Lecture - Public Health (Control of Infectious Disease) Flashcards
What three things differentiates infectious disease epidemiology from that of non-infectious disease?
- The fact that in an infectious disease, the case is a risk factor since prevalence drives incidence. Like, if you’re in a room with a diabetic, you aren’t more likely to get it. If you’re in a room with a flu patient. Control number of prevalant cases to control incident cases
- People maybe immune in infectious diseases but not with non-infectious (like you can’t be immune to diabetes - I mean, yeah sure you can have genes that make you less susceptible but still, vaccines are more intense)
- Epidemiological investigations with infectious diseases may be urgent bc like outbreaks etc
What general factors influence whether an individual (not a pop) will get sick from flu this winter? Aka what’s the epidemiological triangle and one more factor
And what’s the public health version of this question?
Host response
Environment
The agent
Plus chance
• Genera factors influence whether you get sick from flu
o Host/individual factors (eg vaccines or genetic), environment (eg if you’re in a high prev pop), agent (the thing that causes it/the organism) = Epi triad. But also chance – you might be doing the same thing but bad chance
“What influences whether we have a flu epidemic this year?”
If 20 Otago students go and 10 of them get the flu - what determines if this results in a flu epidemic at Otago on their return?
Three things:
- How likely they are to transmit the virus
- For how long they’re infecitous
- How many susceptible people they meet while infectious
Three things influence: likeliness of transmission, how long they’re infectious for (if twice as long then twice as many get infected), and how many susceptible
What happens if these 10 are infectious for one week and infect
- Four others and this transmission pattern continues over the next few weeks (So 10 people infect 4 and that means 4/10 so 0.4 people per person so like the next 4 will infect 4 x 0.4 = 1.6 etc)
- 10 others and this pattern continues?
- 20 others and this transmission pattern continues
(a) 10 wil infect 4, 4 infect 1, and then it goes away
o (b) You have a constant incidence – you have 10 people each week getting it. Not an epidemic
o © 20 people first week, then 40 the following week and it keeps going
So 1. Decreasing incidence and it will soon disappear
- Stable incidence and it remains endemic
- Increasing incidence so an epidemic occurs
What is Basic Reproductive Rate (R0)? Why is it called a rate?
When should you worry?
It is the average number of individuals directly infected by an infectious case, when he/she enters a total susceptible population
Should be called number but called rate by convention
So basically, the key issue is whether your basic repor rate is less than or greater than 1. If less than 1, epi disappears and if greater than 1, then get into endemic state. You’ll have an exponential rise if greater than 1 so worry
What is R0 influenced by? So what’s the formula?
Biological, social and medical factors impact the size of R0, how so?
- Risk of transmission per contact (beta)
- Number of contacts per week (k)
- Duration of infectivity (D)
So biological affects the risk of transmission and duration of infectivity. Social affects risk of transmission per contact and number of contacts per week. Medically basically only affects duration of infectivity.
• Biological, social, medical”
o Risk of transmission can be biological bc like measles more infectious than other organsism etc
o Social factors- what people do like if you have a cold and wear a mask, you’re less likely to infect others etc. SO yes, it depends on the flu virus but also what you do when you have the flu
o Duration – could be biological component of disease but also by treatment (medical influence)
o Size of epidemic is dependent on social, bio and medical factors
So as infection results in immunity, what happens to the number of susceptible contacts? So what happens to R0? And what about incidence?
Drop in susceptible contacts
Impact R0
Hence incidence
Will all the students have been infected when incidence drops to 0?
Everyone is susceptible at first and then it drops off but there will always be some who just by chance never happen to get the disease
So the flu will go away not jsut because everyone’s been affected - it’s because the conc of susceptibles have dropped so much that the epidemic can’t be sustainted.
“Epidemic disappeared before all had been infected because conc of susceptible dropped to a level that resulted in R dropping to below 1”
In Year 1, the flu spread through the Uni and 75% were infected. What’s going to happen in Year 2 and Year 3?
What if there’s an immunisation programme in early Year 3 that raised the proportion immune from 40% to 70% so only 30% are susceptible?
In Year 2, some people will leave and they’ll be replaced by non-immune so proportion of immune drops from 75% to 60%
When only 60% are immune, 40% are susceptible: so k value is 0.4 x 10 per week
Year 3, some more will leave that proportion of immune will drop from 60% to 40%. And when 40% only are immune, 60% are susceptible. This will increase the R0
So with annual introduction of same flu, epidemics may not occur every year
When 70% are immune, R0 goes down. Not all are immunised but there is no epidemic since most of the non-immunised are protected (HERD IMMUNITY)
What is herd immunity?
Control of the disease through increasing the proportion immune in the population to a level such that spread not sustained
Why we can protect pop through immunisation even if you don’t have complete immunisation – protected through those who have vaccine
The basic reproductive rate is conceptually useful because why?
It provides a framework to plan control by minimising “drivers of spread’ (beta, k and d) and for mathematical modelling
BUT IT IS DIFFICULT TO ACTUALLY MEASURE
Individual vs Public Health approach to control:
Individual:
- What does it focus on reducing?
- How does it reduce the risk of acquisition?
Public Health/population approach:
- What sort of aim does it have compared to individual to reduce the spread?
- What two things does it aim to reduce?
-How do you reduce prevalence
Individual:
- Reduce individual risk (risk of acquisition)
- Avoid contact with infection, decrease risk of acquisition if contact occurs
Public Health:
- Much broader apprach
- Decrease prevalence and all “Drivers of Spread” (B = decrease risk of acquisition and transmission, k = decrease mixing between infected and uninfected, D = decrease duration of infection)
To reduce prevalence:
- treat the disease so endure diagnosis and treatment services are available and sometimes screen for carrier
- immunise people: provide immunisation programmes and encourage immunisations to establish herd immunity
So public heal control on infectious diseases aim to control what?
Prevalance and drivers of spread (infectivity, pattern of mixing and duration of infection)
Theoretically, disease eradication through herd immunity is possible if……5
- Immunisation coverage with effective vaccines high enough to maintain R<1 for long enough so organism can’t circulate annnnnnd
- No persistent human carrier state
- No non-human vector
- The agent cannot survive elsewhere
- The agent is not reintroduced
