Lecture 7.1: Control of Infectious Diseases Flashcards
Potential of an infectious agent to spread in a
population is dependent on 4 factors, name them
- Probability of transmission in a contact between an.
infected individual and a susceptible one - Frequency of contacts in the population - contact
patterns in a society - Duration of infectiousness
- Proportion of the population/contacts that are already
immune, not susceptible
What does a R0<1 mean?
The disease will disappear
What is the Basic Reproductive Rate (R0)?
Average number of individuals directly infected by an infectious case (secondary cases) during his or her entire infectious period, when she or he enters a totally susceptible population
What does a R0=1 mean?
The disease will become endemic
What does a R0> 1 mean?
There will be an epidemic
What is an Epidemic?
It is the rapid spread of disease to a large number of hosts in a given population within a short period of time
What does it mean when a disease is Endemic?
Regularly occurring within an area or community
R0 is Affected By…? (3)
- The frequency of contacts in the host population
- The probability of infection being transmitted during
contact - The duration of infectiousness
Basic formula for the actual value of R0?
R0= β * κ * D
What does β stand for in R0= β * κ * D? What can affect β?
- Risk of transmission per contact (i.e. attack rate)
- Condoms, face masks, hand washing –> β ↓
What does κ stand for in R0= β * κ * D? What can affect κ?
- Average number of contacts per time unit
- Isolation, closing schools, public campaigns –> κ ↓
What does D stand for in R0= β * κ * D? What can affect D?
- Duration of infectiousness measured by the same time
units as κ - Specific for an infectious disease
- Early diagnosis and treatment, screening, contact tracing
–> D ↓
What is the Effective Reproductive Number (R)?
It is the expected number of new infections caused by an infectious individual in a population where some individuals may no longer be susceptible
What is Immunisation?
- Immunisation is the process whereby a person is
made immune or resistant to an infectious disease,
typically by the administration of a vaccine - Vaccines stimulate the body’s own immune system to
protect the person against subsequent infection or
disease
What is Herd Immunity?
- Level of immunity in a population which prevents.
epidemics even if some transmission may still occur - Presence of immune individuals protects those who
are not themselves immune
What is the Herd Immunity Threshold?
Minimum proportion (p) of population that needs to be immunised in order to obtain herd immunity
How to calculate the Herd Immunity Threshold?
p = 1 - 1/R0
What is an Outbreak?
Occurrence of more cases of disease than expected in a
given area, among a specific group of people, over a particular period of time
Why Investigate Outbreaks? (8)
- Stop the outbreak
- Find and neutralise the source (cause)
- Prevent additional cases
- Prevent future outbreaks
- Improve surveillance and outbreak detection
- Improve our knowledge
- Keep the public’s confidence
- Training
Steps Involved in Outbreak Investigation (9)
- Confirm outbreak and diagnosis
- Define a case
- Identify cases & obtain information
- Describe data collected and analyse
- Develop hypothesis
- Test hypothesis: analytical studies
- Special studies
- Communicate results
- Implement control measure
Multi Locus Sequence Typing (MLST)
Characterisation of bacterial isolates based on the combination of alleles at multiple loci (gene fragments)
Whole Genome Sequencing (WGS)
- Characterisation of isolates based on unique whole
genome sequences - There is some debate about how to interpret WGS in
their entirety
Examples of Epidemic Curves: Common Point Source
- Commonly associated with foodborne illness, where
many people are exposed for a short period of time - Note lack of person-to-person transmission
Examples of Epidemic Curves: Common Persistent Source
Outbreak due to exposure of persons to a noxious influence that is common to the individuals in the group
Examples of Epidemic Curves: Propagated Source
An epidemic that arises when a primary case introduces an infectious agent to a population and subsequent host-to-host transmission results in ‘waves’ of cases
Lab confirmation of Campylobacter takes….hours?
96