CSU 25/2010: HERD IMMUNITY ACQUIRED INDIRECTLY

Wednesday, 3rd of March 2010

CSU 25/2010: HERD IMMUNITY ACQUIRED INDIRECTLY FROM INTERACTIONS

BETWEEN THE ECOLOGY OF INFECTIOUS DISEASES, DEMOGRAPHY AND ECONOMICS

It is widely known that declines in child mortality lead to declines in fertility. Does high fertility lead to high mortality? According to Bonds and Rohani, 'an important principle that has been entirely absent from demographic transition theories that further explains correlations between fertility and mortality is that, through the ecology of infectious diseases, fertility causes high disease burdens and therefore mortality.'

This statement, if verified, has obvious implications for achievement of MDG 4, usually considered attainable primarily through interventions aimed at nutritional disorders and infectious disease, without achieving fertility reductions.

From the authors' abstract:

'. . . A particularly subtle phenomenon that receives little attention in the epidemiology literature and is especially important for poor communities is the role of the birth rate as an important direct cause of high disease burdens. Because of their high rates of transmission and life-long immunity, the persistence of many child diseases such as measles relies on high rates of reproduction as their source of susceptible individuals. Thus, there are significant direct health benefits of lower fertility rates, which are further enhanced by interactions with economic processes. Indeed, fertility, poverty and disease all interact with each other in important and predictable ways that can be built into traditional disease ecology models. We present such a model here that provides insights into the long-term effect of policy interventions. For example, because of indirect income effects, herd immunity may be acquired with lower vaccine coverage than previously thought. Reductions in the disease burden can also occur through lower fertility. Our model thus provides a disease ecology framework that is useful for the analysis of demographic transitions.'

Full text, with grapics and equations, is at http://rsif.royalsocietypublishing.org/content/early/2009/09/08/rsif.2009.0281.long

Good reading.