Concepts of Epidemiology and Measurement of Disease G

Concepts of Epidemiology and Measurement of Disease G V S Murthy Director, Indian Institute of Public Health, Hyderabad (PHFI)

Epidemiology The logic of observation and the methods to quantify these observations in populations (groups) of individuals. Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Contemporary Definition Epidemiology (John M Last) Epidemiology is the QUANTITATIVE Study of DISTRIBUTION and DETERMINANTS of diseases/ health conditions and their CONTROL Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Epidemiology • Quantitative: Measures occurrence and frequency of disease • Distribution: Who are the People Affected and what are their characteristics? • Determinants: What are the causes? Why do people get disease/ suffer from poor health? • Control: What can be done to reduce burden and effects of disease/ poor health? Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Fundamental Assumptions in Epidemiology • Disease/ Health Outcome does not occur in a VACUUM • Disease/ health outcome is not randomly distributed in a population Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Epidemiology DESCRIPTIVE EPIDEMIOLOGY • How disease or health states are distributed in populations, space and time (Occurrence of outcomes) ANALYTICAL EPIDEMIOLOGY • Testing a hypothesis (describe association between exposure and outcome) Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Epidemiologic Concepts: Distribution of Disease Indicates how diseases/ health conditions/ determinants are distributed in a population: • Time - When affected ? • Place - Where affected ? • Person - Who all are affected ? Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Epidemiological Triad AGENT Weather Housing Geography Work setting Air quality Water Food Infectivity Pathogenicity Virulence Immunogenicity Stability Survival DISEASE ENVIRONMENT Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel Age Sex Genotype Behavior Nutritional status Health status HOST

Agent Characteristics • Infectivity – Ability to cause infection (# infected/# susceptible) • Pathogenicity – Ability to cause disease (# clinical diseased/# infected) • Virulence – Ability to cause death (# deaths/# diseased) • These are dependent on HOST factors Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Measurement of Disease Information on health of a community is needed for : • Measuring health status of communities • Planning and evaluating health services • Ensuring optimal utilisation of limited funds available for programmes The Crux of Epidemiology is Measurement and Comparison Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Epidemiologic Concepts: Disease Frequency Measurement of Disease in Numerical Terms • Absolute numbers • Proportions • Rates Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Mathematical Parameters Used in Epidemiology Used to relate number of cases of disease to size of population and time • Ratio: Division of one number by another where the numbers don’t have to be related • Proportion: numerator is subset of denominator, often expressed as a percentage • Rate: time is an intrinsic part of denominator, term is most misused Need to specify if measure represents events or people Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Absolute Numbers Country Infected Persons TB (2011) Afghanistan 110, 000 Bangladesh 620, 000 Brazil 91, 000 China 1, 400, 000 Ethiopia 200, 000 India 3, 100, 000 Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Absolute Numbers • Have limited use • Only when population is stable • Comparisons over short periods of time • Case comparison during epidemic of cholera Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Proportions Country Infected TB Population (2011) Per Million Afghanistan 110, 000 30, 551, 674 3600 Bangladesh 620, 000 156, 594, 962 3950 Brazil 91, 000 200, 361, 925 454 China 1, 400, 000 1385, 566, 537 1010 Ethiopia 200, 000 94, 100, 756 India 3, 100, 000 1252, 139, 596 2475 Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel 2125

Prevalence Proportion Measures number of cases NOW No. of persons with disease at a specified time ----------------------------- X 10 n No. of people in the defined population Easier to Construct Compared to True Rates Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Factors Affecting Prevalence • • • Increase Long duration • Low cure rate • Low case fatality Increase in new cases Immigration of patients Improved detection Emigration of healthy people • • • Decrease Shorter duration • High cure rate • High case fatality Decrease in new cases Emigration of patients Improved cure rate Immigration of healthy people Changes in prevalence may have many causes and are difficult to interpret Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Incidence • Number of new cases in a given period in a specified population – Time, (i. e. , day, month, year) must be specified • Measures the rapidity with which new cases are occurring in a population • Can be expressed: – In absolute numbers – In terms of cumulated incidence – In terms of incidence density (rate) Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Characteristics of Rates • Numerator should be related to denominator • Cases should relate to specific time periods • All individuals included in the denominator should be exposed to risk of developing disease( different from Proportion - Prevalence) • Rates are the only way to compare trends over time or different regions Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Cumulative Incidence Rate Measures Number of NEW Cases Persons developing disease in a specified period x 10 n Persons free of disease at beginning • Expressed as % per month / year • It measures denominator only at ONE point in time • Attack rate is similar to cumulative incidence rate • Assumes that the entire population at risk at the beginning was followed up for the period of observation Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Incidence Density (Rate) # of new cases ID = ------------------------- x 10 n Total Person Time of observation • Measures occurrence of new cases in population • Also known as Incidence Rate • Reflects more exactly the person-time observed • Indicates the SPEED at which a defined ‘at-risk’ group or population develops a new disease or outcome over a given time period Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Concept of Person Years 100 people studied for one year, or 50 people studied for two years, are both equivalent to 100 person years of observation: – 100 people × 1 years = 100 person years – 50 people × 2 years = 100 person years Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Relationship between incidence & prevalence P=IXD Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Incidence & Prevalence 1 2 3 4 5 6 7 8 9 10 11 12 2009 2010 2011 2012 Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel 2013

