Complex networks such as the sexual partnership web or the Internet often show a high degree of redundancy and heterogeneity in their connectivity properties. This peculiar connectivity provides an ideal environment for the spreading of infective agents. Here we show that the random uniform immunization of individuals does not lead to the eradication of infections in all complex networks. Namely, networks with scale-free properties do not acquire global immunity from major epidemic outbreaks even in the presence of unrealistically high densities of randomly immunized individuals. The absence of any critical immunization threshold is due to the unbounded connectivity fluctuations of scale-free networks. Successful immunization strategies can be developed only by taking into account the inhomogeneous connectivity properties of scale-free networks. In particular, targeted immunization schemes, based on the nodes' connectivity hierarchy, sharply lower the network's vulnerability to epidemic attacks.


Originally published in Physical Review E, v.65 no.3 (2002), 36104. DOI:10.1103/PhysRevE.65.036104. Dr. Vespignani is affiliated with Northeastern University as of the time of deposit.


complex networks, containment, infectious disease, immunization

Subject Categories

Computer viruses, Epidemics, Internet




American Physical Society

Publication Date


Rights Information

©2002 American Physical Society

Rights Holder

American Physical Society

Click button above to open, or right-click to save.

Included in

Physics Commons