Introduction To Epidemiology PDF

INTRODUCTION TO EPIDEMIOLOGY By Dr S H Nzala  Introduction to Epidemiology Health A state of complete physical, mental, and social well- being and not merely the absence of disease or infirmity - World Health Organization, 1948 Public Health Is an effort orga...

INTRODUCTION TO EPIDEMIOLOGY By Dr S H Nzala  Introduction to Epidemiology Health A state of complete physical, mental, and social well- being and not merely the absence of disease or infirmity - World Health Organization, 1948 Public Health Is an effort organized by society to protect, promote, and restore the health of the population  Disciplines of Medicine Basic sciences: anatomy, physiology, pathology etc. Clinical sciences: paediatrics, obstetrics etc - concerned with care for individual patient Public health – community replaces the individual  Background of Epidemiology Epidemiology draws upon the medical, biological, and behavioral sciences (anthropology, psychology, sociology, and education), as well as statistics, demographics health and medical care services and computer sciences.  Background We can study health and disease by -Observing effects on individuals Laboratory investigation of experimental animals Measuring the distribution of health problems in the population  Origin “Epidemiology” from Greek : Epi = upon; Demos = people ‘epidemic’ = “upon the people” , Logos = study Epidemiologists first concern was to investigate, control and prevent epidemics Two basic assumptions about disease: Disease does not occur at random Disease has causal and preventive factors  Definition of Epidemiology Study of the distribution and determinants of health-related states and events in specified populations and the application of this study to the control of health problems We can also say that it is an analytical tool for assessing the effectiveness of medical intervention and health care delivery Epidemiology is the basic science of public health that focuses on the population at large Specialty disciplines: Pharmacoepidemiology, Clinical Epi, Psycho-Social Epi, Nutritional Epi, Molecular Epi, Genetic Epi, Cancer Epi, Environmental Epi, Occupational Epi, etc.,  Scope of epidemiology Shift from infectious diseases initially to chronic diseases of later life and now applied to other areas e.g. -injuries - adverse drug reactions -mental illness -family planning -health services research etc.  Scope of epidemiology Shift also to evaluation of certain exposures (chemicals, ionising radiation) Evaluation of effects of new approaches to prevention Evaluation of treatment effectiveness etc Organization of health care  Scope of epidemiology Epidemiology not only concerned with epidemics but also with inter-epidemic periods and with sporadic and endemic occurrences of diseases  Classification Two broad categories: a) Descriptive epidemiology: the study of the frequency (amount) and distribution of health related states within a population by person, place and time b) Analytic epidemiology: more focused study of health related problems or reasons for relatively high or low frequency in specific groups.  Epidemiological questions To describe the occurrence of disease fully, some broad questions must be asked: a) Who is affected? b) When do the cases occur? c) Where do the cases occur? Other questions can be What health events are occurring Why is it occurring How can it be influenced  Epidemiology Definition (cont’d) Epidemiology Distribution characterizing the distribution of health status in terms of age, gender, race etc. Determinant any factor that brings about a change in a health condition or other defined outcomes.  Epidemiology Definition (cont’d) Epidemiology Disease deviation from physical, mental or emotional health expands to include conditions such as injuries, birth defects, health outcome etc. Population group of people often geographically defined Determinants of disease occurrence: includes both causes and factors that influence the risk of disease  Determinants of disease Disease is as a result of the epidemiologic triad (Agent, Host and Environment) Infection occurs only when the AGENT is encountered by a susceptible Host in an ENVIRONMENT that is favourable  HOST VECTOR ENVIRONMENT AGENT The EpidemiologicTriad  Distribution of disease Frequencies of values or categories of measurement with respect to time, place and persons  Frequency Involves measuring disease distribution Requires information - count (quantification) - size of population - time period Involves measuring disease distribution Requires mathematical calculation - ratio - proportion - rate  Uses of Epidemiology: Historical study - is community health getting better or worse? We can only decide by comparing experiences over time. Community diagnosis Working of health services - availability, accessibility, utilization, effectiveness, efficacy, efficiency  