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

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