Week 2

Question 1

What is a disease?

Answer 1

• Physiological dysfunction/ change in the body (objective term)
• Important for epidemiology, as much of the field deals with communicable (also known as infectious diseases) and non- communicable diseases

Question 2

What are the 2 types of diseases?

Answer 2

• Communicable diseases (also known as infection disease) can be transmitted directly or indirectly to a susceptible person via contact, inhalation or ingestion.
Ex: covid

• Non-communicable diseases can NOT be transmitted directly or indirectly to others.
Ex: cancer, diabetes, vascular diseases

Question 3

What is an illness?

Answer 3

• Ill health (subjective and objective)
• Limit one’s ability to live a normal life
• i.e., psychological distress

Broad term
Physical and psychological

Question 4

What is sickness?

Answer 4

The feeling of not feeling well
It can be subjective or objective

Question 5

What is the chain of infection?

Answer 5

Reservoir (place where the virus or bacteria is) -> Portal of Exit (how they exit the host) -> Mode of
transmission (ex: skin, nose) -> Portal of Entry -> Susceptible Host

Question 6

What are the Modes of disease transmission?

Answer 6

• Direct
– Person to person contact

• Indirect
– Common vehicle
• Single exposure
• Multiple exposures
• Continuous exposure

– Vector
• Carries the infection to the host (animal, mosquito)

Question 7

So many things are responsible for us to contract diseases. What are some examples?

Answer 7

Social environment
Physical environment
Genetic endowment
Health care
Health and function
Prosperity
Well-being
Individual response (behaviour, biology)

Question 8

What are different types of Measures in disease frequency?

Answer 8

• Counts (used very often): # of people with a disease

• Proportions (value from 0-1): A fraction of the population is affected

• Rates (time frame: week, day, month): how fast the disease is occurring in a population (time is considered in the denominator)

• Ratios: what groups are at higher risk of disease than other groups

Question 9

What is the difference between Endemic, Epidemic, and Pandemic?

Answer 9

• Endemic is defined as the habitual presence of a disease within a given geographic area

• Epidemic is defined as the occurrence in a community or region of a group of illnesses of similar nature, clearly in excess of normal expectancy and derived from a common or a propagated source

• Pandemic refers to a worldwide epidemic

Question 10

What is a disease outbreak?

Answer 10

Higher number of cases than expected in a location within a certain time period

Question 11

What are the 3 big Steps of an outbreak investigation?

Answer 11

What is the problem?
What is the cause?
What can we do with it?

Question 12

What are the steps in the main step of “What is the problem?”?

Answer 12

1. Prepare for field work
2. Establish the existence of an outbreak
3. Verify the diagnosis
4. Construct a working case definition
5. Find cases systematically and record information
6. Perform descriptive epidemiology

Question 13

What are the steps in the main step of “what is the cause?”?

Answer 13

7. Develop hypotheses
8. Evaluate hypotheses epidemiology
9. As necessary, reconsider, refine, and re-evaluate hypotheses
10. Compare and reconcile with laboratory and/or environmental studies (dosages)

Question 14

What are the steps in the main step of “What can we do with it?”?

Answer 14

11. Implement control and prevention measures
12. Initiate or maintain surveillance
13. Communicate findings
    * trying to bring the curve back to normal

Question 15

What are the Common stages of most diseases?

Answer 15

• Stage of susceptibility (ex: winter season you are more at risk of getting sick)
• Stage of pre-symptomatic disease (dry mouth, tired, sore throat)
• Stage of clinical disease - commonly diagnosed and treated with a wide range of severity (meet the diagnosis criterias)
• Stage of diminished capacity - a convalescent period/ residual disability

Question 16

At the stage of susceptibility, what is herd immunity?

Answer 16

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
Ex: vaccination during covid to be immune

Question 17

During the Stage of pre-symptomatic disease, what can we find in communicable diseases vs non-communicable diseases?

