transmission 4: Disease control Flashcards
R0 and eradication
Eradication: intervention measures are no longer required, and the agent that previously caused the disease is no longer present.
In order to eradicate a disease, the average number of secondary cases generated by an index case must be reduced to below 1. -> stop the spread and eventually eradicated when those that are infected die
To achieve this the proportion immunised must exceed the ‘fraction immune at equilibrium (HIT) in the ansence of vaccination’.
To work out the proportion that must be immunised, RO must be calculated.
Eradication examples
Small pox
- In 1980 WHO declared smallpox eradicated
- Vaccine developed in 1796
- Possible due to low R0 and easily detectable symptoms
rinderpest
- Eradicated in 2011
- Vaccine developed in 1960
- Possible as it is a ‘dead end disease’ as infects wild cow herd whose low population density inhbit disease spread.
R0 calculation method: Calculate B
R0= B x D
B: rate of new infections arising from one infected person is a totally susceptible population. ( γ = B X I so γ= B in susceptible population)
D: Duration of infection (1/ sigma)
There are multiple equations that can be used to calculate B depending on the dynamics of the pathogen:
SEXUALLY transmitted pathogens: B = βc
(per partnership probaility of tranmission x average rate of partnr change)
VECTOR-BORNE pathogens : B = ma^2bcH
Althoug conceptually useful, they are not precise enough to estimate R0 and then use this estimation to make important immunisation decisions.
R0 calculation method: Visual observation
Place one infected individual in a pool of susceptible and count the number of secondary cases
Example: SEIR model created for Ebola outbreak
R0 calculation method: Phylogenetic reconstruction of epidemics
Phylogenies reconstructed from contemporary gene sequences allow us to estimate the changing R number Through time.
Increasing R0: long terminal branches
Decreasing R0: short terminal branches
Example: Epidemic hisotry of HCV strain
- Inferred the epidemic history of different subtypes by looking at the genetic diversity in the genes, allowing tree reconstruction.
- Simple SI model was fitted and R0 estimated
Example:
- Study constructed maximum liklihood tree of HIV strain from HIV polymerase gene from the Swiss HIV Cohort study
- From the analyses estimated tranmission parameters
- They used the parameters to esimte R0
- true value =2.25, estimated value from 10 trees= 3.02
R0 calculation method: Using Life span and age of infection
Possible for pathogens that confer life long immunity
R0 = Life span / Age of infection
Life span= 1/ death rate
Use age of first infection or Average age of first exposure = 1/ risk of infection
Measure age of infection
o Longitudinally – follow single susceptible indiv until they get infected (laborious + time consuming)
o Average age of 1st exposure (A)– high risk infection -> more likely get disease earlier (inversely proportional)
What does it take to eradicate a disease?
- A technology for quick detection of all infections
- No non-human reservoir of infections
- Limited geographic spread
- A vaccine that disrupts disease spread
Reduce risk of infection
A. Identifying major transmission routes
B. Interrupting these routes
Examples:
- bed nets for malaria
- better sanitation for cholera
- Indeitfying women with HIV, at risk of tranferring vertically to children. Intervention like cesarian or no breast feeding.
Reducing risk can shift age of first infection
Age of first infection= 1/ Risk of infection
Example: Rubella
- Mild disease which is only a risk to mothers
- If get when are young, immune at mothering age
- Reducing risk can increase age of infection
- > If infects women in first trimester of pregnancy then their children can get congenital rubella syndrome (present from birth) => microcephaly, cataracts (also get w/ ZIKA virus)
- Vaccinate as infants only if >80% coverage can be obtained – otherwise may get perverse effects of intermediate transmission where age first exposure likely at pregnancy age
Example: Polio - nearly eradicated
- Worse in adults than children – more likely to get severe paralysis(1/1000 increases to 1/75)
- Perverse consequence of attempted eradication as decreased risk getting it puts age of 1st exposure back to later where adult symptoms will be worse
Example: Chicken Pox
- If the risk of chickenpox infection is reduced through vaccination without achieving high vaccination coverage rates, susceptible individuals may reach adulthood without being naturally exposed to the virus.
- More extreme symtoms at adulthood
Key considerations for control + problems:
1) Increase size of immune pop’n (vaccination)
! But need necessary coverage so that age of first infection is not pushed back to when individuals are susceptible
! Hard to develop vaccines against antigenically variable infectious agents
2) Reduce risk of infection
! Infrastructure
! Perverse consequences of increase in age of infection (rubella, polio)
Work out R0 and HIT-> how will R0 change through the seasons -> Is HIT sufficient in one season but not another?
How will changing the risk of infection affect age of infection -> better to get as a child?
Is the pathogen evolving and antigenic-ally variable??
Consider co-infection -> how will vaccination against one effect another? -> may control it or may provide opportunity
Consider cross immunity between co infected individuals -> if high cross immunity then vaccination may be good against multiple strains, if not it won’t be.
Wipe out co-infection before vaccination (e.g. tape worms)
Summary
Need to know R0 to calculate the number that should be immunised
methods
- Calculate B
- Visual observation
- Phylogenetics
- Life span and age
If this is not possible, reduce risk by:
- identifying routes of tranmission
- Interupting routes of transmission
Must be carelful not to alter the age of infection as can lead to more severe symtoms.
