Abstract
Measles eradication efforts have been successful at achieving elimination in many countries worldwide. Such countries actively work to maintain this elimination by continuing to improve coverage of two routine doses of measles vaccine following measles elimination. While improving measles vaccine coverage is always beneficial, we show, using a steady-state analysis of a dynamical model, that the correlation between populations receiving the first and second routine dose also has a significant impact on the population immunity achieved by a specified combination of first and second dose coverage. If the second dose is administered to people independently of whether they had the first dose, high second-dose coverage improves the proportion of the population receiving at least one dose, and will have a large effect on population immunity. If the second dose is administered only to people who have had the first dose, high second-dose coverage reduces the rate of primary vaccine failure, but does not reach people who missed the first dose; this will therefore have a relatively small effect on population immunity. When doses are administered dependently, and assuming the first dose has higher coverage, increasing the coverage of the first dose has a larger impact on population immunity than does increasing the coverage of the second. Correlation between vaccine doses has a significant impact on the level of population immunity maintained by current vaccination coverage, potentially outweighing the effects of age structure and, in some cases, recent improvements in vaccine coverage. It is therefore important to understand the correlation between vaccine doses as such correlation may have a large impact on the effectiveness of measles vaccination strategies.
Highlights
As of early 2017 measles was officially eliminated from seven countries in the Western Pacific, 24 countries in Europe, and all countries in the Americas, despite occasional outbreaks that threaten this elimination status [1,2,3]
Several resurgent outbreaks have occurred; for example, the 2013 outbreak in Brazil, which lasted more than 1 year and changed Brazil’s elimination status [1, 8], the 2015 outbreak in Mongolia [9], which occurred less than 1 year after Mongolia was certified measles free [10], and the 2017 outbreak in Romania, which threatens much of Europe’s elimination status [3]
For a constant level of first- and second-dose coverages, reducing the correlation between doses increases the proportion of the population that receives at least one dose, but decreases the proportion of the population that receives both doses (Fig. 1), increasing population immunity
Summary
As of early 2017 measles was officially eliminated from seven countries in the Western Pacific, 24 countries in Europe, and all countries in the Americas, despite occasional outbreaks that threaten this elimination status [1,2,3]. Measles is a leading cause of vaccine-preventable childhood death [4], and maintaining successful elimination at the country-level is key to regional elimination goals [5, 6]. As measles is a highly transmissible disease, the herd immunity threshold (that is, the proportion of the population that needs to be immune in order to prevent a reintroduction event sparking prolonged re-emergence of the disease) is commonly considered to be about 95% [13]. In order to maintain high levels of immunity, extensive vaccination efforts are continued in countries that have achieved elimination. In most countries, these vaccination efforts involve administering two doses of measles-containing vaccine (MCV) [6, 15]
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