Balancing Evidence and Uncertainty When Considering Rubella Vaccine Introduction

Tuesday, 3rd of December 2013

[source]PLoS One[|source]

Epidemic theory and empirical observation show that the average age of infection for a vaccine preventable disease conferring lifelong immunity is predominantly determined by the population birthrate, the level of vaccine coverage and the transmissibility of the disease. In most populations the birthrate is known to some degree of accuracy through census data. Because rubella vaccine is most often distributed as part of a bivalent measles-rubella (MR) vaccine or trivalent measles-mumps-rubella (MMR) vaccine, the coverage that will be obtained upon introduction of rubella vaccine is known with reasonable certainty based on current measles vaccine coverage. However, transmissibility, generally characterized by the basic reproductive number, R₀, is not so easily measured.

In this article, the authors present an analysis aimed at helping policy makers, program funders and other stakeholders articulate the utility of introducing rubella vaccination in specific settings while taking into account the uncertainty in the underlying transmission dynamics of the disease.  The report estimates the median R₀ of rubella in the African region to be 5.2, with 90% of countries expected to have an R₀ ranging between 4.0 and 6.7. The authors then conclude that even in low birth rate settings high vaccine coverage must be maintained to avoid the paradoxical increase in CRS, highlighting the importance of maintaining high levels of vaccination coverage before and after RCV introduction. More details on this technical update are available at:  http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3702572/

Abstract

Background: Despite a safe and effective vaccine, rubella vaccination programs with inadequate coverage can raise the average age of rubella infection; thereby increasing rubella cases among pregnant women and the resulting congenital rubella syndrome (CRS) in their newborns. The vaccination coverage necessary to reduce CRS depends on the birth rate in a country and the reproductive number, R₀, a measure of how efficiently a disease transmits. While the birth rate within a country can be known with some accuracy, R₀ varies between settings and can be difficult to measure. Here we aim to provide guidance on the safe introduction of rubella vaccine into countries in the face of substantial uncertainty in R₀.

Methods:  We estimated the distribution of R₀ in African countries based on the age distribution of rubella infection using Bayesian hierarchical models. We developed an age specific model of rubella transmission to predict the level of R₀ that would result in an increase in CRS burden for specific birth rates and coverage levels. Combining these results, we summarize the safety of introducing rubella vaccine across demographic and coverage contexts.

Findings: The median R₀ of rubella in the African region is 5.2, with 90% of countries expected to have an R₀between 4.0 and 6.7. Overall, we predict that countries maintaining routine vaccination coverage of 80% or higher are can be confident in seeing a reduction in CRS over a 30 year time horizon.

Conclusions:  Under realistic assumptions about human contact, our results suggest that even in low birth rate settings high vaccine coverage must be maintained to avoid an increase in CRS. These results lend further support to the WHO recommendation that countries reach 80% coverage for measles vaccine before introducing rubella vaccination, and highlight the importance of maintaining high levels of vaccination coverage once the vaccine is introduced.