases of disease which occurred/diagnosed during the specified period The numerator should not include cases which occurred/diagnosed earlier Denominator is a population at risk The denominator includes all who should be able to develop the disease that is being described during the time period covered

ATTACK RATE An attack rate is a variant of an incidence rate
It is applied to a narrowly defined population observed for a limited time, such as during an epidemic. Attack rate is usually expressed in percentage, so 10n = 100 For a defined population (the population at risk), during a limited time period

ATTACK RATE Example of attack rate
75 persons who attended a church picnic, 46 developed gastroenteritis. Then, the attack rate for gastroenteritis is: # of new cases among the pop during the period Attack rate = X 100 Population at risk at the beginning of the period

ATTACK RATE Attack rate = 46 X 100 = 61% 75
This proportion is a measure of the probability or risk of becoming a case. Interpretation: the probability or the risk of developing gastroenteritis was 61%, among persons who attended the picnic. This can be interpreted that 61% out of 75 who attended the church picnic developed GE

ATTACK RATE Attack rate is a proportion- persons in the numerator are also in the denominator This proportion is a measure of probability or risk of becoming a case In the example, we could say that among persons who attended the picnic, the probability of developing GE was 61% or the risk of developing GE was 61%

SECONDARY ATTACK RATE A secondary attack rate is a measure of the frequency of new cases of a disease among the contacts of known cases. Secondary attack rate = 𝑿 𝒚 x10n # of cases among contacts of primary cases during the period x 100 total number of contacts

SECONDARY ATTACK RATE Example of 2nd attack rate
Seven cases of hepatitis A occurred among 70 children attending a child care center. Each infected child came from a different family. The total number of persons in the 7 affected families was 32. One incubation period later, 5 family members of the 7 infected children also developed hepatitis A. Calculate the attack rate in the child care center and the secondary attack rate among family contacts of those cases.

SECONDARY ATTACK RATE 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 = 𝒙 𝒚 X 10n = 𝟕 𝟕𝟎 x 100 = 10%

SECONDARY ATTACK RATE Secondary attack rate:
x = cases of hepatitis A among family contacts of children with hepatitis = 5 y = number of persons at risk in the families (total number of family members minus children already infected) = 32 − 7 = 25

Secondary attack Rate Secondary Attack rate = 𝒙 𝒚 X 10n
= 20%

2. Person-time incidence
Person-time is an estimate of the actual time-at risk in years, months, or days that all persons contributed to a study. In certain studies people are followed for different lengths of time, as some will remain disease-free longer than others.

A subject is eligible to contribute person-time to the study only so long as that person remains disease-free and, therefore, still at risk of developing the disease of interest. By knowing the number of new cases of disease and the person-time-at-risk contributed to the study, an investigator can calculate the incidence rate of the disease, or how quickly people are acquiring the disease.

Calculating person-time Suppose an investigator is conducting a study of the incidence of type 2 Diabetes . He follows 5 subjects from baseline Diabetes for up to 10 weeks. The results are graphically displayed as follows:

Person-time incidence

IR= No of new cases of disease during a period of time x10n Total person-time-at-risk throughout the observation period \ The denominator for incidence rate (person-time) is a more exact expression of the population at risk during the period of time when the change from non-disease to disease is being measured.

The graph shows how many days each subject remained in the study as a non-case (no second MI) from baseline. Time contributed by each subject: Subject A: 53 days Subject B: 70 days Subject C: 24 days Subject D: 70 days Subject E: 19 days Total person-days in the study: =236 person-days

The total number of subjects becoming cases (subject A, C, and E) Person time incidence = x1000 236 p-d 3 IR = = cases per p-d 236 p- = 12.7 cases per 1000 p-days Type 2 Diabetes may be expressed in cases per person-year (p-y) by: ( cases/p-d) x (365 p-d/1 p-y) = 4.6 cases/p-y

Uses of incidence rate as health status indicator
Useful for taking action : To control diseases For research into aetiology, pathogenesis, distribution of disease and efficacy of preventive and therapeutic measures. e.g. Rise in incidence rate may mean several things: Failure of current control programs. Reporting practices have improved. Change in aetiology of disease, e.g. environmental characteristics 26 26

Uses of Incidence Rates (Incidence Density)
Incidence rate is a health status indicator useful for taking action To control disease since it is a direct measure of risk. For research into aetiology, pathogenesis, distribution of disease and efficacy of preventive and therapeutic measures If the incidence rate is significantly higher in one area, then the cause of that disease can be systematically searched.

