Transmissibility and Dynamics of Infection Flashcards
What is an Infectious Disease?
An illness due to a specific agent or its toxins that arise through transmission of the agent/toxin from an infected person animal or reservoir to a susceptible host
What can a transmissible disease also be known as?
A communicable disease
In terms of the two components of epidemiological studies, what is particular about infectious disease epidemiology?
The infectious diseases are an outcome to be studied but also a risk factor for further disease as they transmission is propagated by disease
Which are the three ways that Infectious disease can be spread?
- Direct (person to person)
- Indirect
- Transmission cycles
List some ways that Infectious disease can be spread directly
sneezing/coughing vertical transmission mucosa to mucosa blood and transplants skin to skin
What propagates indirect transmission of infectious diseases and what can these be?
fomites
Fomites can include, water, food and other objects which are then known as vectors
When are transmission cycles observed?
which things may they require the presence of?
They are often observed with parasitic diseases
they can require the presence of hosts, reservoirs and vectors
Which two things determine the transmissibility of an infection within a population? (concerning the organism)
- The length of time an infected person remains infectious for
- The probability of transmission given contact between an infectious person and a susceptible individual
Which two things determine the transmissibility of an infection within a population ( concerning the characteristics of the environment)
- The type of contact occurring within the environment
2. The number of contacts happening in the environment
What are 2 common measures of infectiousness?
Attack Rates
Reproduction numbers
What is the definition of Attack Rates?
The number of new cases occurring in the duration of the outbreak divided by the population at risk at the start of the outbreak
What is the formula for attack rates?
AR = The number of new cases during the outbreak (D) / the population at risk at the start of the outbreak (N0
AR = D/N
5000 people in a refugee camp, 2000 people developed cholera in an outbreak.
What is the attack rate of cholera in this instance?
AR = D/N
= 2000/5000 = 0.4 = 40%
What is the definition of a secondary attack rate?
The proportion of those exposed to the primary case that develop the disease as a result
Measure of the incidence of new cases over those at risk
When are attack rates used?
Used in outbreaks or epidemics of infectious diseases
when are secondary attack rates used?
Used in infectious disease spread in small communities such as households or schools
147 cases of monkey pox
The number of people who shared a home with a case was 834
Of these 834 people, 36 people became infected.
What is the 2AR in this instance?
Number of people who become infected after contacting a case/the number of people who were in contact with a case
2AR = 36/834 = 0.0432 = 4.3%
Over which time periods are attack rates measured?
Short, specific periods of time, usually the duration of an outbreak or an epidemic
Are attack rates risks or rates?
They are actually risks
Which 3 variables are secondary attack rates dependent on?
- The intensity of the contact - higher for household contacts than neighbour contacts
- The type of contact - some will be higher for some contacts rather than others
- The stage of the illness - there will be variation with the infectiousness of an individual throughout the duration of an illness
What is a basic reproduction number?
The average number of secondary cases per typical case in a totally susceptible population
The number of successful transmissions per 1 case
How can the basic reproduction number be measured directly?
By counting the average number of secondary cases of an infection if introduced into a totally susceptible population
Give two examples of a totally susceptible population.
- The introduction of an infection reintroduced onto an island where it has previously been absent
- The spread of a newly emerging infection
Which three things will the basic reproduction number be dependent upon?
- the probability of infection being transmitted during contact
- The duration of infectiousness
- The contact rate
Using the dependent variables, what is the formula for the basic reproduction rate?
R0 = probability of infection during contact (P) x duration of infectiousness (D) x contact rate (c)
R0 = pcD
What does it mean if R0>1?
An epidemic will occur on average
Means that control measures are necessary to prevent or delay the epidemic
What does it mean if R0<0?
Means that an epidemic will not result on average.
Means that no control measures are necessary to prevent or delay an epidemic
In terms of secondary cases, what happens when a disease has been present in a population for some time and why?
A proportion of the population will become immune or they will already be infected meaning that not all contacts will lead to transmission and therefore the average number of secondary cases will be reduced
What is the Net Reproduction Number?
R(t) = The average number of new cases per case in the population
How can the net reproduction number be denoted?
Rn
Rt
Re
What is the mathematic formula for the net reproduction number?
The basic reproduction number multiplied by the fraction of the host population which is susceptible
Rt = R0 x st R0 = Rt/st
In order to calculate the net reproduction number, what do we have to assume?
Homogenous mixing - we assume that infectious cases are as likely to interact with immune persons as with susceptible people
What is the R0 of a population in which 90% are immune and 10% are susceptible?
The disease is stable
Rt = 1 as the disease is stable
Fraction that is susceptible (st) = 0.1 (10%)
R0 = Rt/st
= 1/0.1 = 10
In order to calculate Rt, we need to know the proportion of the population which is immune.
How do we usually do this?
Where there is good serological correlate of immunity, this can be estimated through serological surveys
