Global Transmission Dynamics of Measles in the Measles Elimination Era

Abstract

Although there have been many epidemiological reports of the inter-country transmission of measles, systematic analysis of the global transmission dynamics of the measles virus (MV) is limited. In this study, we applied phylogeographic analysis to characterize the global transmission dynamics of the MV using large-scale genetic sequence data (obtained for 7456 sequences) from 115 countries between 1954 and 2015. These analyses reveal the spatial and temporal characteristics of global transmission of the virus, especially in Australia, China, India, Japan, the UK, and the USA in the period since 1990. The transmission is frequently observed, not only within the same region but also among distant and frequently visited areas. Frequencies of export from measles-endemic countries, such as China, India, and Japan are high but decreasing, while the frequencies from countries where measles is no longer endemic, such as Australia, the UK, and the USA, are low but slightly increasing. The world is heading toward measles eradication, but the disease is still transmitted regionally and globally. Our analysis reveals that countries wherein measles is endemic and those having eliminated the disease (apart from occasional outbreaks) both remain a source of global transmission in this measles elimination era. It is therefore crucial to maintain vigilance in efforts to monitor and eradicate measles globally.

Keywords: elimination; endemic; globalization; measles; outbreak; phylogeography; transmission.

Figure 1

Maximum clade credibility phylogeny for a portion of the N gene of measles virus (MV). Time-scaled phylogenies were inferred for analysis of measles virus transmissions in the USA using a relaxed molecular clock model in a Bayesian MCMC algorithm that incorporates virus sampling dates (year) and sampling location (i.e., the USA or one of the six WHO regions) to estimate phylogenetic trees concurrently. Tip times reflect the year of sampling, and branches are colored according to the most probable location of their descendent nodes or sampling location site of tips. Genotypes are indicated on the right side of the tree. Areas bordered by broken lines are magnified in the right panel.

Figure 2

Source and destination of global transmission.

Figure 3

Measles virus (MV) source index. Temporal trend in the MV source index for six countries indicating whether a country of interest is a source or destination of global transmission. The MV source index represents the relative frequency of import and export (see Materials and Methods for detail). Data before 1995 are not shown because few statistically credible instances of transmission were identified before that year.