The SIR and SIS models are the canonical model of epidemics of infections that make people immune upon recovery. Many open questions in computational epidemiology concern the underlying contact structure’s impact on models like the SIR or SIS. Temporal networks constitute a theoretical framework capable of encoding structures both in the networks of who could infect whom and when these contacts happen. In this talk, we discuss the detailed assumptions behind such simulations—how to make them comparable with analytically tractable formulations of the SIR model, and at the same time, as realistic as possible. We also discuss fast algorithms for such simulations and the challenges in improving them.

1. Petter
Holme
Temporal
network
epidemiology:
Subtleties
and
algorithms

3. Temporal networks
i j t
1 2 0
1 2 5
2 3 7
1 2 8
3 4 9
⋮ ⋮ ⋮
0
2
3
4
Time
10
1
5

4. SIR on static graphs
‣Infection rate β → Infection across an SI link* is a Poisson process for its
duration.
‣ . . . *Edge in a static graph.
‣Constant recovery rate ν → Exponentially distributed durations of infection.
‣One seed chosen at random among all vertices.
‣Gillespie
‣Event-driven algorithm (Kiss, Miller, Simon, 2017), a.k.a. Next-reaction
‣Composition / rejection (St-Onge, Comp. Phys. Comm. 2019)
Algorithms

5. SIR on temporal graphs
‣Realism | After all, the goal is to simulate reality
‣Continuity | It should be possible to reduce the time dimension and get static
network epidemiology.
‣Simplicity | Keep the same level of abstraction throughout the modeling.
‣Generalizability | It should be easy to extend the model.
‣Speed | As a tiebreaker among design principles.
Design principles:
P Holme, 2021. Fast and principled simulations of the SIR model on temporal
networks. PLoS ONE 16(2): e0246961.

6. Algorithmic model formulation
‣Initialization | Initialize all individuals to susceptible.
‣Seeding | Pick a random individual i and a random time ti in the interval [0,T). At
time ti, infect i.
‣Recovery | Whenever a node becomes infected, let it stay infected for an
exponentially distributed time δ before it recovers.
‣Contagion | If i got infected at time ti and is still infected at time t > ti, and j is
susceptible at time t, then a contact (i,j,t) will infect j with probability β.

7. Straightforward algorithm
1. Initialize all nodes as susceptible.
2. Run through the contacts in increasing order of time.
3. If a there is a contact between a susceptible and infectious node, then infect the
susceptible node with probability β.
4. Whenever a node gets infected (including the source), then draw its time to recovery
from an exponential distribution, and change its state to I.
5. Stop the simulation when there are no infectious nodes.
0
2
3
4
Time
10
1
5
1. Bisection search to
fi
nd the
fi
rst contact that can spread the disease.
2. Stop the simulations when all infected nodes are no longer active.
Could be sped up with several tricks:

8. 0
A
B
5 10 15 20
a
b
c
d
Time
a:
b
c
d
19
2 15
0 4 6
10
b: a 0 4 6 c: a
d
19
2 15
13 16
d: c 13 16
a 10
Internal representation of the temporal network
Contact lists ordered in decreasing order of the last element.
Event-based algorithm

9. Event-based algorithm
1. Use bisection search to
fi
nd the smallest index k of tij such that ti < tij(k).
Where tij(k) denotes the k’th contact of tij.
2. Add a random number K generated by log(1–X) / log(1–β) to k and
denote the sum by k′. (The probability of the k’th event of a Bernoulli process
occurring.) X is a uniform random number in [0,1).
3. If k′ is larger than tij’s number of elements, then return some out-of-bounds
value (to signal that no contact will spread the disease). Otherwise, return k′
—the contact between i and j that could be contagious.
Contagious contact
Finding what contact between two nodes i and j that would spread the disease, if
any.
a:
b
c
d
19
2 15
0 4 6
10
b: a 0 4 6 c: a
d
19
2 15
13 16
d: c 13 16
a 10

10. Event-based algorithm
1. Pop the individual i with the earliest infection time from the heap.
2. Iterate through the neighbors j of i.
a. If j is susceptible, get the time tj when it would be infected by i (by calling
contagious-contact).
b. If it simultaneously holds that
i. There is no earlier infection event of j on the heap.
ii. i’s recovery time is not earlier than tj.
then put the contagion (i infects j at time tj) on the heap.
Infect
Handling the infection of one node i.

11. Event-based algorithm
Taken together:
1. Read the network and initialize everything.
2. Infect the source node.
3. While there are any nodes left on the heap, call infect.
4. Reset the simulation.
5. Go to 2 until you have enough averages.
6. Evaluate the output.

12. Transmission probability
10
0.1
1
0.01 0.1 1
Recovery
rate
Outbreak
size
2
4
6
8
10
12
14
16
18
Example output: SocioPatterns Gallery day 1

13. 1
2
3
4
5
6
7
0.1 1 10
Outbreak
size
Infection rate
Analytical
Straightforward algorithm
Event-based algorithm
A B
Validation of the program

14. A graph with complex behavior w.r.t SIR
7
1 6 7
5
1 6 7
5
1 6
1
2
3
4
5
0.1 1 10
1
2
3
4
5
6
7
0.1 1 10
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0.1 1 10
3
2
6
3 2 5
3
2
7
5
Sentinel
surveillance
Vaccination
Influence
maximization
Ω Ω
τ
2
1
4 5
6 7
3
β β
β
P
Holme,
Phys.
Rev.
E
96,
062305
(2017).

15. Complexity
N = number of nodes
M = number of edges (node pairs with at least one contact)
C = number of contacts
Straightforward algorithms: O(N + C)
Event-driven algorithm: O(L log N log C)

16. 5
10
15
1 10 100 1000
5
10
15
10 100
5
10
15
Relative
speed-up
1000
1 10
Relative
speed-up
Relative
speed-up
Average degree, z Number of nodes, n
Avg. no. contacts per edge, c
B
A C
z = 2
n = 100
c = 100
n = 100
c = 100
z = 2
0 0 0
Speed-up relative to the straightforward algorithm of
arti
fi
cial networks

17. Speed-up relative to the straightforward algorithm
Transmission probability
Recovery
rate
0
10
20
30
40
Speed-up
1000
100
10
1
0.1
0.1 1
0.01
0.001

18. 1
10
100
1000
10000
1000 10000 100000 1×106
1×107
Number of contacts
Relative
speed-up
Speed-up relative to the straightforward algorithm

19. SIR with
fi
xed infection-duration
Everything like before, except the disease lasts a
fi
xed duration T.
Could we then compute the infection probability of a node for all β at once?
Work
in
progress

20. SIR with
fi
xed infection-duration
Everything like before, except the disease lasts a
fi
xed duration T.
This makes the problem more structured and should be much faster. For
example, we then compute the infection probability of a node for all β at once?
Naïve algorithm idea:
1. For every node i, let a variable xi representing the minimum value of T
needed to reach i.
2. Go through every contact (i,j,t) in increasing t. Update xi to max(xi,tj+xj) where
tj is the time since j was infected, and similarly for xj.

21. The catch
1
2
3
4
5
6
7
1
2
3
4
5
6
7
time
time
S I R
T

22. petterhol.me
Code: github.com/pholme/tsir/
Thank you!