Epidemiology Flashcards
Infection
Contamination
Infestation
Infection:
entry and multiplication of infectious agent in body
Contamination:
Presence of infective agent on a surface
Infestation:
lodgement and development/multiplication of an arthropod on body/clothes
Types of infections
- Nosocomial
- Iatrogenic
Eg: ADR - Exotic
- Opportunistic
- Dead-end
Agent determinants
1. Infectivity: invade and multiply Attack rate/2° attack rate 2. Antigenic: Local immune response 3. Pathogenicity: Local inflammatory response or pathological changes 4. Virulence: Fatality rate
Amphixenasis
Infection from animal to human and vice versa
Eg. trypanosomiasis
Epidemiological triad
Agent
Host
Environment
Diseases eradicated in India
- Small pox
- Guinea worm disease
- Wild poliomyelitis
Potentially eradicable diseases
- Measles
- Malaria
- Poliomyelitis
- Leprosy
- Yaws
India declared free from the diseases
- Yaws
- Childhood trachoma
- Neonatal tetanus
Disease transmission types
- Direct: D-tics
droplet, transplacental, inoculation, contact, soil - Indirect: FAV-uv
Fomite, air, vehicle, unclean hands, vector
Transplacental transmission of diseases
TORCH, HIV
Chemicals: thalidomide, stilbesterol, heavy chemicals
1st trimester: rubella
2nd trimester: parvovirus
3rd trimester: syphilis, toxoplasmosis, CMV, hepatitis B
Delivery: hepatitis C, herpes, HIV
Biological transmission of diseases
- Propagative
- Cyclodevelopment
- Cyclopropagative
- Transovarian transmission
- Transtadial transmission
Chemical isolation
Ring isolation
Treatment in homes 🏡 and rendering the patients non-infectious TB, leprosy, STD Contacts of swine flu patients Ring isolation/ ring immunization: A type of chemical isolation Eg., poliomyelitis, measles, small pox
Investigation of epidemic
- Verification of diagnosis
- Confirmation of epidemic
- Defining population at risk
- Rapid search for cases
- Data analysis
- Formulate hypothesis: to find the cause of the disease
- Test hypothesis
- Evaluate ecological factors
- Further investigation for risk
- Report writing and dissemination
Wearing the PPE
Apron ➡️
mask 😷 ➡️
eye goggles 🥽 ➡️
gloves 🧤
Mask 😷 is always removed last
Survival analysis
Tells about probability of surviving over some period.
Using life table analysis and Kaplan Mier analysis
Types of standardization (of rates of different population)
Direct standardization:
When age specific death rate and standard population is available
Indirect standardization:
• When age specific death rate or a standard population is not available
• Reference population is considered
SMR = observed deaths/expected death *100
Prevalence
Special prevalence rate
Prevalence: total number of cases in a population
Special prevalence rate:
1. Point prevalence
2. Period prevalence
Incidence
Special incidence rates
Incidence: number of new cases in a defined population per using time
Special incidence rates:
1. Attack rate
2. Secondary attack rate:
Number of infected individuals from a primary case within one incubation period
Types of epidemiology
1. Observational (non-interventional): • Descriptive • Analytical 2. Experimental (interventional): • non-randomized trials • randomized trials
Descriptive epidemiology
- Defining the population or sample of population
- Defining the disease
- Describing the disease
- Measurement of disease
- Comparing with known indices
- Formulation of hypothesis
Time fluctuations seen in diseases
- Short term: epidemic
- Periodic: cycles or seasonal
- Long term: secular trends
Epidemic
classification
- Slow
- Common source:
• single exposure
• multiple exposure - Propagated
Sites of epidemic depends on
- Herd immunity
- Secondary attack rate
- Density or chance of contact with cases
Types of observational study
- Case study
- Case series
- Cross-sectional
- Case control: retrospective
- Cohort study
Concurrent and non concurrent cohort study
Non-concurrent cohort: fixed study population
Concurrent cohort:
• variable/ dynamic study population
• positive association with time trend of the disease
Nested case control study
A type of cohort study
Nesting of case control within a cohort
Advantage: better analysis
Study of choice for rare and expensive investigations
- Simple cohort study
- Retrospective cohort study
- Ambispective cohort study
- A cohort of exposed and unexposed are chosen and followed up for a particular time and assessed for the number of diseased and healthy individuals
- Calculating the number of diseased and healthy individuals among the exposed and unexposed for past 10 years till date
- Days has been collected for 8 years. Next 2 years is by prospective study
Biases seen in cohort study