Answer 17

communicable disease:
• Incubation period (optimal environment for the agent to work/react): The time between the invasion of an infectious agent and the development of the first signs or symptoms of disease
• Carrier: An individual who has no clinical signs or symptoms of the disease but has the causative agent, which can be transmitted to others

non-communicable disease:
• Latent period (latency period): The period from disease initiation to disease detection
(Can be long. Ex: some cancers have long periods and we won’t know until we feel uncomfortable. Ex: dementia is only diagnosed 2-3 years after starting to forget little things)
• Subclinical disease: The disease is fully developed but produces no signs or symptoms in the host (ex: 25% of people will have winter depression)

Question 18

True or false: different diseases have different ranges but it does not differ between people for the same disease?

Answer 18

False: it differs, every body reacts differently

Question 19

What is the formula for attack rate?

Answer 19

Number of people at risk in whom a certain illness develops
__________________________________________
Total number of people at risk

• i.e., Number of people who ate a certain food and became ill
__________________________________________________
Total number of people who ate that food

Question 20

What are the 3 things we look at when Exploring the occurrence of disease?

Answer 20

• Who (who was exposed. Ex: age, sex, marital status, race/ethnicity, etc)
• Where (where was the exposure)
• When (when did they get exposed/when did they develop. Ex: winter)

Question 21

How does the “who” help us?

Answer 21

Helps us know how to prevent and protect (ex: vaccines for babies vs for adults)

Question 22

What are the levels of prevention for the common stages of most diseases?

Answer 22

• Stage of susceptibility = Primary prevention
• Stage of pre-symptomatic disease = Secondary prevention (how to reduce symptoms)
• Stage of clinical disease = Secondary and tertiary prevention
• Stage of diminished capacity = Tertiary prevention (rehab, comfort)

Question 23

What is surveillance?

Answer 23

• Centers for Disease Control and Prevention (CDC): “ongoing systematic collection, analysis, and integration 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 who need to know”

• World Health Organization (WHO): “The ongoing systematic collection, collation, analysis, and interpretation of data; and the dissemination of information to those who need to know in order that action be taken.” (To make health policies)

Question 24

What is the Public health knowledge delivery chain of order?

Answer 24

Multiple data sources -> public health surveillance and public health research (both contribute to information) -> Synthesis and translation -> Population health assessment -> ‘Actionable’
public health knowledge

Question 25

What are the Conditions under surveillance?

Answer 25

• Initially infectious diseases (population shifted towards more non-communicable disease) • Now other conditions and exposures – Acute injury and diseases – Chronic diseases – Critical exposures • Influenced nature of data collected – Hospital statistics – Repeated surveys

Question 26

What is the surveillance cycle?

Answer 26

Data collection (ex: survey) -> data cleaning (what’s important) -> data analysis -> interpretation -> dissemination and public health action

Question 27

Why do surveillance?

Answer 27

• Detect outbreaks – identify cases/contacts who need treatment – prevent further transmission • Quantify burden of disease & monitor trends (ex: how many got diagnosed with diabetes) • Evaluate interventions & monitor changes, e.g., new drugs, vaccines (before vs after to see the potential) • Support disease elimination & eradication • Natural history (helps us know what to look for from the past) • Health planning and to inform policy • Applied research & testing hypotheses

Question 28

What are the Surveillance essentials?

Answer 28

• Timeliness of data collection (quick action in public health) • Sensitivity of system to detect a case – Low ‘undercount’ • Representativeness of cases reported (ex: HIV) • Timely dissemination of information collected: – Back to those who reported the data – On to those who will take action • Cost of the system

Question 29

What are different Forms of surveillance …?

Answer 29

• Passive (Limited efforts to stimulate reporting) • Active (Regular outreach to potential reporters) • Sentinel (Subset of reporters in population) • Digital (Internet-based) • Mass gathering (At major events to gather info) • Rumour (Reading newspapers. Social media not always real) • Risk factor (Surveillance of population behaviours) • Hazard (Monitoring of environmental hazards)

Question 30

What is the Exposure–disease–diagnosis pathway?

Answer 30

Person is exposed -> Person develops symptoms -> Person seeks medical attention (clinics, hospital) -> Healthcare provider notifies the case

Question 31

Public health importance is defined by a range of which factors?

