Age-specific mixing generates transient outbreak risk following critical-level vaccination.

Monday, 3rd of October 2016

Epidemiol Infect. 2016 Sep 28:1-11. [Epub ahead of print]

Age-specific mixing generates transient outbreak risk following critical-level vaccination.

Bhattacharyya S¹ Ferrari MJ².

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Abstract

Measles elimination goals have been adopted in a range of countries sub-regions and regions since the WHO declared an elimination goal by 2015 or 2020. All countries attempt to achieve and maintain high coverage through routine immunization programmes. This routine strategy however does not ensure the elimination goal of measles. Many developed countries such as the United States that have succeeded in interrupting measles transmission earlier are now experiencing outbreaks with an increasing number of cases. Using a stochastic age-structured model of measles vaccination dynamics we explore and characterize the transient dynamics of measles susceptibility in the years following the implementation of routine vaccination at the herd immunity threshold. We demonstrate how a population could face risks of potentially large outbreaks even within few years of vaccination. We characterize different risk profiles depending on the incidence pattern in the years prior to vaccination. These results suggest that the classic critical vaccination threshold is necessary to achieve herd immunity but not sufficient to prevent long periods of transient super-critical dynamics. Our results suggest the need of future work for more careful monitoring of the impacts of current immunization programmes and developing models that take into account more complicated vaccination strategies demographic factors and population movements.