Uses of Epidemiology: Individual risks and chances of getting disease Completing the clinical picture - constructing a model Identification of syndromes “lumping and splitting” Search for causes Evaluation of presenting signs/symptoms of disease - by analysing data in hospital charts Clinical decision making - involves use of decision trees  Specific objectives of Epidemiology: to identify aetiology of disease and risk factors to determine extent of disease found in the community (disease burden) to study the natural history and prognosis of disease to evaluate new preventive and therapeutic measures and new modes of health care delivery to provide foundation for developing public policy and regulatory decisions relating to environmental problems. Epidemiology: a scientific tool –To describe –To understand –To propose and test hypotheses –To validate or challenge public policy MEASURES OF DISEASE OCCURRENCE MORBIDITY MEASURES Frequency: Disease Occurrence N u m e r a t o r ( events, cases ) Proportion Ratio Rate Event x Time D e n o m i n a t o r ( sample, population )  MORBIDITY MEASURES RATIO: expresses relationship between two numbers e.g. x:y or x/y X k. example male: female PROPORTION: This is a specific type of ratio in which the numerator is included in the denominator and the resultant value is expressed as a percentage  MORBIDITY MEASURES RATE is a special form of proportion that includes specification of time. CRUDE RATE: Summary rates based on the actual number of events in total population over a given time period. SPECIFIC RATES : Summary rates based on the actual number of events in a population subgroup over a given time period.  ADJUSTED RATES To adjust is to “remove the effect of …” ADJUSTED RATES are rates that have undergone statistical transformation to permit fair comparison between groups differing in some characteristics that may affect risk of disease.  INCIDENCE INCIDENCE measure the probability that health people will develop a disease during a specific period of time; i.e. it is the number of new cases of a disease in a population over a period time IR =No. of new cases of disease x time p. Population at risk  CALCULATING INCIDENCE - REQUIREMENTS Need to follow prospectively a defined group of people and determine the rate at which new cases appear  CALCULATING INCIDENCE - REQUIREMENTS a) Knowledge of the health status of the study population: b) Determine time of onset c) Specification of numerator i.e. number of persons versus number of conditions. If numerator is not specified then it is taken as persons (risk per person)  CALCULATING INCIDENCE - REQUIREMENTS d) Specification of denominator – always a defined population - Use population at midpoint for time - Only those at risk form the denominator e) Always state definite period of time. (period of observation)  PREVALENCE Measures the number of people in a population who have the disease at a given time PR =No. existing cases of disease x time Total Population Prevalence depends on a) The number of people who have been ill in the past (previous incidence) b) The duration of their illness  PREVALENCE POINT PREVALENCE – measures the probability of people having a disease at a given point in time PERIOD PREVALENCE = Prevalence at a point in time, plus new cases (incidence) and recurrences during a succeeding period (e.g. one year)  REQUIREMENTS FOR CALCULATING PREVALENCE Similar to those for incidence EXCEPT a) Knowledge of time of onset is not required b) Denominator always includes the entire related population since numerator contains both old and new cases  Measures at Population Level incidence prevalence All (old + new) cases mortality emigrations + recoveries (in a dynamic/population model)  USES OF PREVALENCE Determine workload, especially in chronic diseases Planning of facilities and manpower needs To express the burden of some attribute or condition in a population To monitor control programmes for chronic conditions; e.g. mental illness (reflects duration as well as incidence) To estimate importance of a disease in a population (calculation of incidence)  USES OF PREVALENCE Useful in tracking changes in disease patterns over time (can determine point prevalence by a series of cross sectional surveys  USES OF INCIDENCE Fundamental tool for aetiologic studies of both acute and chronic diseases (as direct indicator of risk of disease) it is the basis for statements about probability or risk of disease NB: Incidence is direct measure of risk. High prevalence does not necessarily signify high risk. (it may signify increase in survival) Low prevalence may reflect a rapidly fatal process or rapid cure of disease as well as low incidence.  INCIDENCE AND SCREENING First screening picks up both prevalent as well as incident cases of diseases Re-screening detects only incident cases (I.e. those that developed disease between first and subsequent screens).  