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Epidemiology Introduction


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Epidemiology Introduction

  1. 1. Mr. Kuldeep Vyas Asst.professor – EPIDEMIOLOGY
  2. 2. INTRODUCTION • Epidemiology is derived from Greek word “epidemic” EPI – upon DEMOS – people LOGOS – study • Epidemiology means the study of disease in human population.
  3. 3. Cont... • The Greek physician Hippocrates is sometimes said to be the father of epidemiology. He is the first person known to have examined the relationships between the occurrence of disease and environmental influences. HIPPOCRATES
  5. 5. Definition of Epidemiology The study of frequency, distribution and determinants of diseases is known as epidemiology. (International epidemiological association) “Epidemiology is defined as the study of the distribution and determinants of health related states or events in specified populations and the application of this study to control the health problems”. LAST-(September 22, 1926 - September 11, 2019)
  6. 6. Key Words of Epidemiology • Epidemic : ( Epi = upon : demos = people ) An outbreak of disease in a community in excess of “normal expectation ” • Endemic: (En = in; demos = people). The constant presence of disease within a geographic area or the usual prevalence of a given disease in a particular area. malaria, tuberculosis, etc. • Pandemic: (Pan = all: demos = people) An epidemic which spreads from country to country or over the whole world, as for example, the recent epidemic ofAIDS.
  8. 8. Aims of Epidemiology • To describe the distribution and size of disease problems in human population. • To identify etiological factors in the pathogenesis of disease. • To provide data essential to the planning, implementation, and evaluation of services for the prevention, control and treatment disease and to setting up the priorities among these services.
  12. 12.  Search of cause / causes of diseases  Helps to describe the health status of population  Helps to discover and bridge gaps in natural history of diseases.  Helps in controlling diseases  Helps in better management of health services  To study the effects of disease state in a population over a time and predict future health.  To diagnose the health of the community.  To evaluate health services.  To estimate the individual risk from group experiences.  To identify the syndrome.  To search for causes of disease. USE OF EPIDEMIOLOGY
  14. 14. The entry and development or multiplication of an infectious agent in the body of man or animal. INFECTION
  15. 15. ENDEMIC  The constant presence of a disease or within ainfectious geographical agent area or population given group, without importation from outside. Eg. Common cold.
  17. 17. SPORADIC  The cases occurs irregularly, haphazardly from time to time and generally infrequently. E.g.. Polio
  18. 18. PANDEMIC  An which from epidemic spreads country to country or over the whole world. Eg. AIDS
  19. 19. ZOONOSES  Diseases or infections which are naturally between transmitted vertebrate animals and men. eg. Rabies, plague
  20. 20. INCUBATION PERIOD  This is the time interval between the entry of the disease agent into the body and the appearance of first sign and symptom of the disease.
  21. 21. CARRIER  A person harbors the who disease agent without having any outward signs and symptoms.
  22. 22. ISOLATION  A separation person infectious of a with disease contact human with beings from other for the period of communicability.
  23. 23. PATHOGENICITY cause Ability to the disease.
  24. 24. CONTAGIOUS that is through A disease transmitted contact. Eg. STD and Leprosy.
  25. 25. RESERVOIR  The habitat ( place for living) where an infectious living, grows agent and multiplies.
  26. 26. VIRULENCE  Measure the severity of disease.
  27. 27. FOMITES  Inanimate articles than food or contaminated other water by the infectious discharges.
  28. 28. VECTOR  Usually an arthropod eg. Mosquito which transfers an infectious agent from an infected person to a healthy person.
  30. 30. LEVELS OF PREVENTION •Primary prevention • Health promotion Specific protection •Secondary prevention • Early diagnosis (screening tests, case finding programmes ) Adequate treatment •Tertiary prevention • Disability limitation Rehabilitation – Functional rehabilitation – Vocational rehabilitation – Social rehabilitation – Psychological rehabilitation
  31. 31. Epidemiological approach 1. Asking questions 2. Making comparisons 1. Asking questions • Epidemiology has been defined as a means of • learning or asking questions.. and getting answers that lead to further questions • RELATED TO HEALTH EVENTS: •What is the event? (the problem) •Where did it happen? •When did it happen? •Who are affected? •Why did it happen?
  32. 32. Conti.. RELATED TO HEALTHACTIONS •What can be done to reduce this problem and its consequences ? •How can it be prevented In the future ? •What action should be taken by the community ? By the health services? By other sectors ? •What resources are required ? How are the activities to be organized ?
