Lecture 2: "recap" Flashcards
What other factors than transmission routes are important in containment strategies?
When to consider something a relevant exposure. E.g. with chlamydia unsafe sex is a relevant exposure. With COVID this varies (1 meter, 2 meter, 1.5 meter)
Why was John Snow important in containment strategy/epidemiology?
John Snow was important in outbreak containment and epidemiology. He traced the cholera epidemic in London to the infected water pump on Broad Street, instead of confirming to the popular belief at the time that cholera was caused by bad air
What is outbreak investigation?
Investigating the cause of the outbreak by investigating the people who were affected and the people who were unaffected. What did they eat, where did they go, what did they do, etc? Screening and surveillance
What can happen when you get exposed to an infectious agent?
You can get ill (infected, show symptoms, infectious), nothing (uninfected, no symptoms, not infectious), you can get asymptomatic/subclinical disease (infected, no symptoms, infectious). Whether you’re infectious thus does not always mean you are sick. Sometimes you are infectious before you start having symptoms or after they end. If you get infected, you can develop immunity or not
What is the latent period of a disease?
Latent: exposed, infected, not infectious. Usually before start symptoms
What is an incubation period?
Incubation period: exposed, infected, no symptoms YET
What is important to realise when comparing the incubation period and latent period?
Disease can be passed on before there are clinical symptoms!
What is the difference between a primary case and an index case?
Index case: first case
Primary case: First case in a certain society or community
What does the R-number mean?
The reproductive rate. The rate at which the infection spreads. E.g. R=2 means that every infected person will infect two people. R<1 means the new cases are decreasing, R>1 means the new cases are rising.
Which generic things influence the reproductive rate of a disease?
The infectiousness, (the virulence), the susceptible population, the amount of relevant contacts
What would happen to a disease without containment strategies?
It would disappear. The susceptible population will either be dead or will have built immunity
What disease fits with a quick rise in cases before dying out?
Point-source outbreak, an outbreak where there was one point in time where people were exposed to the disease. The distribution of cases in time is attributed to the variation in incubation time. An example is infected food causing diarrhoea.
What disease fits with a more or less steady number of cases?
An continuing-source outbreak with no propagation (= e.g. human to human spread). An example of this is an infected well or shower causing disease. The source is continuing to infect people, but people don’t infect each other.
What disease fits with a number of “waves” of cases?
Propagated source, e.g. influenza, because that is human to human transmission.
How do you calculate an odds ratio?
An odds ratio (OR) is a statistic that quantifies the strength of the association between two events, A and B. The odds ratio is defined as the ratio of the odds of A in the presence of B and the odds of A in the absence of B, or equivalently (due to symmetry), the ratio of the odds of B in the presence of A and the odds of B in the absence of A. Two events are independent if and only if the OR equals 1, i.e., the odds of one event are the same in either the presence or absence of the other event. If the OR is greater than 1, then A and B are associated (correlated) in the sense that, compared to the absence of B, the presence of B raises the odds of A, and symmetrically the presence of A raises the odds of B. Conversely, if the OR is less than 1, then A and B are negatively correlated, and the presence of one event reduces the odds of the other event.
Note that the odds ratio is symmetric in the two events, and there is no causal direction implied (correlation does not imply causation): an OR greater than 1 does not establish that B causes A, or that A causes B.
Suppose a radiation leak in a village of 1,000 people increased the incidence of a rare disease. The total number of people exposed to the radiation was V_{E}=400 out of which D_{E}=20 developed the disease and H_{E}=380 stayed healthy. The total number of people not exposed was V_{N}=600, out of which D_{N}=6 developed the disease and H_{N}=594 stayed healthy.
The odds of getting the disease if exposed is D_{E}/H_{E}=20/380=approx .052, and the odds if not exposed is D_{N}/H_{N}=6/594=approx .010. The odds ratio is the ratio of the two: (D_{E}/H_{E})/( D_{N}/H_{N})=(20/380)/(6/594)=approx. 5.2
