Epidemiology Part 3 (Public Health Measures, Measures of Risk)

Prof (Dr) RN Basu TOPIC Slide No. 9. Public health measures for control 101  Public health measures 101 10. Epidemic disease occurrence 106  Level of disease 106  Endemic level 106  Sporadic 107  Hyperendemic 107  Epidemic 107  Outbreak 108  Cluster 108  Pandemic 108 11. Measures of risk 111  Frequency measures 112  Ratio 113 ◊ Properties and uses of ratios 114  Proportion 116  Rate 121 ◊ Incidence rate 122 ◊ Prevalence rate 124 ◊ Case fatality rate 125  Morbidity frequency measures 125 Morbidity ◊ 127  Attack rate 135  Secondary attack rate 136  Years of potential life lost 143  Natality (Birth) Measures 152 • Public health measures • The knowledge about the portal of exit and method of transmission can decide on a public health strategy to control a disease • Some part of the chain of transmission can be more easily susceptible to intervention • Public health measures should preferably be directed to that part of the chain of transmission • Most appropriate intervention in some diseases is controlling the agent at the source

103 • A sick patient may be treated with suitable antibiotics to eliminate the infection • Similarly an infected but asymptomatic person can be treated to eliminate the infection and/or by isolation to eliminate the risk of transmission to others • Some interventions may be directed towards mode of transmission • For example, by isolating or advising the patient to avoid specific type of contact associated with transmission • Vehicleborne transmission may be intervened by elimination or decontamination of the vehicle • Faeco-oral transmission can be eliminated by rearranging environment or by behavioural change such as handwashing

104 • For airborne diseases, ventilation systems may be appropriately designed • Vector borne transmission can be interrupted by controlling the vector population, such as by preventing breeding • Some simple measures can be used to protect the portal of entry • For example, mosquito nets can be used to prevent mosquito bites • Use of masks can be effective for droplet and airborne infection to prevent entry into the respiratory system • Using long sleeve shirts and long pants can prevent mosquito and tick bites

105 • Infectious agents need a susceptible host for establishing infection in the host • Some public health measures can be directed towards this • Vaccinations are used for many diseases to improve the immunity status • Herd Immunity concept is another way to deny availability of a susceptible host in the community • The concept works this way: • If a large number of population is immune to a disease either by infection or by vaccination, then the agent will not be able to find a susceptible individual to infect

106 • The degree of herd immunity necessary to halt an outbreak varies with the disease • Herd immunity means not everyone needs to be immune to the disease to prevent spread of disease in an outbreak • Herd immunity has not prevented outbreaks of measles and rubella even when the immunization level is as high as 85% to 90% • It is seen that a minor cluster of non-immune persons may be staying in conditions dictated by socioeconomic or cultural factors • If the pathogen gains entry into these clusters outbreak may occur

107 • Level of disease • Endemic level • The amount of a particular disease that is present in a community is the endemic level or base line level • It is not the desired level. The desired level should be total absence of disease • If the level is not high enough in absence of intervention, this may continue to provide a pool of susceptible persons • Thus the baseline level is the expected level of the disease in the community

108 • Sporadic • Some rare diseases may occur in a community occasionally • These diseases occur infrequently or irregularly • In these cases, occurrence of a single case warrants an epidemiologic investigation • Hyperendemic • This refers to persistent high levels of disease occurrence • Epidemic • This refers to a sudden increase in the number of cases above what is normally expected in the population in that area

109 • Outbreak • Outbreak and epidemic are the same with the same definition • It is usually to a limited geographic area • Cluster • This refers to an aggregation of cases grouped in place greater than the number expected • Expected number may not be known • Pandemic • Refers to an epidemic that has spread over several countries or continents • Large number of people are affected

110 • Epidemic occurs when agent and susceptible hosts are present in large numbers • And also the agent can be easily transmitted from source to the susceptible host • The conditions favourable to occurrence of an epidemic are : • A recent increase in the virulence of the agent • The recent introduction of the agent into a setting where it has not been before • An enhanced mode of transmission so that more people are exposed

111 • A change in the susceptibility of the host response to the agent, and/or • Factors that increase host exposure or involve introduction throw new portal of entry • Up to this was for the infectious diseases • But non infectious disease can also occur in an epidemic proportion • Example: • Diabetes • Obesity • Road traffic accidents

