News Release 

Knowing its position on the 'canonical path' could help a country better target measles

American Association for the Advancement of Science

As countries around the world make progress towards eliminating measles, the dynamics of the disease change in a globally consistent and predictable way, researchers report. The study presents a "canonical path" that countries typically follow, starting out with periods of high incidence of the disease, which then go down sharply as high vaccination rates mean fewer susceptible people, but, with higher year-to-year variability in measles outbreaks in the subsequent years, before a final elimination stage. By looking at their place in the general canonical path, the authors say, countries can know the underlying distribution by age of susceptibly to measles in their population, which can help them target control efforts. Measles is a highly infectious and dangerous disease that has been difficult to eliminate in regions around the globe despite effective vaccination efforts. Nevertheless, the World Health Organization has established ambitious goals to eliminate measles in all 6 of its regions by 2020. Matthew Graham and colleagues evaluated patterns in global measles dynamics for countries aspiring to these goals, using more than 30 years (1980-2017) of data on disease incidence and vaccination activities worldwide. They report that countries undergo predictable changes in the size and frequency of measles outbreaks, in general. For example, countries just starting on the canonical path are generally characterized by a high incidence of the disease with low year-to-year variability. However, as the incidence of the disease declines, yearly variability increases, resulting in more erratic outbreaks in some years - even if there were no outbreaks in others - and changes in susceptible populations. Graham et al. calculated the position of each country they studied along the canonical path by calculating its location in "incidence space," a metric derived from a country's magnitude of and variation in measles incidences; this metric can provide critical information for guiding tailored vaccination and public health programs, as well as for identifying countries deviating from these general expectations. In a related video, co-author Justin Lessler describes the canonical path and deviations from it, in more detail. Countries may deviate from this path for good reasons, he says, like very strong vaccination programs that allow them to move faster to elimination, or for bad reasons, because their programs are failing and they move backwards, as in some parts of the Americas.

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