  33. 33. 2. Making comparisons •This approach is to make comparisons and draw inferences. •Comparison may be made between different population at a given time eg. Rural with urban population •Between sub group of population eg. Male with female population •Between various periods of observation eg. Different seasons Conti..
  34. 34. Toolsof measurement 1. Rates 2. Ratios 3. Proportions 1. Rates A rate measures the occurrence of some particular event (development of disease or the occurrence of death) in a population during a given time period. Number of deaths in one year Death rate = X 1000 Mid - year population
  35. 35. (1) Crude rates: These are the actual observed rates such as the birth and death rates. Crude rates are also known as unstandardized rates. (2) Specific rates: These are the actual observed rates due to specific causes (e.g., tuberculosis); or occurring in specific groups (e.g., age-sex groups) or during specific time periods (e.g.. annual, monthly or weekly rates). (3) Standardized rates: These are obtained by direct or indirect method of standardization or adjustment, e.g., age and sex standardized rates. Conti..
  36. 36. 2.RATIO Another measure of disease frequency is a ratio. It expresses a relation in size between two random quantities. examples include: •The number of children with malnutrition at a certain time •sex-ratio, doctor population ratio, child woman ratio, etc Conti..
  37. 37. 3.PROPORTION A proportion is a ratio which indicates the relation in magnitude of a part of the whole. The numerator is always is usuallyincluded in the denominator. A proportion expressed as a percentage. The number of children with scabies at a certain time Example x 100 The total number of children in the village at the same time Conti..
  38. 38. MORTALITY The occurrence of death in a population. Mortality data are relatively easy to obtain, and, in many countries, reasonably accurate. Many countries have routine systems for collecting mortality data each year, information on deaths is analyzed and the resulting tabulations are made available by each government.
  39. 39. Limitations of mortality data • Incomplete reporting of deaths • Lack of accuracy:-inaccuracies in the recording of age and cause of death • Lack of uniformity:- There is no uniform and standardized method of collection of data • Changing:- Changing coding systems and changing fashions in diagnosis may affect the validity. • Diseases with low fatality:-the disease is associated with low fatality (e.g., mental diseases, arthritis).
  40. 40. MORTALITYRATESANDRATIOS 1. Crude death rate The simplest measure of mortality is the crude death rate It is defined as the number of deaths (from all causes) per 1000 estimated mid-year population in one year, in a given place. Number of deaths during the year = X 1000 Mid-year population
  41. 41. 2. Specific death rates When analysis is planned to throw light on aetiology, it is essential to use Specific death rates. The specific death rates may be (a)cause or disease specific e.g., tuberculosis cancer, accident; (b)related to specific groups e.g. age-specific, sex-specific , age and sex specific, etc. Conti..
  42. 42. 3. Case fatality rate (Ratio) Case fatality rate represents the killing power of a disease.it is simply the ratio of deaths to cases. The time interval is not specified. Case fatality rate is typically used in acute infectious diseases (e.g., food poisoning, cholera, measles) Total number of deaths due to a particular disease = X 100 Total number of cases due to the same disease Conti..
  43. 43. 4. Proportional mortality rate (Ratio) •It is sometimes useful to know what proportion of total deaths are due to a particular cause (e.g.. cancer) or •what proportion of deaths are occurring in a particular age group (e.g., above the age of 50 years). •Proportional mortality rats expresses the number of deaths due to a particular cause (at in a specific age group) per 100 (or 1000) total deaths. Thus we have: Conti..
  44. 44. (a) Proportional mortality from a specific disease Number of deaths from the specific disease in a year = X 100 Total deaths from all causes in the year Conti..
  45. 45. 2. Under 5 proportionate mortality rate Number of deaths under 5 years of age in the given year. = X 100 Total number of deaths during the same period 3. Proportional mortality rate for aged 50 years and above Number of deaths at persons aged 50 years and above = X 100 Total deaths at all age groups in that year Conti..
  46. 46. 5.Survival rate It is the proportion of survivors in a group, (e.g. of patients) studied and followed over a period tag, a 5 year period. Survival rates have received special attention in cancer studies. Total number of patients alive after 5 years Survival rate = X 100 Total number of patients diagnosed or treated Conti..