112 Measures of Risk

113 • Frequency Measures • A measure of central location provides a single value that summarises the entire data in a single value • Measures of central locations are: • Mean, Mode and Median • But Frequency measures characterises only part of the distribution • Frequency measures compare one part of the distribution to another part of the distribution or to the entire part of the distribution • Common frequency measures are: ratio, proportion, and rates • All three frequency measures have the same basic form

114 • Ratio • A ration is the relative magnitude of the two quantities or a comparison of any two values • The numerator and denominator need not be related • Therefore one could compare apples with oranges • Method of calculating a ratio

115 • Properties and uses of ratios • Ratios are common descriptive measures, used in all fields • In epidemiology ratios are used as both descriptive measures and as analytic tools • As a descriptive measure ratios can describe, e.g., male-to-female participants in a study, or • Ratio of control to cases • As an analytic tool, ratios can be calculated for occurrences of injury, illness or death between two groups

116 • Usually the values of both the numerator and denominator of a ratio are divided by the value of one or the other • The purpose is that either the numerator or the denominator equals 1 • Commonly used epidemiologic ratio: death-to-case ratio • It is the number of deaths attributed to a particular disease during a specified period divided by the number of new cases identified during the same period • It is used as a measure of the severity of illness • Example: • Death to case ratio in rabies is 1 but death to case ration in common cold is 0

117 • Proportion • A proportion is the comparison of a part to a whole • It is a type of ratio in which the numerator is included in the denominator

118 • For a proportion, 10n is usually 100 (or n=2) • It is usually expressed as a percentage • Properties and uses of proportions • These are common descriptive measures used in all fields • In epidemiology it is usually used as descriptive measures • Example: 1. Proportion of persons enrolled in a study among all those eligible 2. Proportion of children administered polio vaccine

119 • In a proportion, the numerator must be included in the denominator • i.e., numerator is a subset of denominator • Example: • Number of apples divided by the number of oranges is not a proportion, but • Number of apples divided by total number of fruits of all kinds is a proportion • A proportion can be expressed as a fraction, a decimal, or a percentage • The statements “one fifth of the residents became ill” and “twenty percent of the residents became ill” are equivalent

120 • Proportions can easily be converted to ratios • Example: • Numerator is number of women attended clinic = 179 • Denominator all clinic attendees = 341 • Proportion of women attendees = 179/341, or 52% • To convert to a ratio who are not women = (341-179) = 162 (men) • Thus ratio of women to men could be calculated from the proportion as: • 179/(341-179) = 179/162 = 1.1 to 1 female to men ratio

121 • A specific type of epidemiologic proportion : proportionate mortality • Proportion of death in a specified population that are attributable to different causes • Each cause is expressed as a percentage of all deaths • Sum of all causes add up to 100% • These proportions are not rates because the denominator is all deaths, not the size of the population in which the death occurred

122 • Rate • In epidemiology, a rate is a measure of the frequency with which an event occurs in a defined population over a specified period of time • Rates put disease frequency in the perspective of the size of the population • Rates are particularly useful for comparing disease frequency in different locations, at different times, or among different groups of persons • The population may also be of different size • Thus, a rate is a measure of risk

123 • Incidence Rate • There is some variation amongst epidemiologists how the term rate is used • Like a layman’s use of the term rate, some epidemiologists use the term in the same sense • A non-epidemiologist by the term rate means how fast something is happening or going • E.g., the speedometer of a car • The speedometer indicates how fast the car is moving, i.e., kilometre per hour • This rate is always mentioned per some unit of time • These epidemiologists use rate to mean how quickly a disease is occurring in a population

124 • This measure conveys a sense of speed with which disease occurs in a population • Example: • Number of new cases of prostate cancer per year in men above the age of 65 years • Other epidemiologists use the term a bit loosely • They refer the term to mean a proportion with • Number of cases in the numerator, and • Size of the population in the denominator

125 • Attack rate • The proportion of population that develops illness during an outbreak • Example: • 30 children of a class of 60 in a school developed diarrhoea after eating mid day meal • More accurate phrase for attack rate is incidence proportion • Prevalence rate • A prevalence rate is the proportion of the population that has a health condition at a point in time • Example: • 30 cases of hypothyroidism in December 2019 in locality A

126 • Case-fatality rate • It is the proportion of patients died of a particular disease • Example: • One death due to meningitis in the population of locality A • All of these are proportions • None are expressed per units of time • Therefore many do not consider these as “true” rates • However these these terminologies are used widely

