Week 2 Flashcards
What is a disease?
• 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
What are the 2 types of diseases?
• 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
What is an illness?
• Ill health (subjective and objective)
• Limit one’s ability to live a normal life
• i.e., psychological distress
Broad term
Physical and psychological
What is sickness?
The feeling of not feeling well
It can be subjective or objective
What is the chain of infection?
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
What are the Modes of disease transmission?
• Direct
– Person to person contact
• Indirect
– Common vehicle
• Single exposure
• Multiple exposures
• Continuous exposure
– Vector
• Carries the infection to the host (animal, mosquito)
So many things are responsible for us to contract diseases. What are some examples?
Social environment
Physical environment
Genetic endowment
Health care
Health and function
Prosperity
Well-being
Individual response (behaviour, biology)
What are different types of Measures in disease frequency?
• 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
What is the difference between Endemic, Epidemic, and Pandemic?
• 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
What is a disease outbreak?
Higher number of cases than expected in a location within a certain time period
What are the 3 big Steps of an outbreak investigation?
What is the problem?
What is the cause?
What can we do with it?
What are the steps in the main step of “What is the problem?”?
- Prepare for field work
- Establish the existence of an outbreak
- Verify the diagnosis
- Construct a working case definition
- Find cases systematically and record information
- Perform descriptive epidemiology
What are the steps in the main step of “what is the cause?”?
- Develop hypotheses
- Evaluate hypotheses epidemiology
- As necessary, reconsider, refine, and re-evaluate hypotheses
- Compare and reconcile with laboratory and/or environmental studies (dosages)
What are the steps in the main step of “What can we do with it?”?
- Implement control and prevention measures
- Initiate or maintain surveillance
- Communicate findings
* trying to bring the curve back to normal
What are the Common stages of most diseases?
• 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
At the stage of susceptibility, what is herd immunity?
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
During the Stage of pre-symptomatic disease, what can we find in communicable diseases vs non-communicable diseases?
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)
True or false: different diseases have different ranges but it does not differ between people for the same disease?
False: it differs, every body reacts differently
What is the formula for attack rate?
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
What are the 3 things we look at when Exploring the occurrence of disease?
• 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)
How does the “who” help us?
Helps us know how to prevent and protect (ex: vaccines for babies vs for adults)
What are the levels of prevention for the common stages of most diseases?
• 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)
What is surveillance?
• 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)
What is the Public health knowledge delivery chain of order?
Multiple data sources -> public health surveillance and public health research (both contribute to information) -> Synthesis and translation -> Population health assessment -> ‘Actionable’
public health knowledge
What are the Conditions under surveillance?
• 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
What is the surveillance cycle?
Data collection (ex: survey) -> data cleaning (what’s important) -> data analysis -> interpretation -> dissemination and public health action
Why do surveillance?
• 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
What are the Surveillance essentials?
• 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
What are different Forms of surveillance …?
• 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)
What is the Exposure–disease–diagnosis pathway?
Person is exposed -> Person develops symptoms -> Person seeks medical attention (clinics, hospital) -> Healthcare provider notifies the case
Public health importance is defined by a range of which factors?
– 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
What are different Sources of surveillance data?
• Statutory notifications
• Emergency departments
• Hospital admissions
• Mortality data
• Repeated surveys
• General practices
What are types of Measures of disease frequency?
• Incidence and Prevalence
• Mortality Rates
• Morbidity Measures
• Standardized Rates
What is the difference between prevalence and incidence?
• 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
How do you calculate prevalence?
Number of people with disease at a given point in time
__________________________________________________
Total number of people in the population
How do you calculate incidence?
Number of people who develop disease in a year
____________________________________________________
Average number of people in the population in same year
What is Period prevalence?
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…?
What is the incidence rate?
• 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
What is the Population at risk?
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
Why does prevalence always change?
- because incidence is added
- because some people are cured or dead
What is the Relationship between incidence and prevalence?
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
Prevalence (P) is affected by what?
– 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)
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?
• Sick on day 1 9 ÷ 100 = 9% = Prevalence
• ‘At risk’ of getting sick 100 – 9 = 91
• Became sick 7 ÷ 91 = 7.7% = Incidence proportion
What is the incidence proportion?
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)
What are the criterias for incidence proportion?
• 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
What are Problems with incidence proportion?
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?
What is the Person-time measurement?
• 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
What is the incidence rate?
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
What is the Incidence rate versus incidence proportion?
• 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
Incidence rates can also be estimated from what?
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
What are important points regarding the numerator and denominator?
• 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.)
What does “crude” mean in “crude mortality rates”?
No adjustments (ex: age, sexe, etc)
How do you calculate the Annual mortality rate for all causes (per 100, 000 population)?
Total # of deaths from all causes in 1 year
————————————————————-
# of persons in the population at midyear X100,000
How do you calculate the Annual mortality rate for a specified disease (per 100, 000 population)?
Total # of deaths from this disease in 1 year
—————————————————————-
# of persons in the population at midyear X100,000
What is the difference between proportion and rate?
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
T or F: age and sets are the most common for standardized factors?
True
What’s the differences between Direct versus indirect standardisation?
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
What is case-fatality and how do you calculate it?
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
How do you calculate proportionate mortality?
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
How do you determine the Years of potential life lost?
• 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
What is Disability-adjusted life years (DALY)?
• 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.