  47. 47. MORBIDITY The occurrence of an illness or illnesses in a population. Morbidity has been defined as any departure, subjective or objective, from a state of physiological well-being. The term is used equivalent to such terms as sickness. illness, disability etc. WHO Expert Committee on Health Statistics noted in its 6thReport that morbidity could be measured in terms of 3 units (a) persons who were ill (b)the illnesses (periods or spells of illness) that these persons experienced; and (c) the duration (days, weeks, etc.) of these illnesses
  48. 48. INCIDENCE Incidence rate is defined as the number of NEW cases occurring in a defined population during a specified period of time . It is given by the formula : Number of new cases of specific disease during a given time period Incidence = X 1000 Population at risk during that period For example:- if there had been 500 new cases of an illness in a population of 30,000 in a year, the incidence rate would be: = 500/30,000 x 1000 = 16.7 per 1000 per year
  49. 49. PREVALENCE The term "disease prevalence" refers specifically to all current cases (old and new) existing at a given point in time, or over a period of time in a given population. (a) Point prevalence (b) Period prevalence (a) Point prevalence •Point prevalence of a disease is defined as the number of all current cases (old and new) of a disease at one point of time, in relation to a defined population. several days, or even a few weeks.
  50. 50. Point prevalence is given by the formula: Number of all current cases (old and new) of a specified disease existing at a given point in time X 100 Estimated population at the same point in time Conti..
  51. 51. (b) Period prevalence A less commonly used measure of prevalence is period prevalence. It measures the frequency of all current cases (old and new) existing during a defined period of time (e.g., annual prevalence) expressed in relation to a defined population. cases arising during the year. Period prevalence is given by the formula: Number of existing cases (old and new) of a specified disease during a given period of time interval X 100 Estimated mid interval population at risk Conti..
  52. 52. METHODS OFEPIDEMIOLOGICAL STUDIES Epidemiologists employ three different• Epidemiologists employ three different methods or approaches for epidemiological studies which are: 1. Descriptive method 2. Analytical method. 3. Experimental method
  53. 53. 1. DescriptiveMethod Descriptive method of epidemiological study is concerned with the study of frequency and distribution of disease and health related events in population in terms of person, place and time. This method gives information about which all are affected by a particular disease or health related event or problems, where the cases occur and when they occur. The data is collected about:
  54. 54. • Personal characteristics such as age, sex, race, marital status, occupation, education, income, Class, dietary pattern, habits. • Place distribution of cases i.e. areas of high concentration, low concentration and spotting in the map. • Time distribution/trends such as year, season, month, week, day and hour of onset of the disease. Conti..
  55. 55. Such information’s give clue to possible associated factors such as age with specific disease e.g. measles, diphtheria, pertusis in early childhood, cancer in middle age, arterioscleroses in old age, some habits like smoking with lung cancer, dietary pattern with obesity. The data collected are analyzed and presented in terms of percentages, rates and ratios. Conti..
  56. 56. - Cross-sectionalstudies In this design of descriptive method of study, the data is collected from a cross-section of population at a one point in time. The results of the study are applied on the population. The cross-section of the population is sampled carefully so that it is representative of the whole population. Cross-section study is like a snapshot and provides information about the prevalence of a disease. It is also called as prevalence study. example study of diabetes or hypertension by personal characteristics and life style
  57. 57. - LongitudinalStudies Longitudinal studies art useful for studying the natural history of diseases, finding out incidence rates of diseases and identifying risk factors of diseases. Longitudinal studies are more expensive and time consuming than cross- sectional studies.
  58. 58. 2. AnalyticalMethod You have learn that descriptive studies generate etiological clues for various diseases help in formulating a guess or hypothesis for further vigorous study or testing e.g. "Cigarette smoking (10 to 20 in a day) causes lung cancer in 10 to 15% of smokers after 20 years of exposure”, wife battering is related to violence in victim's childhood family of origin". These types of hypothesis are further studied and tested by analytical studies to determine the association of cause with the effect.
  59. 59. - Case control study In this method a group of people who have been diagnosed as having a particular problem e.g. lung cancer (cases) are compared with a group of people who are similar in characteristics to that of cases but they are free from the problem i.e. free from lung cancer under study (controls).
  60. 60. - Cohort Study A cohort is a specific group of people, at a certain time, sharing common characteristics or experience e.g. people born on the same day or the same year (birth cohort), couple married in the same year (marriage cohort), a class of nursing students (experience cohort), people With same occupation (occupation cohort) etc.