Uses of Incidence Rates (Incidence Density
Increasing rate may indicate failure or ineffectiveness of current control programmes Might suggest need for new disease control or preventive programmes or that reporting practices have improved A change or fluctuations in the incidence may also mean a change in the aetiology of disease e.g. change in the agent, host and environmental characteristics

3. Prevalence Disease Prevalence
Refers to specifically to all current cases/events (old and new) existing at a given point in time or over a period of time in a given population A total number of individuals who have attribute or disease at a particular time (or during a particular period) divided by the population at risk of having the attribute or disease at this point in time or midway through the period Although referred to as a rate, prevalence rate is a ratio

3. PREVALENCE The proportion of all individuals who have a disease (or an attribute) at a particular point in time in relation to the population at risk. Note: cases are both old and new. Prevalence = Number of existing cases (old and New) X 100 Total population at risk

3. Prevalence Prevalence is of two types Point Prevalence
Period prevalence This is the number of all current cases (old and new) of a disease at one point of time, in relation to defined population Point prevalence deals with cases at one point in time.

i. Point Prevalence Point prevalence = 𝑿 𝒚 X 10n Prevalence = # of all current cases (old and New) X100 Total population at risk “Prevalence rate” is mostly used to mean point prevalence Point prevalence can be made specific for age, sex and other attributes

ii. PERIOD PREVALENCE Measures the frequency of all current cases (old and new) existing during a defined period of time expressed in relation to a defined population # of existing cases of a specified disease during a given time interval PP = x100 estimated mid-interval population at risk 33

Incidence and prevalence
Jan Dec 31 Case 1 Case Case 3 Case 4 Case 5 Case 6 C Case Case 8 34

Incidence and Prevalence
= Start of the disease = Duration of the disease Incidence cases – 3, 4, 5, 8 Point prevalence (Jan 1) cases – 1, 2, 7 Point prevalence (Dec 31) cases – 1, 3, 5, 8 Period prevalence (Jan-Dec) cases – 1, 2, 3,4, 5, 7, 8. Number of cases of a disease beginning, developing and ending during the period

Incidence and Prevalence
A disease will have relatively lower prevalence than incidence if: It is of short duration either due to recovery or death. Decrease in Prevalence caused by: Decrease in incidence Decrease in duration (recovery or death)

Incidence and prevalence
The longer the duration the greater its prevalence. Example TB has high prevalence than incidence because: New cases keep cropping up through out the year. Old cases persist for months. 37

Relationship Between Incidence and Prevalence
Recovery

Relationship between Incidence and Prevalence
Prevalence depends on two factors: Incidence Duration of illness Assuming incidence and duration of illness are unchanging can be expressed as: Prevalence = Incidence x Duration of illness P = I x D 39

Relationship between Incidence and Prevalence
Example Incidence = 10 cases per 1000 pop per yr. Mean duration = 5 years. P = I x D = 10x5 = 50 cases per 1000 population =5%

Prevalence vs Incidence
Prevalence is the number of existing cases of disease in the population during a defined period. Incidence is the number of new cases of disease that develop in the population during a defined period.