127 Condition Ratio Proportion Rate Morbidity Risk ratio Attack rate Person-time incidence rate (Relative risk) (Incidence proportion) Odds ratio Secondary attack rate Period prevalence Point prevalence Attributable proportion Mortality Death-to-case-ratio Proportionate mortality Crude mortality rate (Death) Case fatality rate Case-specific mortality rate Age-specific mortality rate Maternal mortality rate Infant mortality rate Natality Crude birth rate (Birth) Crude fertility rate 128 • Morbidity Frequency Measures • Morbidity • Any departure, subjective, or objective, from a state of physiological or psychological well being • It encompasses: • Disease, injury, and disability • It can also be used to describe the periods of illness that these persons experienced, or • The duration of these illnesses

129 • Measures of morbidity frequency is the number of persons in a population who become ill (incidence),or • Are ill at a given time (prevalence) • Frequently used measures of morbidity are: • Incidence proportion • Secondary attack rate • Incidence rate • Point prevalence • Period prevalence

130 Measures Numerator Denominator Incidence proportion Number of new cases of disease Population at start of time interval (or attack rate or risk) during specified time interval Secondary attack rate Number of new cases among Total number of contacts contacts Incidence rate (or person-time rate) Number of new cases of disease Summed person-years of during specified time interval observation or average population during time interval Point prevalence Number of current cases (new and Population at the same specified pre-existing) at a specified point in point in time time Period prevalence Number of current cases (new and Average or mid-interval population pre-existing) over a specified period of time

131 • Incidence • Three types of uses of this term • Occurrence of new cases of disease or injury in a population over a specified period of time • Also used to mean the number of new cases in a community, • Also it means the number of new cases per unit of population • Two types of incidence are commonly used: 1. Incidence proportion 2. Incidence rate

132 • Incidence proportion or risk • It is the proportion of initially disease-free population that develops disease, becomes injured, or dies during a specified (usually limited) period of time • Synonyms include: • Attack rate • Risk • Probability of getting disease • Cumulative incidence

133 134 • Properties and uses of incidence proportion • It is a measure of risk of disease, or • The probability of developing the disease during the specified period • As a measure of incidence, it includes only new cases of disease in the numerator • The denominator is the number of persons in the population at the start of the observation period • This measure is considered a proportion because all of the persons with new cases of the disease (numerator) are also represented in the denominator • A risk is also a proportion

135 • More about denominators • The denominator of an incidence proportion is the number of persons at the start of the observation period • It should be limited to the population at risk for developing the disease • It means the persons who have the potential to get the disease and be included in the numerator • Example: • Say, the numerator represents the new cases of cancer of ovaries • The denominator should be restricted to women • Because, men do not have ovaries

136 • Attack rate • In the outbreak setting, this term is used as a synonym for risk • It is the risk of getting the disease during a specified period • Different types of attack rates are calculated • Overall attack rate • It is the total number of new cases divided by the total population • Food-specific attack rate • Number of persons who ate a specified food and became ill divided by the total number of persons who ate that food

137 • Secondary attack rate • This rate is sometimes calculated to document the community transmission of illness versus illness in a household, barrack, or other closed population • It is calculated as:

• Often the total number of contacts in the denominator is calculated as the total households of the primary cases minus the number of primary cases • For secondary attack rate, 10n usually is 100%

138 • Example: Calculating Secondary Attack Rates • Consider an outbreak of shigellosis • 18 persons in 18 different households became ill • Population of the community is 1000 • Overall attack rate = 18 / 1000 x 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 then • Secondary attack rate = (17 / (86-18)) x 100 % = (17/680 x 100% = 25%

139 • Incidence rate • Also called person-time rate • It is a measure of incidence that incorporates time directly into the denominator • It is generally calculated from a long-term cohort follow-up study • In this type of study, the enrolees are followed over time and the occurrences of new cases are documented • There are four end points in this study: 1. Onset of disease 2. Death 3. Migration (lost to follow up), and 4. The end of the study

140 • The incidence is the ratio of the number of cases to the total time the population is at risk of the disease • Formula:

• In a long-term follow-up study of morbidity, each participant may be followed up for several years • One person followed up for 5 years contribute 5 person-years • A person lost to the study is considered contributing ½ year to the denominator in the year the participant was lost

