38. Epidemiology and Population Health (HT) Flashcards
What is epidemiology?
The study of health and disease in populations (epi = on; demos = population).
Define prevalence.
- How commonly a disease or condition occurs at a given time.
- Prevalence = Number of diseased persons / Population size
Define risk.
- The probability of an event in a specified interval of time.
- Risk = Number of people who become diseased in a given period of time / Number of people in the beginning
What is the limitation of risk as a concept?
- It only works in ‘closed’ populations, so it does not account for births or deaths.
- Incidence is better.
Define incidence.
- Probability of developing a disease in a very small time interval.
- Incidence = Number of people who become diseased while observed / Sum of observation times of all people
This is better than risk because it does not assume a closed population.
Give an example of prevalence, risk and incidence.
- Prevalence -> 5,875 cases of diabetes in a population of 45,193 = 0.13 (13%)
- Risk -> 226 new cases of diabetes in one year in a town with a population of 45,193, equivalent to a risk of 0.005
- Incidence -> 5.2 per 1000 per year cases of diabetes
How are incidence and prevalence related?
- Incidence is positively correlated with prevalence
- e.g. An incidence of 5.2 per 1000 per year cases of diabetes with an average disease length of 25 years results in a prevalence of 13% (0.52 x 25).
What are the different types of study done in epidemiology?
Observational:
- Case report
- Case series
- Cross-sectional (or prevalence)
- Case-control (or retrospective) [IMPORTANT]
- Cohort (or prospective) [IMPORTANT]
Eexperimental (interventional):
- Randomised controlled trials (RCTs) [IMPORTANT]
What is a case report?
Study of one individual with a disease, timely or rare information. This is often done if the disease is rare.
What is a case series?
- A study of several patients with similar symptoms. May lead to general hypothesis. No controls.
- It is essentially a set of case reports.
What is a cross-sectional (a.k.a. prevalence) study?
- A study that measures exposure and disease in study group at one time point (“snapshot”).
- Frequently takes the form of survey, e.g. questions about diagnosed coronary heart disease, family history, diet.
What is the purpose of cohort and case-control studies?
They aim to explore what causes a disease.
How do case control studies work?
- They are a form of observational study where a group of cases (i.e. people with the disease) are compared with controls (i.e. people without the disease)
- They try to find relationships between risk factors and the likelihood of developing the disease
Describe the different steps in a case-control study.
- Choose a well-recorded population containing some people who now have or who did have a precisely defined disease
- Define Cases (those with the disease)
- Hypothesise (“guess”) what may have caused illness (morbidity) or death (mortality)
- Select matched population who might have developed pathology but didn’t (i.e. Controls, who stayed healthy)
- Obtain data about the guessed cause (put same questions
- Determine incidence of supposed causative factor
- Compare the results from the two groups
Give some examples of classic observational studies in epidemiology and the researchers who carried them out.
[EXTRA]
- Ignaz Semmelweis -> Child bed fever in Vienna’s maternity wards
- John Snow -> Cholera outbreak in London
- Joseph Goldberger -> Pellagra
- Alice Stewart -> Childhood Leukaemia
Describe Ignaz Semmelweis’ study about child bed fever. What type of study was this?
[EXTRA]
- Child bed fever (puerperal fever) is a bacterial infection frequently developed after childbirth. It can result in septicaemia.
- Semmelweis’ noticed that incidence of child bed fever was much higher in a Vienna hospital ward 1 than 2.
- He noted that ward 1 was attended by doctors who also performed autopsies, while ward 2 was attended by midwives who did not perform autopsies
- He hypothesised that this had something to do with the matter
- This was sort of a CASE-CONTROL (retrospective) study
- Semmelweis then initiated a controlled trial using chloride of lime solution (bleach) for hand-washing.
- Mortality rate fell to 2% in the First Maternity Ward (same as Second).
- However, his ideas were not accepted by the medical community since there was no evidence of germ theory yet.
Describe John Snow’s study about cholera in London. What type of study was this?
[EXTRA]
- John Snow lived when there was a high incidence of cholera in London
- He noticed that there were two companies supplying water from different sources (one cleaner than the other)
- He used this as an opportunity for a COHORT (prospective) study
- He compared the cholera death rates of people supplied by the two water supplies, finding that one water supply lead to much higher incidence of cholera, suggesting that this water was contaminated
Describe Joseph Goldberger’s study about pellagra.
[EXTRA]
- Pellagra is a disease characterised by the 4 D’s (diarrhea, dermatitis, dementia and death)
- It is caused by chronic lack of niacin (vitamin B3) in the diet
- It was throught that pellagra was an infectious disease.
- But in the first three weeks of studying Pellagra in the Southern parts of the US, Goldberger noticed that Pellagra was:
- a) Present almost exclusively in rural areas
- b) Associated with poverty
- c) Associated with the relative cheap and filling diet consisting of meat, meal (corn meal) and molasses (3 Ms)
- d) Not acquired by nurses, attendants or employees of hospitals or orphanages whose inmates had the disease
- He therefore hypothesised that pellagra is not an infectious disease and showed this by (1) inducing pellagra in prisoners by giving them mostly corn-based diets and (2) by giving “filth” from pellagra patients to healthy volunteers
In case-control studies, what are odds?
Odds are the probability of an event occurring, divided by the probability of the event not occurring.
What is odds ratio?
A statistic that quantifies the strength of the association between two events, A and B.
Calculate the odds ratio for this data.
Odds ratio = Odds that child with leukaemia had been X-rayed / Odds that child without leukaemia had been X-rayed
What are the strengths and weaknesses of case-control studies?
Strengths:
- Good for rare disease
- Relatively fast
- Relatively inexpensive
- Can look at the association between the disease of interest and many kinds of exposures
Weaknesses:
- Susceptible to bias
- May be hard to find suitable controls
- Time relations may not be clear
- Associations that are revealed may not be causal
What is the general principle of a randomised control trial?
The subjects are randomly allocated into two groups and then one group is given an experimental treatment. Outcomes are then assessed.
What is relative risk?
Ratio of the risk of developing a disease in the group exposed to a factor divided by the risk in the unexposed group.
For example, the risk of developing CVD in a statins group divided by the risk in the control group.
Compare relative risk and odds ratio, including when each is used.
- Relative risk is used in RCTs, while odds ratio is used in case-control studies
- Note that odds are calculated differently than risk
What are some important aspects of RCTs?
Compare the incubation period and latent period.
- Latent period = The time between catching an infection and CLINICAL SIGNS (i.e. immune response) appearing.
- Incubation period = The time between catching an infection and SYMPTOMS appearing.
Give an equation for R0.
What are some side effects of vaccines?
Give an equation for the critical level of immunisation need to eradicate a disease.
- pc = Critical rate of immunisation
- The equation shows that if there is a very infectious, quickly spreading virus, high levels of immunisation are required for eradication
State some of the main routine vaccinations in children in the UK.
Also: Meningitis B, Rotavirus
Summarise measles, mumps and rubella infection symptoms.