Prevalence Increased by: Longer duration of the disease
Prolongation of life without cure Increase of new case (i.e. Incidence) In- migration of cases Out-migration of healthy people In-migration of susceptible people Improved diagnostic

Increased by: FACTORS INFLUENCING OBSERVED PREVALENCE RATE
1. Longer duration of disease 2. Prolongation of life of patients without cure 3. Increase new cases (incidence) 4. In-migration of cases 5. Out-migration of healthy people 6. In-migration of susceptible people 7. Improved diagnostic facilities (better reporting)

Prevalence Decreased by: Shorter disease duration
High case fatality from disease Decrease in new cases In-migration of healthy people Out-migration of cases Improved cure rate of case

Decreased by: FACTORS INFLUENCING OBSERVED PREVALENCE RATE
1. Shorter duration of disease 2. High case-fatality rate from disease 3. Decrease in new cases (incidence) 4. In-migration of healthy people Since prevalence rates are influenced by so many factors unrelated to disease causation, do not usually provide strong evidence causality Prevalence rates are used to measure the occurrence of chronic conditions 5. Out-migration of cases 6. Improved cure rate of cases

improved treatment vs. change in prevalence
Improvement in treatment Decrease in duration of illness prevalence If improved treatment Deaths prevented & no recovery Increase in prevalence 51

Uses of Prevalence Describe the current disease burden in a population i.e estimates the magnitude health/disease problems in a population Identifying potential high-risk population Useful for administrative, planning purposes and resource allocation Monitoring of control of Chronic disease control programmes such as HIV

b) Mortality Frequency Measures
Mortality Rates A measure of the frequency of occurrence of death in a defined population during a specified interval This is the number of deaths occurring in a specified population in a given time period

Mortality Rates Mortality rates and ratios measure the occurrence of deaths in a population using different ways. Rates whose denominators are the total population are commonly calculated using either the mid - interval population or the average population. This is done because population size fluctuates over time due to births, deaths and migration

Size of the pop among which deaths occurred
Mortality Rate Mortality Rate = 𝑿 𝒚 X 10n MR = # of deaths occurring during a given period X1000 Size of the pop among which deaths occurred The denominator mostly used is the size of the pop at the middle of the time period.

Mortality Frequency Measures
1. Crude Mortality Rate (CDR) This is the mortality rate from all causes of death for population Rates can also be calculated for sub-groups within the population For 10n, 1,000 or 100,000 can be used CDR = No. of deaths during the year X 1000 mid- year population

The crude death rates have a major disadvantage: Lack comparability for communities with pop that differ by age, sex, race, etc

Types of Mortality Rates
i) Cause-specific Mortality Rate This is Mortality Rate from a specific cause from a population Numerator is number of deaths attributed to a specific cause Denominator is the size of the pop at midpoint of the time period For 10n, 100,000 is be used = # of deaths from specific cause during time period X100,000 Estimated size of pop at midpoint of time period

ii) Age-specific Mortality Rate This is mortality rate limited to particular age group Numerator is the number of deaths in that age group Denominator is of persons in that age group in the population 𝑿 𝒚 X 10n = No. of deaths from a specific age during time period x100,000 No of persons in that specific age group in a population

iii) Infant Mortality Rate (IMR) This is the number of deaths of infants under 1 year in a given year per 1, 000 live births. IMR = # deaths of infants under 1 yr during time period x 1,000 Total number of live births during same period IMR is expressed per live births IMR is a ratio and not a proportion The infant mortality rate for Zambia is high and now stands at 45 deaths per 1000 live births (ZDHS ). That means out of 1000 live births about 45 die before they celebrate their first birth day.

Uses of Infant Mortality Rate (IMR)
It measures the risks of dying during the first year of life A high IMR indicates unmet health needs and poor environmental conditions Most commonly used as a measure for comparing health services among nations

iv) Neonatal Mortality Rate This is the number of deaths of children under 28 days of age during a in a given time period. = # deaths of babies under 28 days during time period x 1,000 Total number of live births during same period Expressed as per 1,000 live births

v) Under Five Mortality Rate This is the number of children who die before the age of 5 years Includes infant and neonatal mortality U5MR = No. of deaths 0>5 yrs of age during a given time x1000 Average (mid-year) population of >5 years at the same time Zambia Under five mortality is high, 75 deaths per 1000 live births (ZDHS )

vi) Maternal Mortality Rate This is the number of women who die as a result of complications of pregnancy or childbearing in a given year per 100,000 women of childbearing age in the population MMR=No pregnancy related deaths maternal deathsX100,000 No of women in child bearing aged 15 – 49 years

