Epidemiological Measures Lecture Notes PDF

By Dr. Ashraf Hussain MBChB. PhD/Com.Med Epidemiology is the study of the distribution and determinants of health-related states or events in a specified population, and the application of this study to the control of these health problems.  Epidemiology is concerned with the frequency and...

By Dr. Ashraf Hussain MBChB. PhD/Com.Med Epidemiology is the study of the distribution and determinants of health-related states or events in a specified population, and the application of this study to the control of these health problems.  Epidemiology is concerned with the frequency and pattern of health events in a population:  Frequency refers not only to the number of health events such as the number of cases of meningitis or diabetes in a population, but also to the relationship of that number to the size of the population.  The resulting rate allows epidemiologists to compare disease occurrence across different populations.  Pattern refers to the occurrence of health-related events by person, place, and time.  Time patterns may be annual, seasonal, weekly, daily, hourly, weekday versus weekend, or any other breakdown of time that may influence disease or injury occurrence.  Place patterns include geographic variation, urban/rural differences, and location of work or schools.  Personal characteristics include demographic factors which may be related to risk of illness, injury, or disability such as age, sex, marital status, and socioeconomic status, as well as behaviors and environmental exposures.  All these are supplied by the descriptive epidemiology  Epidemiology is also used to search for determinants, which are the causes and other factors that influence the occurrence of disease and other health-related events.  epidemiologists use analytic epidemiology or epidemiologic studies to provide the “Why” and “How” of such events.  What = health issue of concern  Who = person  Where = place  When = time  Why/how = causes, risk factors, modes of transmission  The pattern of disease may be described by the time of its occurrence, i.e., by week, month, year, the day of the week, hour of onset, etc.  It raises questions whether the disease is seasonal in occurrence; whether it shows periodic increase or decrease; or whether it follows a consistent time trend.  Such studies may yield important clues about the source or aetiology of the disease, thereby suggesting potential preventive measures. Three kinds of time trends or fluctuations in disease occurrence. I. Short-term fluctuations II. Periodic fluctuations, and III. Long-term or secular trends  The best known short-term fluctuation in the occurrence of a disease is an epidemic.  Epidemic is defined as "the occurrence in a community or region of cases of an illness or other health-related events clearly in excess of usual expectancy".  Epidemicity is thus relative to usual frequency of the disease in the same area, among the specified population, at the same season of the year. Three major types of epidemics: A. Common-source epidemics (single sourse) 1. Single exposure or "point-source" epidemics. 2. Continuous or multiple exposure epidemics B. Propagated epidemics 1. Person-to-person 2. Arthropod vector 3. Animal reservoir C. Slow (modern) epidemics  Also known as "point-source" epidemics.  The exposure to the disease agent is brief and simultaneous.  The epidemic tends to be explosive, there is clustering of cases within a narrow interval of time.  The resultant cases all develop within one incubation period of the disease  An example is an epidemic of food poisoning.  The exposure from the same source been prolonged - continuous, repeated or intermittent – not necessarily at the same time or place.  A prostitute may serve as a common source of a gonorrhoea outbreak, but since she will infect her clients over a period of time there may be no explosive rise in the number of cases.  This outbreak will continue beyond the range of one incubation period.  A propagated epidemic is most often of infectious origin  Results from person-to-person transmission of an infectious agent (e.g., epidemics of hepatitis A and polio).  Transmission continues until the number of susceptibles is depleted or susceptible individuals are no longer exposed to infected persons or intermediary vectors.   The speed of spread in propagated epidemics depends upon 1. Herd immunity, 2. Opportunities for contact 3. Secondary attack rate. Herd Immunity  Herd immunity is defined as the 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.  The presence of a large proportion of immune persons in the population lessens the likelihood that a person with the disease will come into contact with a susceptible individual.  The presence of a large proportion of immune persons in the population lessens the likelihood that a person with the disease will come into contact with a susceptible individual  When we carry out immunization programs, it may not be necessary to achieve 100% immunization rates to immunize the population successfully.  We can achieve highly effective protection by immunizing a large part of the population; the remaining part will be protected because of herd immunity.  Seasonal variation is a well-known characteristic of many communicable diseases  For example, measles is usually at its height in early spring and so is varicella.  Upper respiratory infections frequently show a seasonal rise during winter months.  Bacterial gastrointestinal infections are prominent in summer months because of warm weather and rapid multiplication of flies.  Noninfectious diseases and conditions may sometimes exhibit seasonal variation, e.g., sunstroke, hay fever, snakebite.  Some diseases occur in cycles spread over short periods of time which may be days, weeks, months or years.  For example, measles in the prevaccination era appeared in cycles with major peaks every 2-3 years and rubella every 6-9 years.  This was due to naturally occurring variations in herd immunity.  Non-infectious conditions may also show periodic fluctuations, e.g., automobile accidents are more frequent on working days than week-ends.  The term "secular trend" implies changes in the occurrence of disease (i.e., a progressive increase or decrease) over a long period of time, generally several years or decades.  Secular trend implies a consistent tendency to change in a particular direction or a definite movement in one direction.  Examples include coronary heart disease, lung cancer and diabetes which have shown a consistent upward trend in the developed countries during the past 50 years or so, associated with a decline of such diseases as tuberculosis, typhoid fever, diphtheria and polio. Geographic patterns provide an important source of clues about the causes of the disease. Comparisons may examine mortality and morbidity in relation to socio-economic factors, dietary differences and the differences in culture and behaviour. These variations may be classified as : a. International variations b. National variations c. Rural-urban variations d. Local distributions  Chronic bronchitis, accidents, lung cancer, cardiovascular diseases, mental illness and drug dependence are usually more frequent in urban than in rural areas.  