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Saving Lives and Livelihoods Amidst a Once-in-a-Century Crisis 9

disease to push R below 1 can eliminate the risk of a large epidemic. This indicates that
0
around the critical value R = 1, it can be worth investing large amounts of effort even to
0
produce small shifts in the basic reproductive number by controlling each of the two factors.
Both (p) and (k) would be impacted by the network structures in a population. Infectious diseases
spread through the human social network, and network effects are significant in influencing the
spread of disease (David Easley & Jon Kleinberg, 2010). The patterns of spread of epidemics are
determined not just by the properties of the pathogen carrying it — including its contagiousness,
the length of its infectious period, and its severity — but also by network structures within the
population it is affecting. The social network within a population, i.e., the modes of interaction
determines a lot about how the disease is likely to spread from one person to another.

The opportunities for a disease to spread are given by a contact network: there is a node
for each individual/organization, an edge if two people come into contact with each other
in a way that makes it possible for the disease to spread from one to the other and a path
linking nodes to edges. A network is said to be connected if any individual (or node)
can be reached from any other by following network links; epidemiologically, this is
equivalent to infection being able to reach the entire population from any starting point.
In this way, each infected individual is linked to one other from whom they caught the
infection, and additionally, to a variable number of others to whom they transmitted the
disease, thus providing a ‘transmission network’ consisting of all the links through which
infection spread in a single outbreak. For a highly contagious disease, involving airborne
transmission based on coughs and sneezes, the contact network will include a huge number
of links, including any pair of people who sat together on a bus or an airplane. Thus,
network structures in a society become very significant in modelling the spread of a
contagious disease and probability of its turning into an epidemic/pandemic.

Mode of Contagion of an Epidemic

High Contagion Probability Low Contagion Probability,
A Contact Network - the Infection Spreads the Infection is Likely to Die
Widely Out Quickly

Adapted from David Easley & Jon Kleinberg, 2010
Note: Bold lines implies spread of infection in the contact network
These epidemic models on networks help to determine the features affecting spread, how
interaction within networks can be restricted, and in particular, how it is possible to reduce
spreading by means of public health measures such as vaccination, (quicker) diagnosis and
treatment, isolation, travel restrictions and so on. A key priority is, therefore, the early and
rapid assessment of the transmission potential of any emerging infection.