Measures of Mortality Mortality refers to death (cf morbidity for illness)  MORTALITY RATES Annual (crude) mortality rate =Total number of deaths from all causes in one year/number of persons in the population at midyear x 1,000 Age-specific mortality rate =Number of deaths from all causes in children under five years old in one year/number of children less than 5 years old in the population at midyear x 1,000  MORTALITY RATES Disease or cause-specific mortality rate =Number of deaths from cancer in one year/number of persons in the population at midyear x 1,000 Case-Fatality rate (percent) =Number of individuals dying during a specified period of time after disease onset or diagnosis/individuals with the specified disease x 100  MORTALITY RATES Case-Fatality rate (percent) =Number of individuals dying during a specified period of time after disease onset or diagnosis/individuals with the specified disease x 100  Example: Case Fatality Rate Assume a population of with 1000 deaths in the year. In one year, 20 individuals became sick with cholera and 6 died from the disease The cause-specific cholera mortality rate for that year was: 6/1000 = 0.006 = 0.6% The case-fatality rate from cholera for that year was: 6/20 = 0.3 = 30%  MORTALITY is an index of the severity of a problem both from the clinical and public health standpoint may also be used as an index of risk of disease e.g. when case fatality is high and when the duration of illness (survival) is short. DESCRIPTIVE EPIDEMIOLOGY PERSON, PLACE AND TIME  DESCRIPTIVE EPIDEMIOLOGY Examining the distribution of disease in a population in terms of descriptive characteristics in order to: 1) Identify subgroups at highest risk 2) Find clues about possible causes (hypothesis generating studies) In order to describe the occurrence of disease fully, it is necessary to specify Person, Place and Time  DESCRIPTIVE EPIDEMIOLOGY Descriptive epidemiology identifies nonrandom variations in the distribution of disease to enable an investigator to generate testable hypotheses regarding aetiology.  PERSON: Three characteristics are of vital importance: Age, Sex and Ethnic group or race. e.g. death rates fairly high in infancy, decreasing to reach lowest point between ages 5 - 14 and climbs gradually up to age 40. Thereafter - exponential.  PERSON: In general chronic conditions tend to increase with age. Frequency and Severity of disease may also depend on age. Age-sex distribution of some diseases may reflect occupational exposure.  PERSON: Sex Death rates tend to be higher in males than females but morbidity rates are generally higher in females. Depression: Much higher in females than in males  Person – Race/ethnicity Race/ethnicity Major differences exist between culture, behavior, health events and related activities with subsequent impact on disease and mortality patterns It has to be considered in all analysis of studies and its effect controlled for  PERSON: (Ethnic Group and Race) Common in the USA where we have large numbers of racial populations.  PERSON: Classifying disease by race has been controversial, but a) Many diseases differ markedly in frequency, severity, or both in different racial groups. b) Statistics by race are helpful for identifying health problems. Race may be an indicator of groups with particular deficiencies in health care.  PERSON: Ethnic Group and Race: Environmental factors, however, are still important in the aetiology of some diseases e.g. cancer of the stomach in Japanese. Other variables would include: a) social class (education, area of residence, income, life style etc) b) b) Occupation,  PERSON- Other variables: c) Marital status- death rates vary from lowest to highest in the order: married, single, widowed, and divorced  Person – Family Variables a) Family size - associated with social class, b) Birth order - first borns at higher risk of asthma, schizophrenia, peptic ulcer and pyloric stenosis c) Maternal age - e.g congenital abnormalities d) Parental deprivation - e.g psychiatric and psychosomatic disorders, attempted suicide  PERSON: Other variables a) Blood Type- e.g ‘A’ - increased risk of gastric cancer ‘O’ - increased risk of duodenal ulcer etc. b) Environmental exposures chemicals (tobacco, asbestos), infectious diseases (specific immunity)  PLACE May be Geographical, urban- rural differences etc. Urban vs. Rural Occupational exposures Stress Pollution Self-selection Population density Habits  TIME Occurrence usually expressed on a monthly or annual basis Major types of variation over time are: secular, cyclic, and short term fluctuations.  TIME Secular trends: these are long term variations- years or decades. Cyclic trends : recurrent alterations in the frequency of disease. Cycles may be annual (seasonal) or have other periodicity.