  61. 61. a) Prospective cohort study: Cohort study is prospective in nature because the group under study is free from the disease but exposed to risk factor and epidemiologist study the development of a condition over time. example-lung cancer b) Retrospective cohort study: This is also called as historical cohort study. In this type of study designs the event has already occurred Conti..
  62. 62. ExperimentalMethod • Experimental studies are similar in approach to cohort studies except that conditions are under the careful control of investigator. • Experimental studies are done to confirm the etiology of diseases,' establish the efficacy of preventive or therapeutic measures and evaluate health care services. • These' studies are done under controlled conditions. • The investigator administers an intervention/gives treatment to the experimental group which is either exposure to causative factor of disease or preventive/therapeutic measure to improve or influence health or prevent disease but not to the control group which is similar to experimental group in all its aspects.
  64. 64. 2
  65. 65. 1st - The Infectious Agent 3 -any disease-causing microorganism (pathogen) Infectivity Pathogenicity Virulence
  66. 66. 2nd: Source or Reservoir • The starting point for the occurrence of a communicable disease Source of infection :• • the person, animal, object or substance from which an infectious agent passes or is disseminated to the host (immediate source) • RESERVOIR: • “any person, animal, arthropod, plant, soil, or substance, or a combination of these, in which an infectious agent normally lives and multiplies, on which it depends primarily for survival, and where it reproduces itself in such a manner that it can be transmitted to a susceptible host. It is the natural habitat of the infectious agent.” 4
  67. 67. •5 Types of Reservoirs Human reservoir Animal reservoir Non-living reservoir
  68. 68. “A person in the populationor study group Identifiedas having particular disease, health disorder or • condition • Under investigation” 7
  69. 69. The clinical illness maybe mild or moderate, typical or atypical, severe or fatal. Epidemiologically, mild cases may be more impor tant sources of infection than severe cases because they are ambulant and spread the infection wherever they go, whereas severe cases usually confined to bed. 8
  70. 70. Subclinical cases Inapparent, Covert, Missed or Abortive Cases • disease agent multiply in the host but does not manifest by S/S. • But contaminates the environment in the same way as clinical cases. • Subclinical cases play a dominant role in maintaining the chain of infection in the community. 9
  71. 71. subclinical cases detected only by laboratory tests occurs in most infectious disease. Eg  Rubella,  Mumps,  Polio,  Hepatitis A and B,  Influenza,  Diphtheria 10
  72. 72. Latent infection • infectious agent lies iinn aa nonon-n-iinfnfectectiioousus ffoorm-rm- ddormormantant within the host without sysymmptptoommss with no shshededdidinngg (and often without demonstrable presence in blood, tissues or bodily secretions of the host) egeg..  HSV and VZV: nerve ganglia cells,  CMV: kidney and salivary glands cells,  EBV: lymphocytes 11
  73. 73.  Index Case ◦ Person that comes to the attention of public health authorities  Primary Case ◦ First case of a communicable disease introduced into the population unit bring studied ◦ Attack rate  Secondary Case ◦ Person who acquiresthe disease from an exposure to the primary case ◦ Secondary attack rate 12
  74. 74. Carriers  13 Reason : due to inadequate treatment or immune respons the disease agent is not completely eliminated leading to a carrier state. An Infected Person or Animal that harbours a specific infectious agent in the absence of discernible (Visible) clinical disease and serves as a potential source of infection to others
  75. 75. Three elements in a carrier state: 14
  76. 76. CARRIER S • As a rule carriers are less infectious than cases, but epidemiologically, they are more dangerous than cases • because ◦ escape recognition ◦ continuing to live a normal life among population or community ◦ readily infect the susceptible individuals ◦ over a wider area and longer period of time under favorable conditions. 15
  77. 77. IncuIncubatbatoorryy CCaarrrrieierrs:s: those who shed the infectious agent during the incubation period. This usually occurs during last few days of IP  Measles- the period of communicability is 4 days before the rash.  Mumps- usually 4-6 days before onset of symptoms  Polio- 7-10 days before onset of symptoms  Hepatitis B- for a month before jaundice  Pertusis  Influenza Classification of Carrier
  78. 78. Carrier May Be Classified : ByType Convalescent Carriers:  those who continue to shed the disease agent during the period of convalescence  In the disease, clinical recovery does not coincide with bacteriological recovery.  Serious threat to HH members  Highlights importance of bacteriological surveillance of carriere state after recovery ◦ typhoid fever ◦ cholera, ◦ diphtheria, ◦ bacillary dysentery ◦ pertusis 17
  79. 79. Carrier may be classified : BY TYPE 18 Healthy Carriers: victims of subclinical infection who have developed carrier state without suffering from overt disease, but are nevertheless shedding the disease agent ◦ poliomyelitis, ◦ cholera, ◦ meningococcal meningitis, ◦ salmonellosis, ◦ diphtheria. Note:- Person whose infection remains subclinical may or may not act as carrier (eg.- in polio inf may remain subclinical but person act as temp carrier due to shedding of virus in stool..while TB most of us with +ve Mt, do not disseminate bacillie- so not labelled as carrier.