On the other hand, skin and zoonotic diseases and soil-transmitted helminths may be more frequent in rural areas than in urban one.  These variations may be due to differences in population density, social class, deficiencies in medical care, levels of sanitation, education and environmental factors. The disease is further characterized by defining the persons who develop the disease by age, sex, occupation, martial status, habits, social class and other host factors.  Age: measles in childhood, cancer in middle age and atherosclerosis in old age.  Sex or Gender: Certain chronic diseases such as diabetes, hyperthyroidism and obesity are strikingly more common in women than in men, and diseases such as lung cancer and coronary heart disease are less frequent in women.  Below are three key terms taken from the definition of epidemiology, followed by a list of activities that an epidemiologist might perform. Match the term to the activity that best describes it. You should match only one term per activity. A. Distribution B. Determinants C. Application _B___ 1. Compare food histories between persons with Staphylococcus food poisoning and those without __B__ 2. Compare frequency of brain cancer among anatomists with frequency in general population ___A_ 3. Mark on a map the residences of all children born with birth defects within 2 miles of a hazardous waste site __A__ 4. Graph the number of cases of congenital syphilis by year for the country _C___ 5. Recommend that close contacts of a child recently reported with meningococcal meningitis receive Rifampin __A__ 6. Tabulate the frequency of clinical signs, symptoms, and laboratory findings among children with chickenpox in Cincinnati, Ohio  It refers to the constant presence of a disease or infectious agent within a given geographic area or population group, without importation from outside;  may also refer to the "usual" or expected frequency of the disease within such area or population group.  The term "hyperendemic" expresses that the disease is constantly present at a high incidence and/or prevalence rate and affects all age groups equally;  An endemic disease when conditions are favourable may burst into an epidemic (e.g., cholera, measles).  As new control or preventive measures are applied, the endemic status of a disease may change.  The word sporadic means scattered  The cases occur irregularly, haphazardly from time to time, and generally infrequently e.g., polio, tetanus, herpes-zoster and meningococcal meningitis.  A sporadic disease may be the starting point of an epidemic when conditions are favourable for its spread.  Many zoonotic diseases are characterised by sporadic transmission to man.  The "unusual“ occurrence in a community or region of disease, specific health-related behaviour (e.g., smoking) or other health related events (e.g., traffic accidents) clearly in excess of "expected occurrence".  Some use the term "outbreak" for a small, usually localized epidemic.  The above definition covers not only the usual epidemic diseases such as measles, chickenpox and cholera which are compressed in time, but also the modern "slow" epidemics of non- communicable diseases (e.g., CHO, lung cancer) in which the time scale of the epidemic is shifted from days or weeks to years. Figure shows the trend and epidemic of measles in Iraq 2000-2018  An epidemic usually affecting a large proportion of the population occurring over a wide geographic area or all over the world  Control means reduction of disease incidence, prevalence, morbidity, and/or mortality to a locally acceptable level as a result of deliberate efforts.  Continued intervention measures are required to maintain the reduction.  Elimination of transmission (also referred to as interruption of transmission) to mean reduction to zero of the incidence of infection caused by a specific pathogen in a defined geographical area, with minimal risk of reintroduction, as a result of deliberate efforts;  Continued actions to prevent re-establishment of transmission may be required.  Eradication to mean permanent reduction to zero of a specific pathogen, as a result of deliberate efforts, with no more risk of reintroduction.  Extinction to mean eradication of the specific pathogen so that it no longer exists in nature or the laboratory, which may occur with or without deliberate efforts.  The Centers for Disease Control and Prevention (CDC) defined Epidemiologic Surveillance as the  “Ongoing systematic collection, analysis, and interpretation of health data essential to the planning, implementation, and evaluation of public health practice closely integrated with the timely dissemination of these data to those responsible for preventing and controlling disease and injury”  Because surveillance can directly measure what is going on in the population, it is useful both for measuring the need for interventions and for directly measuring the effects of interventions. The purpose of surveillance is to empower decision makers to lead and manage more effectively by providing timely, useful evidence.  Passive surveillance denotes surveillance in which available data are used in which disease reporting is mandated or requested by the government or the local health authority, with the responsibility for the reporting often falling on the health care provider or district health officer.  The completeness and quality of the data reported thus largely depend on this staff, who often take on this role without additional funds or resources.  As a result, underreporting and lack of completeness of reporting are likely.  to minimize this problem, the reporting instruments must be simple and brief.  Examples of reportable diseases include common sexually transmitted infections (syphilis, gonorrhea  Active surveillance: a system employing staff members to regularly contact heath care providers or the population to seek information about health conditions. Active surveillance provides the most accurate and timely information, but it is also expensive. The information collected is then used in a number of ways to 1. Evaluate the effectiveness of control and preventative health measures 2. Monitor changes in infectious agents e.g. trends in development of antimicrobial resistance 3. Support health planning and the allocation of appropriate resources within the healthcare system. 4. Identify high risk populations or areas to target interventions 5. Provide a valuable archive of disease activity for future reference.  Monitoring is the performance and analysis of routine measurements aimed at detecting changes in the environment or health status of the population.  Monitoring can be of variety of activities like: Health program Air quality Water quality Intrauterine growth Nutritional status Functioning of a health facility etc