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Identifying
high-risk carriers of infectious diseases is worth the effort.
http://www.nature.com/nsu/010823/010823-6.html
20 August 2001
PHILIP BALL
Sexually transmitted
diseases (STDs) call for
discrimination. Containing
the spread of an STD by
focusing on promiscuous
individuals, who are most
likely to pass it on, should
be cheaper and more
effective than large-scale
random campaigns,
according to two new
mathematical analyses1,2.
Why?
Because the web of
human sexual contacts is
scale-free - there is no
typical number of sexual
partners3.
Many people have
few partners; a few have many. And diseases propagate
differently
through scale-free networks than through
networks
in which contacts between individuals are purely
random.
An epidemic spreads through a random network only when
the disease is transmitted faster than a certain
threshold
value. A disease can be eliminated from a randomly
connected population by keeping the transmission rate
below
this threshold, for example by immunization.
But there is no such threshold in scale-free networks,
so
even a very slow-spreading disease can be sustained at
a
low incidence throughout the population4. And
calculations
now show that uniform, random immunization would fail
to
eradicate the disease.
The upside to scale-free networks is that they are
characterized by a scattering of very highly connected
nodes- 'hubs' that hold the web together. The hubs in this
case are
individuals who have many sexual contacts.
So immunizing promiscuous individuals could effectively
curtail transmission of an STD at relatively
little cost. In other
words, by severing the hubs' connections, the web
rapidly
falls apart, say Romualdo Pastor-Satorras of the
Universitat
Politčcnica de Catalunya in Barcelona, Spain, and
Alessandro
Vespignani of the Abdus Salam International Centre for
Theoretical Physics in Trieste, Italy.
The problem is finding the hubs - promiscuous
individuals
are notoriously hard to identify. Fortunately, as Zoltán
Dezsö
and Albert-Láˇszló Barabási of the University of
Notre Dame
in Indiana show, any targeting of hubs, however
imperfect,
raises the threshold spreading rate above zero,
offering a
chance to stamp out the disease for good.
"Even modestly effective attempts to uncover and
treat the
hubs, if carried out systematically, are more
successful than
policies based on large-scale but random distribution
of the
available treatments," say Dezsö and Barabási.
If strongly
focused, control and prevention campaigns should work
even
if they don't always hit their targets, both teams of
researchers agree.
References
1.Pastor-Satorras, R. & Vespignani, A. Optimal
immunisation
of complex networks. Preprint, July, (2001).
2.Dezsö, Z. & Barabási, A.-L. Can we stop the
AIDS epidemic?
Preprint, July (2001).
3.Liljeros, F., Edling, C. R. , Amaral L. A. N.,
Stanley H. E. &
Aberg Y. The web of human sexual contacts. Nature, 411,
907 - 908 (2001).
4.Pastor-Satorras, R. & Vespignani, A. Epidemic
spreading in
scale-free networks. Physical Review Letters, 86, 3200
-
3203 (2001).
© Nature News Service / Macmillan Magazines Ltd 2001
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