Demographics epidemiology and the impact of vaccination campaigns in a measles-free world – Can elimination be maintained?

Tuesday, 4th of April 2017

Demographics epidemiology and the impact of vaccination campaigns in a measles-free world – Can elimination be maintained?

• J.M. Prada^a   
• C.J.E. Metcalf^{a b c}
• S. Takahashi^a
• J. Lessler^d
• A.J. Tatem^{e f}
• M. Ferrari^g
• ·^a Department of Ecology and Evolutionary Biology, Princeton University, USA
• ·^b Office of Population Research, WWS, Princeton University, USA
• ·^c Fogarty International Center, National Institutes of Health, USA
• ·^d Department of Epidemiology, Johns Hopkins Bloomberg School of Public Health, Baltimore, USA
• ·^e WorldPop, Department of Geography and Environment, University of Southampton, UK
• ·^f Flowminder Foundation, Stockholm, Sweden
• ·^g Center for Infectious Disease Dynamics, Pennsylvania State University, USA

Received 26 August 2016, Revised 26 January 2017, Accepted 3 February 2017, Available online 16 February 2017

Abstract

Introduction

All six WHO regions currently have goals for measles elimination by 2020. Measles vaccination is delivered via routine immunization programmes which in most sub-Saharan African countries reach children around 9 months of age and supplementary immunization activities (SIAs) which target a wider age range at multi-annual intervals. In the absence of endemic measles circulation the proportion of individuals susceptible to measles will gradually increase through accumulation of new unvaccinated individuals in each birth cohort increasing the risk of an epidemic. The impact of SIAs and the financial investment they require depend on coverage and target age range.

Materials and methods

We evaluated the impact of target population age range for periodic SIAs evaluating outcomes for two different levels of coverage using a demographic and epidemiological model adapted to reflect populations in 4 sub-Saharan African countries.

Results

We found that a single SIA can maintain elimination over short time-scales even with low routine coverage. However maintaining elimination for more than a few years is difficult even with large (high coverage/wide age range) recurrent SIAs due to the build-up of susceptible individuals. Across the demographic and vaccination contexts investigated expanding SIAs to target individuals over 10 years did not significantly reduce outbreak risk.

Conclusions

Elimination was not maintained in the contexts we evaluated without a second opportunity for vaccination. In the absence of an expanded routine program SIAs provide a powerful option for providing this second dose. We show that a single high coverage SIA can deliver most key benefits in terms of maintaining elimination with follow-up campaigns potentially requiring smaller investments. This makes post-campaign evaluation of coverage increasingly relevant to correctly assess future outbreak risk.

Keywords

• Measles;
• Epidemiology;
• SIR;
• Mathematical models;
• Vaccines