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Epidemiology > Transmission Models > Flashcards

Transmission Models Flashcards

1
Q

Infectious disease events

A

—> encounter –> entry –> spread –> multiplication –> damage –> outcome

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2
Q

Communicability

A

Ease with which a disease agent is spread within a population

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3
Q

What does transmission require?

A
  • susceptible host
  • an agent
  • the right environment to establish an effective contact
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4
Q

Effective contact

A
  • contact between infected and susceptible hosts

- contact that results in transmission of disease agent

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5
Q

Mode of transmission

A

Reservoir –> portals of exit –> portals of entry –> new host

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6
Q

Reed Frost model

A

Used to predict the number of new cases in the next incubation period interval
- envisions primary elements in an outbreak

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7
Q

The RF model is primarily ________

A

Conceptual

  • can describe the behavior of a highly communicable disease, or introduction of a new agent
  • rarely a good estimate in endemic diseases
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8
Q

C(t+1) = St(1-q^Ct)

A

C(t+1): cases in time (t+1) where 1 represents an interval of time equal to the incubation period of the agent
St: # of susceptible at time t
q: probability of no contact
Ct: cases at time t

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9
Q

What are 3 factors that affect the RF equation?

A
  • number of susceptible
  • probability of contact
  • number of cases at the start
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10
Q

_______ drives the equation faster

A

Low probabilities of contact

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11
Q

________ equals a faster epidemic rate

A

Higher number of susceptibles

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12
Q

What does the RF model assume?

A
  • infection is spread solely by adequate contact between individuals (no indirect, no fomite or vector)
  • all individuals are susceptible (no prior immunity)
  • individual develops infection after contact
  • infected individuals will recover and be non-susceptible
  • fixed probability of adequate contact
  • population is closed
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13
Q

The RF model is a ______ model

A

Deterministic

- based on fixed probability of adequate contact, which does not vary over time or between individuals

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14
Q

Stochastic model

A

Allows probability of contact to be defined by a distribution of possible values

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15
Q

What are the 3 classes an individual could be in?

A
  • susceptible
  • infected
  • recovered
  • used by both RF and R0 models*
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16
Q

R0 model

A

Intrinsic (basic) reproductive rate
- average number of secondary cases that develop from one primary case during its entire communicable period in a population of susceptible hosts

17
Q

R0 formula

A

= # of contacts per unit time x probability of transmission per contact x duration of infectiousness
*first 2 inputs relate to host/environment, last one relates to agent/host

18
Q

R0 = 1

A

If during the period of communicability, one infected animal can infect one susceptible

19
Q

R0 = 2

A

If during the period of communicability, one infected animal can infect 2 susceptible

20
Q

R0 = 0.5

A

If during the period of communicability, 2 infected animals can infect 1 susceptible

21
Q

R0 > 1

A

Infection is maintained

22
Q

R0 < 1

A

Infection can not be maintained

- number of new cases are greater than number of old cases (?)

23
Q

How to reduce R0 with number of cases per unit time

A
  • reduce number of animals
  • add nonsusceptible animals
  • change susceptible to nonsusceptible
  • reduce animal density
  • restrict animal-animal contact
24
Q

How to reduce R0 with probability of transmission per contact

A
  • restrict contact to be non-effective

- alter mucosal disease

25
Q

Duration of infectiousness

A

Property of agent

  • antimicrobial/antiviral therapy
  • immune stimulation
  • environmental survival of agent (sanitation, decontamination)
  • dust and aerosol exposure
  • vector control/preventing exposure to vector
26
Q

Agent pressure

A
  • number of cases of active disease

- duration of disease/infectiousness (within host/in environment)

27
Q

Herd immunity

A

Ability to protect susceptible individuals within a group due to the high proportion of immune individuals in the same population

  • below a critical number of susceptible animals, outbreaks are not supported
  • innate or acquired
28
Q

Herd immunity reduces the number of ________

A

Susceptible (St)

- alters probability of transmission per contact/effective contact

29
Q

Herd immunity of _____ is usually sufficient

A

70-80%

  • rabies vax is 70%, epidemic will not propagate if 39-57% are vaccinated
  • human measles requires 94%
30
Q

What does herd immunity depend on?

A

Properties of agent

  • infectious dose
  • duration of communicability
  • methods of contact, routes of entry

Epidemiology (19 decks)

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