Answer 31

– Incidence and/or prevalence – Likelihood of an outbreak – Severity of disease or outcome – Disease burden (DALYs) – Cost to health system, society, industry – Availability of an intervention, e.g. vaccine, prophylaxis – Affects vulnerable populations – Public or political interest

Question 32

What are different Sources of surveillance data?

Answer 32

• Statutory notifications • Emergency departments • Hospital admissions • Mortality data • Repeated surveys • General practices

Question 33

What are types of Measures of disease frequency?

Answer 33

• Incidence and Prevalence • Mortality Rates • Morbidity Measures • Standardized Rates

Question 34

What is the difference between prevalence and incidence?

Answer 34

• Prevalent cases are existing cases • Prevalence measures the proportion of people in a population who have the disease at a given point in time:* (any time of the calendar year (Jan 1st, July 1st, Dec 1st). Could have different times because there’s changes depending on season and death) • Incident cases are new cases • Incidence measures how quickly people are developing disease

Question 35

How do you calculate prevalence?

Answer 35

Number of people with disease at a given point in time __________________________________________________ Total number of people in the population

Question 36

How do you calculate incidence?

Answer 36

Number of people who develop disease in a year ____________________________________________________ Average number of people in the population in same year

Question 37

What is Period prevalence?

Answer 37

Period prevalence measures the proportion of people in a population who have the disease at any time during a specified period – e.g. Did your child have any diarrhoea during the past 7 days? • It includes anyone with disease at the start of the period AND anyone who developed it during the period • Often used for conditions of short duration • An extreme example is lifetime prevalence – i.e. Have you ever had…?

Question 38

What is the incidence rate?

Answer 38

• The number of new cases of a disease that occur during a specified period of time in a population at risk for developing the disease – Nominator: New cases – Denominator: Population at risk during the specified period of time

Question 39

What is the Population at risk?

Answer 39

Not everyone is ‘at risk’ of disease – e.g. women who have had a hysterectomy cannot develop uterine cancer – The incidence rate of uterine cancer among all women will be lower than that among women with an intact uterus (Why? - you can’t use all women, only women who still have a uterus. If not, it’s underestimated) • When comparing rates check they have used the same denominator

Question 40

Why does prevalence always change?

Answer 40

- because incidence is added - because some people are cured or dead

Question 41

What is the Relationship between incidence and prevalence?

Answer 41

Prevalence= incidence x duration * in a steady-state situation, in which the rates are not changing and in-migration equals out-migration, and when the prevalence is not too high

Question 42

Prevalence (P) is affected by what?

Answer 42

– Incidence rate (IR) of disease – Duration (D) of disease (prior to cure or death) We can estimate: P = IR x D Or, more accurately: P/(1-P)

Question 43

Ex: 100 children in total. • 9 sick on day 1 • 7 got sick during the week What percentage: • was sick on day 1? • of those at risk and who became sick during the week?

Answer 43

• Sick on day 1 9 ÷ 100 = 9% = Prevalence • ‘At risk’ of getting sick 100 – 9 = 91 • Became sick 7 ÷ 91 = 7.7% = Incidence proportion

Question 44

What is the incidence proportion?

Answer 44

Measures the proportion of people (at risk) that developed disease during a specified period of time: Number of people who develop disease in a specified period (7) _____________________________________________________ Number of people at risk of developing the disease at the start of the period (91)

Question 45

What are the criterias for incidence proportion?

Answer 45

• Must specify the relevant time period • More accurate for short time periods or rare diseases • Assumes no-one is lost to follow-up during the period

Question 46

What are Problems with incidence proportion?

Answer 46

Imagine you identified 5000 healthy men in 2013 and followed them for 5 years to see how many had a heart attack • By 2018, 250 had had a heart attack Incidence proportion = 250 ÷ 5000 = 5% in 5 years • But what if: – Some men move away and you don’t know what happens to them? (You’ll never be able to follow 100% of the men in your study) – Some men die from other causes? (Drop out) • Will the incidence proportion still be accurate?

Question 47

What is the Person-time measurement?