vii) Maternal Mortality Ratio (MMR) This is the number of women who die as a result of complications of pregnancy or childbirth in a given year per 100,000 live births in that year. MMR=No pregnancy related deaths maternal deaths X100,000 Total number of live births during same time MMR is expressed per 100,000 live births Numerator is the deaths related to pregnancy causes during that period Denominator is the number of live births during the same period MMR in Zambia high with 390 deaths per 100,000 live births ( Zambia Demographic Health Survey ). It has been reduced from 729 per 100,000 live births from , and 649 per 100,000 live births in 1996.

viii) Maternal Mortality Ratio (MMR) MMR is a ratio used to measure mortality associated with Pregnancy It measures the risks of dying from complications of pregnancy and child birth MMR reflects access to health care and social services Represents the risk associated with each pregnancy, i.e., the obstetric risk Maternal Mortality Rate reflects the standards of all aspects of maternal care (antenatal, delivery and postnatal). Factors associated with MMR includes maternal age, socio-economic status, nutritional status and access

ix) Case fatality rate (CFR) Case fatality represents killing power of a disease CFR = Total no. of deaths due to a particular disease x 100 Total number of cases due to the same disease CFR is a measure of the severity of the disease Expressed as a percentage 67

ix) Case fatality rate (CFR)
CFR is simply the ratio of deaths to cases CFR typically used in acute infectious diseases e.g. (food poisoning, cholera, measles) CFR of same disease may vary in different epidemics because of changes in host, environment or agent factors.

Use of Mortality rates Mortality rates are used to estimate disease frequency when: incidence data are not available, case-fatality rates are high, goal is to reduce mortality among screened or targeted populations

Use of Mortality rates Importance of specific rates
Useful when focusing on aetiology of disease. Common examples: Cause or disease specific – TB, Cancer, Malaria Group specific Age specific Sex specific Importance of specific rates Identification of groups at risk. Comparisons within and between populations.

Summary of Measures of Disease Frequency in Descriptive Epidemiology
Counts of disease and other health events are important in epidemiology Counts are the basis for disease surveillance and or allocation of resources Counts alone are not sufficient for describing the characteristics of pop and for determining risk. For these purposes we use ratios, proportions and rates as well as measures of central location and dispersion

Summary Ratios and proportions are useful for describing the characteristics of populations. Proportions and rates are used for quantifying Morbidity and Mortality From proportions, can predict risk among different groups, detect high risk groups and develop hypotheses about causes i.e why these groups are at increased risk

Summary The two measures of morbidity are incidence rates and prevalence Incidence rates reflects the occurrence of new diseases in the population: prevalence reflects the presence of disease in the population. To quantify the association between disease occurrence and possible risk factors or causes we commonly use two measures, relative risk and odds ratio

Summary Mortality rate are the standard measures of mortality, while years of potential life lost and years of potential life lost rate which focuses on premature and most preventable mortality All these measures are used when we perform the core epidemiologic task known as: Descriptive Epidemiology

Exercise “If a chicken and one-half lays an egg and one-half in a day and one-half, then how many eggs does one chicken lay in one day?”

Exercise What is the rate of egg-laying expressed in eggs per chicken-day? Express the rate as the number of eggs in numerator and number of chickens-days in denominator We have: eggs 1.5 chickens x1.5 days = eggs 2.25 chicken days = 2/3 egg per chicken-day

Types of Incidence and Prevalence Measures
Rate Type Numerator Denominator Mortality rate Incidence Number of deaths from a disease (or all causes) Person-years at risk in the population Infant mortality rate Number of deaths in a year of children less than 1 year of age N. Live births in the same period, usually per 1,000 annually Case-fatality rate Number of deaths from a disease Number of cases with that disease Attack rate Number of new cases of a disease Total population at risk, for a limited period of observation Period prevalence Prevalence Number of existing plus all new cases during given time period Total population (at risk)

Mpundu MKC MSc Epidemiology and Biostatistics, BSc Nursing, RM, RN

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