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Sexual Networks in Contemporary Western
Societies
Fredrik Liljeros
Karolinska institutet
Stockholm University
(Supported by the Swedish Institute for Public Health)
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How is it possible for Sexually transmitted Infections (STI’s) to reproduce themselves when the average reported life number of contacts are as low as 6-12 in western societies?
Why do we have STI’s at all?
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Data sources
National surveys
+ Representative
-Validity?
-Low Response rate
-Only ego network data
Clinical data
+ Network data
-Validity?
-Representative?
Local networks
+ Network data
-Validity?
Representative?
Online networks
+ Network data
-Validity?
Representative
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Representative
Sample Population
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Network data vs ego network data
MaleFemale
?
?
??
?
?
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Validity
Women report twice as few partners on average than men do
Find at least five explanations!
Low response rate
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Data sources
National surveys
+ Representative
-Validity?
-Low Response rate
-Only ego network data
Clinical data
+ Network data
-Validity?
-Representative?
Local networks
+ Network data
-Validity?
Representative?
Online networks
+ Network data
-Validity?
Representative
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Triangulation
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Basic epidemiology
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Reproduction Rate, R
Average number of contacts * probability for transmission * duration of infectiousness
Low for STIs !
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Basic Reproduction Rate, R0
Gottland
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Reported HIV infected in Sweden
www.smittskyddsinstitutet.se S-GEM
Reported Chlamydia infected in Sweden
www.smittskyddsinstitutet.se
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Explanations for the current situation
Probably no single explanation!
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On average, your partners have more partners than you !
8.19
818
k
5.416
8818
partnersk
Newman Social Networks (2003)
May and Anderson Nature (1987)
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The Core-Group Theory
”Scale free distribution of partners ”
Liljeros et al Nature (2001)
National survey data S-GEM
Gay men attending a STI clinic in London
Colgate et al PNAS (1989)S-GEM
Internet contact site data
Holme, Edling and Liljeros Social networks 2004
Prefferential attachent?
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Concurrency A B
C
D E
AC AB
DC DE
2b.
Contact
network
Line graph
Morris and Kretzchmar Social Networks (1995) S-GEM
Concurrency
A B
C
D E
Contact
network
Line graph
Kretzchmar and Morris Social Networks (1995)
?
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Concurrency A B
C
D E
A B
C
D E
AC BC
DC EC
2a.
AC AB
DC DE
2b.
Contact
network
Line graph
Kretzchmar and Morris Social Networks (1995) S-GEM
Assortative interaction
Newman PRL 2002Liljeros Edling and Amaral (2003)
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Bearman, Moodey and Stovel Forthcoming in AJS(With permission from P. Bearman)
Local network motifs
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Australia
Thailand
USA
Holland
Norway
Spain
England
Greece
Switzerland
231 cases
ÖrebroAustria
Falk et al STI (2003)
Örebro hospital: Local of infection with Chlamydia
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Conclusions
• Standard deviation is of high importance
• Contact tracing are likely to be effective
• Targeted interventions
Further reading….
Liljeros F, Edling CR, Amaral LAN ”Sexual networks: implications for the transmission of sexually transmitted infections”
MICROBES INFECT 5 (2): 189-196 FEB 2003 S-GEM
Take home message
Your partners have more partners than you have yourself!
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