Polynomial epidemics and clustering in contact networks.

It is widely known that the spread of the human immunodeficiency virus was slower than exponential in several populations, even at the very beginning of the epidemic. We show that this implies a significant reduction in the effective reproductive rate of the epidemic, and describe a general mechanism, related to the clustering properties of the disease transmission network, that is capable of explaining this reduction. Our considerations provide what is, to our knowledge, a new angle on polynomial epidemic processes, and may have implications for the choice of strategy against such epidemics.