- Attrition bias: failure to follow-up
- Hawthorn effect
- Selection bias
No recall bias
Types of non-randomized experimental trials
1. Pre & post test study designs with or without control: Usually 2 groups 2. Uncontrolled trials: Historical controls (values) 3. Natural experiments: Calamity becomes intervention
Randomized clinical/control trials
features
- It removes selection bias
- It is done at the level of allocation, not done in the level of selection
- It is known as chance and equal chance
Cross over study design
Multi factorial study design
It is classified under randomized trial (but it can be randomized or not)
2 groups, both are subjected to 2 drugs (not simultaneously) separated by a washout period
Advantage:
🔽 chance of biological variation
Disadvantages:
Cannot be done when:
1. Drug has long t1/2 or the drug is curative
2. If disease is acute/fatal
Types of randomized interventional trials
- RCTs
- mFSD or cross over study design
- Preventive trial
- Risk factor trials
- Cessation experiment
- Trial of etiological agents
Preventive trial /
Vaccine trial
Risk factor trial
On healthy individuals in field trials
Objections of the study is to keep the risk factor under control, to prevent the disease
Attrition
Cross over/ contaminant
in a RCT
Attrition:
people who were lost in follow-up of a study
Cross-over/ contaminant:
people who cross over to the opposite group
Per protocol analysis
Intention to treat analysis
- Per protocol analysis:
• The modified group due to -cross-overs and attrition- is taken into account
• Not adjusted for randomization - Intention to treat analysis:
• Original group -irrespective of cross over and attrition- is taken into account
• Randomization is preserved
Different types of bias
- Observer bias
- Interviewer bias
- Experimental bias
- Surveillance bias
- Recall bias
- Berksonian bias
- Neyman bias
- Hawthorne effect
- Golem effect
- Pygmalion effect
Berksonian bias
Found in case control studies which are hospital based
Differential rate of admission in hospitals
In certain cases, the cases can be equal to control
Hawthorne effect
Neyman bias
Hawthorne effect: change in attitudes under observation
Usually in follow-up studies
It’s a subject bias
Neyman bias: differential mortality pattern b/w two groups
Golem effect 🆚 Pygmalion effect
Golem effect: blunted effect of research when the researcher is unmotivated
Pygmalion effect: the opp
Blinding
Treatment of bias Types: 1. Single blind: subject 2. Double blind: M/C Subject + doctor 3. Triple blind: Subject + doctor + analyzer Most effective
Confounding 🆚
Effect modification
Confounding and effect modification are associated with disease and risk factor
Confounding on stratification shows no association
Effect modification on stratification shows positive effect
Treatment of confounding
- Known: by matching
- Unknown:
• randomization
• standardization
• stratification
• regression
Types of association
1. Direct association: • one-to-one • multifactorial 2. Indirect/non-causal association 3. Spurious association
Causal association or Hill’s criteria of causality
- Biological plausibility
- Coherence of association
- Specificity of association
- Validity of association
- Temporality
- Strength of association
Hierarchy of association
- Case studies
- Case series
- Cross sectional
- Case control
- Cohort studies
- POSD
- RCT
- MFSD
- Systematic review
- Meta-analysis
Increasing downwards
Types of evidence based medicine
- Systematic review:
Preliminary study to assess her effect of an intervention/drug
Structured approach - Meta-analysis:
Statistical analysis for final assessment of effect of risk factor/drug/intervention in disease or health status
Types of screening
- Mass screening
- High risk screening:
- Multi phasic screening:
Two different tests in the same population - Presumptive/pricing:
Primary level of prevention
HIV screening for ANC,… - Prescriptive:
Early diagnosis
PAP smear
Latent period
Lead time
Screening time
Latent period:
Onset of pathogenesis to usual point of diagnosis
Lead time:
1st possible point of diagnosis to usual point of diagnosis (usually after critical point)
Screening time:
1st possible point of diagnosis to critical point of diagnosis
• A better investigation is the one with longer lead time and shorter screening time
Lead time bias
An apparent 🔼 in survival rates because of higher prevalence and an investigation with long lead time without change in treatment modality
Difference between screening test and diagnostic test
Screening test: Sensitivity 🔽 false negative Diagnostic test: Specificity 🔽 false positive False positive errors are much more dangerous than false negative
