Swedish Institute for Infectious Disease Control,

Karolinska Institutet,

Stockholm University

Martin Camitz

A stochastic model of a moderately A stochastic model of a moderately contagious diseasecontagious disease11 in Sweden and in Sweden and the effect of restricting travel as a the effect of restricting travel as a

control strategycontrol strategy

1Read SARS

A stochastic model etc…A stochastic model etc…

• About the model– Hufnagel et al’s model1

• Results

1Hufnagel, L., D. Brockmann, and T. Geisel, Forecast and control of epidemics in a globalized world. Proceedings of the National Academy of Sciences of the United States of America, 2004. 101(42): p. 15124-15129.

I

Stochasticitics… what?Stochasticitics… what?• What happens?

• When does this happen?

+I

R

I

Very randomVery random

SLIR-modelSLIR-model

IS L R

3 events

•Number of infectious

•Infectiousness

•Incubation time •Recovery time

etc…

×289

SLIR-modelSLIR-model

IS L R

3 events

•Incubation time •Recovery time

in Solnain Solna

•Infectious in other municipalities

•Travel intensity

•Number of infectious

•Infectiousness

in Solna

IntensitiesIntensities

IS L R

•Incubation time •Recovery time

•Infectious in other municipalities

•Travel intensity

•Number of infectious

•Infectiousness

QL QI QR

in Solna

1. Pick an event

QL QR

QL QI QR

QL QI

2. Pick a time step t

3. Update intensities

QIStockholm

4. Repeat from 1.

Kalmar

Solna

Run it on a really big Run it on a really big PC…PC…

You might get something like thisYou might get something like this

Or thisOr this

Just for sports, let’s not stop this time.Just for sports, let’s not stop this time.

• 1000 runs

• 60 days

• Average it all out

Much later…Much later…

Two questionsTwo questions

1. What happens if we restrict travel?– Say longer journeys than 50 km or 20 km no

longer permitted.

2. What if traveling doesn’t spread SARS as much as we thought?

Restricting travelRestricting travel

Restricting travelRestricting travel

Fiddling with inter-municipal Fiddling with inter-municipal infectiousnessinfectiousness

• Things that probably affect – Total travel intensity– Medium of travel– Type of transmission

• Does it matter?

Fiddling Fiddling with with

ResultsResults

• It works!

• Travel restrictions slow the spread– Lower incidence after 60 days– Globally and locally

• Comparatative results independent of