  80. 80. Temporary carriers are those who shed the infectious agent for short period of time. Chronic carriers are those who excretes the infectious agent for indefinite periods 19
  81. 81. Chronic carriers Chronic carriers are far more important sources of infection than cases. The longer the carrier state, the greater the risk of community-- reintroduce disease into areas which are otherwise free of infection The duration of the carrier state varies with the disease.  In typhoid fever and hepatitis B, the chronic carrier state may last for several years.  In chronic dysentery it may last for year or longer.  In diphtheria, the carrier state is associated with infected tonsils, in typhoid fever with gall bladder disease. 20
  82. 82.  Mary Mallon (1869 –1938), better known as Typhoid Mary, was the first person in the US identified as an asymptomatic carrier of the pathogen associated with typhoid fever.   She was presumed to have infected some 50 people, three of whom died, over the course of her career as a cook. She was forcibly isolated twice by public health authorities and died after a total of nearly three decades in isolation. 21
  83. 83.  Respiratory carrier: e.g.influenza  Fecal (intestinal) carrier: e.g. typhoid, choler a  Blood carrier: e.g. hepatitis B andHIV  Urinary : e.g.Typhoid  sexual Carrier: gonococcus and HIV 22 Carrier classified : By Portal Of Exit of Infectious Agent
  84. 84. Animal reservoirs • infection that is transmissible under natural conditions from animals to man. • e.g. – Bacterial: Leptospira, plaguefrom Rat. – Viral : Rabies from dog. – Protozoa: Leishmaniasisfromdog. – Helminths : Hydatiddiseasefromdog – Tape worms: Cattle,Pig. •23
  85. 85. Reservoir in non-living things Some organisms are able to survive and multiply in nonliving environments such as soil and water Clostridium that causes tetanus and botulism can survive many years in the soil Hookworms deposit their eggs into the soil Water contaminated by human or animal feces cause GI tract disease (list includes bacteria, viruses, protozoa) •24
  86. 86. 3rd - The Portal of Exit 25 • Route of escape of the pathogen from the reservoir-IA entersintosurrounding env-transfertohostattheirportalof entry Examples: respiratorysecretions, GI bloodexposure, breaksin skin
  87. 87. 4th –Mode of Transmission 26 Direct transmission Indirect transmission Direct contact Droplet infection Contact with soil Inoculation into skin or mucosa vertical Vehicle-borne :Vector-borne • Air-borne Fomite-born Unclean hands and fingers
  88. 88. Direct Transmission •27
  89. 89. DirectContact •Inf spread by direct contact of skin-skin, skin mucosa, mucosa- mucosa of same or other person •by touching, kissing,, bites, or sexual intercourse •Direct & immediate transfer of IA from reserviour –host (no intermediate agency) •So it introduces larger dose of IA •No time interval of survival in environment.. •Overcrowded place or where place with lack of ventilation •28 Scabies Pediculosis STD’s Skin/eye inf leprosy
  90. 90. Droplet spread: • • Direct projection of droplets of saliva/nasopharynge al secretion by Sneezing, Speaking, Coughing Droplets directly impinge on conjunctiva, nasal mucosa or skin •29
  91. 91. •30 H1N1 Tubercle bacilli Measles • • • • Chickenpox
  92. 92. • Inoculation: Pathogen injected into tissues. – Tetanus spores – Arboviruses (Insects). •31
  93. 93. Vertical transmission Transplacental  To R C H  HIV  HBV •32 15
  94. 94. Indirect Transmission 5 ‘F’ 33 food, flies, fomite, finger, fluid
  95. 95. Vehicle transmission • • Water: Cholera, H A V , H E V, Typhoidetc. FOOD: Staphylococci, Cl. Botulinum. • • Blood/serum-HIV, HBV,HCV Organ- cmv 34    Clusteringof cases Distancebwsecondarycasesmore Commonsourcecanbetraced