Is numerator included in denominator? Yes No Is time included in the denominator? Ratio Yes No Incidence Rate (Density) Incidence Proportion (Cumulative Incidence) Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Mortality Rate • Frequency of death in a population during a specified period of time • Crude mortality rate • The mortality rate from all causes of death for a population. Deaths during calendar year x 10 n Population at mid-year Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Disease Specific Mortality Rate # deaths from a disease in a specified period x 10 n Average total population • Reflects the impact of a disease on a population in terms of death Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Proportionate Mortality Ratio Deaths from a specific cause x 100 Deaths from all causes • Not a RATE because it does not have a population denominator Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Attack Rate • Incidence rates which are calculated for defined populations, during limited periods of time, such as an epidemic # cases developing during defined period x 100 or 10 n # in pop. at risk at start of period Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Secondary Attack Rate • Computes the potential for transmission of the infection after exposure to primary cases # cases developing during incubation period among those exposed to an infected person x 100 or 10 n # persons exposed to primary cases Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Case Fatality Relates the number of deaths from a disease to the number of cases • Reflects severity • Can be expressed as: – Proportion – Ratio – Not as rate (although most commonly referred to as case fatality rate) • Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Calculating Case Fatality Proportion CF =# deaths among the cases identified x 10 n # of cases • Applies to a situation where follow-up information is available individually for all case-patients • The numerator is a part of the denominator • Time is generally unspecified as it refers to short periods Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Reviewing Definitions for measuring disease Prevalence = people people Cumulative incidence = people Incidence rate = people-time no dimension is time Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Disease States • Endemic: Transmission occurs but number of cases fairly constant- Conforms to expected frequency of cases • Epidemic: Transmission occurs with increased number of cases compared to what is normally seen (Can be defined as increase in absolute numbers; % rise; 2 SD etc. ) • Pandemics: Transmission leads to disease epidemics across continents Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Number of Cases of a Disease Epidemic Versus Endemic Epidemic Time Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Dynamics of infectiousness Infection Timelines for Infections Latent period Infectious period Non-infectious Susceptible Dynamics of disease Infection Time Incubation period Symptomatic period Susceptible Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel Non-diseased

Infectious Disease Epidemiology • Infectious Period: Length of time that an infected person is shedding organisms and so can infect others. • Incubation Period: Time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease • Latent Period: Period between exposure and the onset of infectiousness • Serial interval (Generation time): Interval between receipt of infection and maximal infectivity of the host [Gap between onset of primary and secondary cases] Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Infectious Disease Epidemiology CLINICAL SYMPTOMS INFECTION DISEASE INCUBATION PERIOD TIME LATENT PERIOD OF COMMUNICABILITY/ INFECTIOUSNESS INFECTIOUS Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Infectious Disease Epidemiology • Primary Case: Person who brings the disease/infection into a population • Index case: First case discovered during an outbreak (First case to have come to health system’s attention • Secondary cases: Persons who are infected by primary case • Reproductive Rate: Potential of an infectious disease to spread in a population. Depends on: – Probability of contact with infective cases – Frequency (no. ) of infective contacts in population – Duration of infectiousness – Proportion of population already immune Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Outcome of Exposure to Infectious Agent EXPOSURE TO INFECTIOUS AGENT No infection Clinical Sub-clinical Carrier Death Carrier Immunity No immunity Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel

Using epidemiologic principles for infectious disease control Preventing spread by tracking primary and index cases and breaking transmission chains Susceptible Immune Sub-clinical Clinical

Summarizing Epidemiology DESCRIPTIVE ●What (case definition) ●Who (person) ●Where (place) ●When (time) ●How many (measures) MEASURES ●Counts ●Times ●Rates ●Risks/Odds ●Prevalence ANALYTIC ●Why (Causes) ●How (Causes) METHODS ●Design ●Conduct ●Analysis ●Interpretatio n ALTERNATIVE EXPLANATIONS ●Chance ●Bias ●Confounding INFERENCES ●Epidemiologic ●Causal Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel ACTION ●Behavioral ●Clinical ●Community ●Environmental

Thank You Obama-Singh Initiative: IITD-IIPHH-NEERI-Drexel