141 • Properties of incidence rates • This rate describes how quickly disease occurs in a population • Person-time is calculated for each person individually • It can accommodate persons leaving or joining the study at any time during the study period • There is a drawback, it assumes that each person has the same probability of disease during the study period is constant • This assumption may not be valid in some chronic diseases as the risks increases with age

142 • This type of long-term cohort study are not common • An easier method of calculating incidence rates is based on numerator of cases observed or reported • The denominator is the mid-year population • Both the incidence rate and person-time rate are comparable • Understanding of person-time rate by non-epidemiologist common people poses some difficulty • Example: Incidence rate say of heart disease 2.5 per 1000 person-years is not easily understood • To make it understandable, person-years may be replaced with persons per year

143 • Prevalence • Sometimes referred to as prevalence rate • It is the proportions of persons in a population who have a particular disease or attributes at a specified point in time or over a specified period of time • Prevalence differs from incidence • Prevalence includes all cases both new and pre-existing • Incidence is limited to only new cases only • Point prevalence • Prevalence measured at a particular point in time (particular date)

144 • Period prevalence • It is measured over an interval of time • It is the proportion of persons with a particular disease or attribute at any time during the interval • Formula:

• The value of 10n is usually 1 or 100 for common attributes. This value may be 1,000, 10,000 or even 100,0000 for rare attributes and for most diseases

145 • Properties and uses of prevalence • The terms incidence and prevalence sometimes may be confusing • Proportion refers to persons who have the disease at or during a particular time period • Incidence refers to persons who develop the disease or condition during a particular time period • The key difference is in their numerator • Prevalence is bases on both incidence and duration of illness • High prevalence of a disease within a population might reflect high incidence or prolonged survival without cure or both

146 • Low prevalence might indicate low incidence, low incidence, a rapid fatal process, or rapid recovery • Prevalence rather than incidence is often measured for chronic diseases such as diabetes or hypertension • These have long duration and the dates of onset are difficult to pinpoint • Mortality frequency measure • Mortality rate • It is a measure of the frequency of occurrence of death in a defined population during a specified interval • Mathematically, mortality and mortality measures are the same

147 • It only depends on what is being measured, illness or death • The Formula is:

• When mortality rates are based on vital statistics (No of death certificates) the denominator most often used is the size of the population at the middle of the time period • in India, the value of 10n most often used is 1000 • Frequently used mortality measures are:

148 Measure Numerator Denominator 10n Crude Death Total number of deaths during a given Mid interval population 1,000 or Rate time interval 100,000 Case-specific Number of deaths assigned to a specific Mid-interval population 100,000 death rate cause during a specific time interval Proportionate Number of deaths assigned to a specific Total number of deaths from all 100 or mortality cause during a given time period causes during the same time 1000 period Death to case Number of deaths assigned to a specific Number of new cases of same 100 ratio cause during a specific time interval disease reported during the same time interval Neonatal Number of deaths among children < 28 Number of live births during the 1,000 mortality rate days of age during a given time interval same time interval Post neonatal Number of deaths among children 28-364 Number of live births during the 1,000 mortality rate days of age during a given time interval same time interval Infant mortality Number of deaths among children Number of life births during the 1,000 rate same time interval Maternal Number of deaths assigned to pregnancy- Number of births during the same 100,000 149 mortality rate related causes during a given time time interval interval • Years of potential life lost (YPLL) • This is an impact of premature death on population • In India, life expectancy in 2020 is 69.73 • If a person dies before this age, then some years of potential life is lost • There are other measure that incorporate disability and other measures of quality of life • YPLL is calculated as the sum of differences between life expectancy and the ages of death before the age of life expectancy • Another end point sometimes used is the 65 years of age

150 • YPLL is affected by these methods of calculation • More weight is given to earlier age of death • Thus death at older age is ”devalued” • YPLL rate represents years of potential life lost per 1000 population below the end point age, such as 65 years • YPLL rates should be used to compare premature mortality in different populations • YPLL does not take into account difference in population sizes

151 • Natality measures are population measures of birth • These measures are used primarily in the field of maternal and child health Frequently Used Measures of Mortality Measure Denominator 10n Crude birth rate Number of live births during a Mid-interval population 1,000 specified time interval Crude fertility Number of live births during a Number of women aged 15-44 years 1,000 rate specified time interval at mid-interval Crude rate of Number of live births minus number Mid-interval population 1.000 natural increase of deaths during a specified time interval Low-birth weight Number of live births <2,500 grams Number of life births during the 100 ratio during a specified time interval same time interval

152 End of Part 3