Infectious disease epi Flashcards
When are you dealing with epidemic vs endemic?
More than expected cases: epidemic. If it is not (with or without peak): endemic
What are the five stages of an epidemic?
Spark -growth – peak – decline – dies out eventually
At the spark, there is often an interest in..
Often there is an interest in index cases and primary cases
What is an index case?
- Index case: First noticed outbreak patient by authorities > awareness that outbreak might be emerging. Also outbreaks of disease not spread from human-human.
What is a primary case?
- Primary case: First documented individual who brings the disease into a population (e.g., school class, all passengers on a ship, an entire country). Can only apply to infectious diseases that spread human-human.
Why is it not helpful to list conditions that led to start of epidemic to predict the next?
for every successful zoonotic jump from animal to human, there are millions that fail to infect people at all
How to forecast epidemics?
- Forecasting epidemics is possible by measuring their growth:
- Initial growth is exponential
What is the latent period?
Period between infection and the onset of the infectious period
What is the infectious period?
Time period where transmitting the infectious agent to another host is possible
What is the incubation period?
Infection till onset of symptoms
What is a serial interval?
Time between appearance of similar symptoms in successive generations
What is the generation time?
beginning of infection in an individual to the time that person transmits to another individual
What does the basic reproduction number R0 indicate?
Transmission potential of an infectious agent
How is R0 defined?
- expected number of new infectious hosts
- directly infected by a randomly selected infectious host
- during period of infectiousness
- when randomly mixing in a large, homogenous population
- where everyone is completely susceptible
By what is the R0 influenced?
DOTS
- Duration of infectiousness
- Opportunity: number of contacts in the host population during the time of infectiousness
- Transmission probability: probability of infection being passed on during contact
- Susceptibility: probability of becoming infectious after infection
What can you do to intervene with the duration of infectiousness?
Rapid diagnosis
Rapid isolation following diagnosis
What can you do to intervene with Opportunity: number of contacts in the host population during the time of infectiousness?
Social distancing
Quarantine
Isolation in single-bed, negative pressure rooms
What can you do to intervene with Transmission probability: probability of infection being passed on during contact?
Personal hygiene
Avoiding handshakes
What can you do to intervene with Susceptibility: probability of becoming infectious after infection
Vaccine
Prompt and effective treatment
R0 does not necessarily concern velocity over time, why?
(r0 = 7, means that within the infectious period, 7 people will be infected. Measles has a very short infection time, therefore the disease will spread quicker. R0 = 7 for leprocy, with a long infectious period for a year, will infect 7 people in a year.
R0 < 1 = ?
R0 = 1 = ?
R0 > 1 = ?
- R0 < 1: the disease will eventually disappear (pathogen dies out)
- R0 = 1: the disease will become endemic
- R0 > 1: the epidemic will continue to grow
What is the effective reproduction number (Re)?
▪ Effective reproduction number, Re : as R0, but in a population that includes people who have developed immunity to the pathogen.
Fraction that needs to be immunized = ?
f = 1-1/R0
What does the SIR model stand for?
Susceptible -r- Infectious -a- Recovered and immune
r = rate of transmission between ‘susceptibles’ and ‘infectious’
a = recovery rate of infected individuals into recovered/immune
What are six assumptions of SIR closed cohort?
- Population is constant (closed cohort) and comprises only ‘susceptibles’, ‘infectious’ and ‘recovered’: 𝑆 + 𝐼 + 𝑅 = 1
- Rate of increase in the infectious is constant and proportional to contacts between susceptibles and infectious
- Infectious recover and become immune at a constant rate
- (People meet each other at random)
- (Latent period is zero, i.e., people become infectious as soon as they are infected)
- (The duration of infectiousness is equal to the time of illness)
What is herd immunity?
Protection against a pathogen due to a high proportion of immune population (vaccination/previous infections), reducing the likelihood of infection for susceptibles.