Unintended Consequences and the Paradox of Control: Management of Emerging Pathogens with Age-Specific Virulence

Abstract

We project the long term incidence of Zika virus disease (ZVD) under varying hazards of infection and consider how the age-distribution of disease burden varies between these scenarios. Pathogens with age-structured disease outcomes, such as rubella and Zika virus, require that management decisions consider their impact not only on total disease incidence but also on distribution of disease burden within a population. In some cases, reductions of overall transmission can have the paradoxical effect of increasing the incidence of severe disease despite decreasing the total incidence. This happens because of corresponding increases in the average age of infection. Beginning with the current population structure and demographic rates of Brazil, we project forward total ZVD burden as measured by cases occurring in pregnant women and document the scenarios under which a paradox of control for Zika management emerges. We conclude that while a paradox of control can occur for ZVD, the higher total costs from increasing the average age of infection will only be realized after several decades and vanish under conservative discounting of future costs. This indicates that managers faced with an emerging pathogen should prioritize current disease incidence over potential increases in severe disease outcomes in the endemic state.\n\nAuthor SummaryThe intuitive response to an emerging outbreak is to halt, or at least reduce, transmission. However, in some circumstances, reducing overall transmission and incidence may be counterproductive from a public health perspective as public health interventions affect both the total level and the distribution of disease burden. We consider the scenarios under which reducing transmission of an emerging pathogen such as Zika virus may increase the costs associated with disease in the most vulnerable segments of the population - in this case, reproductive-age women. We conclude that after applying standard discounting rates to future cases, the \"paradox of control\" vanishes and reducing hazard of infection uniformly reduces the total costs associated with severe disease.