Network Epidemic - cs.rpi.edu
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Network Epidemic
F e i m i Yu 11 / 2 9 / 2 0 1 8
11/29/2018 Network Epidemic 2
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Recall of epidemic models without networkβ basic states
Susceptible (healthy)
Infected (sick)
Removed (immune / dead)
S I R Infection
Recovery
Recovery
Removal
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Recall of epidemic models without networkβ definitions
Each individual has <k> contacts (degrees)
Likelihood that the disease will be transmitted from an infected to a healthy individual in a unit time: Ξ²
Number of infected individuals: I. Infected ratio i = I/N
Number of susceptible individuals: S Susceptible ratio s = S/N
11/29/2018 Network Epidemic 4
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Recall of epidemic models without network β comparisons
Susceptible (healthy)
Infected (sick)
Removed (immune / dead)
S I R Infection
Recovery
Recovery
Removal
Susceptible (healthy)
Infected (sick)
Removed (immune / dead)
S I R Infection
Recovery
Recovery
Removal
Susceptible (healthy)
Infected (sick)
Removed (immune / dead)
S I R Infection
Recovery
Recovery
Removal
Time (t)
Frac
tion
Infe
cted
i(t
) Fr
actio
n In
fect
ed i(
t)
Time (t)
Exponential regime: ππ = ππ0πππ½π½ ππ π‘π‘
1βππ0+ππ0πππ½π½ ππ π‘π‘
Final regime: ππ β = 1
Epidemic threshold: No threshold
Exponential regime: ππ = 1 β πππ½π½ ππ
πΆπΆππ π½π½ ππ βπ’π’ π‘π‘
1βπΆπΆππ π½π½ ππ βπ’π’ π‘π‘
Final regime: ππ β = 1 β πππ½π½ ππ
Epidemic threshold: π π 0 = 1
Exponential regime: No closed solution
Final regime: ππ β = 0
Epidemic threshold: π π 0 = 1
11/29/2018 Network Epidemic 5
Real networks are not homogeneous
Real contagions are regional β Individuals can transmit a pathogen only to
those they come into contact with β Epidemics spread along links in a network
Real networks are often scale-free β Nodes have different dgrees
β β¨kβ© is not sufficient to characterize the topology
The black death pandemic
Homogeneous mixing model is not realistic!
11/29/2018 Network Epidemic 6
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Comparison between homogeneous mixing and real network
11/29/2018 Network Epidemic 7
Degree block approximation
Degrees is the only variable that matters in network epidemic model
β Place all nodes that have the same degree into the same block
β No elimination of the compartments based on the state of an individual
β Independent of its degree an individual can be susceptible to the disease (empty circles) or infected (full circles).
11/29/2018 Network Epidemic 8
Network epidemic β SI model
SI model:
Density of infected neighbors of nodes
with degree k
Average degree of the
block k
Split nodes by their degrees
I am susceptible with k neighbors, and Ξk(t)
of my neighbors are infected.
ππππ = ππ0(1 + ππ ππ β1ππ2 β ππ
(πππ‘π‘ππππππ β 1)) ππππππ =
πππ½π½( ππ2 β ππ )
characteristic time for the spread of the pathogen
ππ = β ππππππππ = ππ0(1 + ππ 2β ππππ2 β ππ
(πππ‘π‘ππππππ β 1))
11/29/2018 Network Epidemic 9
Network epidemicβ SI model
An ER random network with <k> = 2
At any time, the fraction of high degree nodes that are infected is higher than the fraction of low degree nodes.
All hubs are infected at any time
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Network epidemic β SIS model
SI model:
Density of infected neighbors of nodes
with degree k
Average degree of the
block k
Split nodes by their degrees
I am susceptible with k neighbors, and Ξk(t)
of my neighbors are infected.
ππππππππ =ππ
π½π½ ππ2 β ππ ππ characteristic time for the spread of the pathogen
Recovery term
ππ =π½π½ππ
spreading rate: High Ξ» indicates high spreading likelihood
Threshold Ξ»c
11/29/2018 Network Epidemic 11
Network epidemicβ SIS model on random network
Epidemic threshold β ππππ = 1
ππ +1
β Exceeds threshold: spread until it reaches an endemic state
β Less than threshold: dies out
11/29/2018 Network Epidemic 12
Network epidemicβ SIS model on scale-free network
Epidemic threshold β ππππ = ππ
ππ2
β In large networks: ππππ β 0, ππ β 0 β This is bad: even viruses that are hard to
pass from individual to individual can spread successfully!
β Why? Hubs can broadcast a pathogen to a large number of other nodes!
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Comparison of the network epidemic models
Model Continuum Equation Ο Ξ»c
SI ππππππππππ
= π½π½ 1 β ππππ ππππππ ππ
π½π½( ππ2 β ππ )
0
SIS ππππππππππ
= π½π½ 1 β ππππ ππππππ β ππππππ ππ
π½π½ ππ2 β ππ ππ
ππππ2
SIR ππππππ
ππππ= π½π½ 1 β ππππ ππππππ β ππππππ π π ππ = 1 β ππππ β ππππ
ππ π½π½ ππ2 β (ππ + π½π½) ππ
1
ππ2ππ β 1
11/29/2018 Network Epidemic 14
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Summary
Network epidemics behave very differently than simple epidemic models
Nodes with higher degree (hubs) are more vulnerable than those with lower degree
In SIS and SIR models, even pathogen with low spreading rate persists iin scale-free networks because of the broadcasting ability of the hubs