Answer 47

• Instead of counting people at risk, count the amount of time they were at risk* for: e.g. 5000 men x 5 years = 1000 men x 25 years = 25 000 men x 1 year = 1 man for 1 year + 1 man for 1.7 years + 1 man for 2.5 years + 1 man for 2.7 years ++… = 25 000 person years • Can be measured in person-years, person-months, person-weeks etc. * When someone develops disease they are no longer ‘at risk’ so no longer contribute person-time

Question 48

What is the incidence rate?

Answer 48

Measures how quickly people are developing disease Number of people who develop disease in one year ———————————————————————— Number of people (at risk) in the population during the same year OR Number of people who develop disease ———————————————————————— Number of person-years when people were at risk of getting the disease

Question 49

What is the Incidence rate versus incidence proportion?

Answer 49

• Incidence proportion measures risk or probability of getting disease • Incidence rate measures how quickly (the rate at which) cases occur • Incidence rate is easier to compare with other studies than incidence proportion if they have a different length of follow-up

Question 50

Incidence rates can also be estimated from what?

Answer 50

routinely collected data • Epidemiological studies: No. new cases during follow-up/Total person-time at risk e.g. 49/105 person-years • Routine data: No. new cases in period (1 yr) / Average or mid-year population e.g. 49/105 people/year • These are equivalent: If mid-year pop. = 500 000 Þ 500 000 person-years

Question 51

What are important points regarding the numerator and denominator?

Answer 51

• Clear definitions of numerator (cases) and denominator (population at risk) are essential • Numerator and denominator must refer to the same time period • Everyone in the numerator must also be in the denominator (persons or person-time) • All individuals in the denominator must be ‘at risk’ of being counted in the numerator • When comparing measures, check they refer to the same number of people (e.g. per 1000, 100 000 etc.)

Question 52

What does “crude” mean in “crude mortality rates”?

Answer 52

No adjustments (ex: age, sexe, etc)

Question 53

How do you calculate the Annual mortality rate for all causes (per 100, 000 population)?

Answer 53

Total # of deaths from all causes in 1 year ————————————————————- # of persons in the population at midyear X100,000

Question 54

How do you calculate the Annual mortality rate for a specified disease (per 100, 000 population)?

Answer 54

Total # of deaths from this disease in 1 year —————————————————————- # of persons in the population at midyear X100,000

Question 55

What is the difference between proportion and rate?

Answer 55

Proportion Numerator: Part of the total (included in denominator) Denominator: Entire population at risk Time component: No explicit time dimension Measure examples: Case fatality Application: Static measures Rate Numerator: Events, in the population Denominator: Population at risk over time Time component: Explicitly includes time Measure examples: Incidence rate Application: Dynamic processes or trends over time

Question 56

T or F: age and sets are the most common for standardized factors?

Answer 56

True

Question 57

What’s the differences between Direct versus indirect standardisation?

Answer 57

Direct Measure: Age-standardised rate Estimates: Rate you would see in a standard population if it had the same age-specific rates as the study population Data required: Age-specific rates in study population Age distribution of standard population Advantages: By using the same standard population we can compare age-standardised rates between populations Indirect Measure: Standardised incidence/mortality ratio (SIR / SMR) Estimates: Ratio of events observed in the study population to the number expected if it had had the same age-specific rates as the standard population Data required: Age-specific rates in standard population Age distribution and total events in study population Advantages: Can be used for small study populations but we cannot compare SIR/SMR for different populations

Question 58

What is case-fatality and how do you calculate it?

Answer 58

case-fatality % for a specified disease Total # of individuals dying during a specified period of time after disease onset or diagnosis ———————————————————————— # of individuals with this specified disease X100

Question 59

How do you calculate proportionate mortality?

Answer 59

Proportionate mortality for a specified disease Total # of deaths from the specified disease in a given time period ———————————————————————— # of deaths in the same given time period X100

Question 60

How do you determine the Years of potential life lost?

Answer 60

• Step 1: Pre-determined “average” age at death – If the average age at death: 80, for an infant dying at 1 year of age, the years of loss is 79 • Step 2: Add up the individual years of life loss for the population

Question 61

What is Disability-adjusted life years (DALY)?

Answer 61

• the number of years of life lost to premature death and years lived with a disability of specified severity and duration • a DALY is 1 lost year of healthy life.