  96. 96. Mechanical Hf Diarrhea Dysentery Typhoid Trachoma Biological Propagative Only multiplication No developmental Plague bacilli in rat flea Cyclo propagative Multiplication developmental Malaria parasites in mosquito Cyclodevelopmental No multiplication developmental Filaria parasite In mosquito Vector- insects 35
  97. 97. Trans-ovarian transmission  Inf agentverticaltransmittedfromfemale mosquitotoherprogeny ◦ Scrub typhus ◦ Rickettsialpox ◦ Indianticktyphus ◦ Q fever ◦ RMSF Trans-stadial transmission- Lymedisease,infectstickvectorasalarva,andthe infectionismaintainedwhenitmoltstoanymphand laterdevelopsasanadult 36
  98. 98.  host feeding preference  infectivity-ability to transmit disease agent  susceptibility –ability to becomeinfected  survival rate of vectors in environment  Domesticity  Seasonal factors… 37
  99. 99. Fomites: Contaminated Nonliving Objects likeCup, towel, napkin, linen, Clothing,glass, Toys,Pencils,doorhandle, surgicalinstruments,syringes,dressingmaterials… Ex: Diphtheria, Trachoma influenza scabies 38
  100. 100. 5th - The Portal of Entry •39 -route through which the pathogen enters its new host
  101. 101. Respiratory System •40 Upper respiratory tract Diphtheria Lower respiratory tract Tuberculosis
  102. 102. Gastrointestinal System ingestion Feco-Oral Route Infectious agent excreted in faeces & transmitted to the oral portal of entry through contaminated food, water, milk, drinks hands • Typhoid fever • Shigella • Cholera • Polio • Rotavirus • Hepatitis A, Hepatitis E •41
  103. 103. Urinary & Reproductive Tracts •42 Gonorrhea Syphilis HIV
  104. 104. Breaks in Protective Skin Barrier •43 Percutaneous Leptospirosis Percutaneous (bite of arthropod) Yellow fever
  105. 105. 6th - The Susceptible Host 44 • A person or an animal that afford lodgment to an infectious agent under natural conditions. •Accepts the pathogen •The support of pathogen life & its reproduction depend on the degree of the host’s resistance.
  106. 106. •Cancer Patients •HIV-AIDS Patients •Transplant Patients •On steroids.. •Infant & Elderly Patients 45
  107. 107. HOST Obligate host : the only host Eg: Man in measles & typhoid Primary /definitive host: in which parasite attains maturity or passes its sexual stage Secondary or intermediate hosts: the parasite is in a larval or asexual state •46
  108. 108. Life cycle Sporozoits Liver Ring Trophozoits Marozoits RBC Mature Schizont Mature Gametocyte Male / Female MosquitoZygote Ookinete Oocyte Salivary Gland Exflagellation
  111. 111. 10 5 0 15 Time 1 3 50% 50%Probable exposure Median incubation time 2 No of cases 1 2 3 4 5 6 7 8 9 10 11 12 1314 15 16 17 18 19 20 21 22
  112. 112. Period From Disease Initiation To Disease Detection For NCDs
  113. 113. 0 5 10 Zeit No of cases Generation time 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 GENERATION TIME INTERVAL OF TIME BETWEEN RECEIPT OF INFECTION BY A HOST ANDMAXIMAL INFECTIVITY OF THAT HOST
  115. 115. It is defined as the time during which an infectious agent may be transferred directly or indirectly from an infected person to another person, from an infected animal to man , or from an infected person to an animal, including arthropods
  116. 116. Ate the food (exposed) Did not eat the food (not exposed) Ill Well Total Attack Rate Ill Well Total Attack Rate 10 3 13 76% 7 4 11 64% Attack Rate = Ill / (Ill + Well) x 100 during a time period Attack rate = (10/13) x 100 = 76% ( 7/11) x 100 = 64%
  117. 117. It is defined as the number of exposed persons developing the disease within the range of the incubation period, following exposure to the primary case
  118. 118.  Used to estimate to the spread of disease in a family, household or other group environment.  Measures the infectivity of the agent and the effects of prophylactic agents (e.g. vaccine) 58 SAR (%) Totalnumberof cases– initialcase(s) Numberof susceptiblepersonsinthegroup– initialcase(s) = x100