As countries around the world seek to make progress toward eliminating measles, the dynamics of the disease change in a globally consistent and predictable way, researchers report in the May 10 issue of Science.
The study, which identified global patterns in measles infection worldwide, revealed a generalized "canonical path" characterized by a series of stages, based on the size and frequency of measles outbreaks, which countries addressing infections typically progress through as they make their way toward their ultimate goal to eliminate measles.
Knowing their place in the general canonical path, public health officials can gain valuable insight into their country's current and future measles threat, including the specific groups most susceptible to infection, which can help them target control efforts, the authors say,
This knowledge can also help countries identify "peer groups" — other countries that now or in the past were at a similar place on the path of measles elimination — helping them to see what worked and what did not at specific points on the path.
Like a wildfire, which requires fuel to burn, an infectious disease requires a steady supply of individuals susceptible to the disease to survive and sustain its spread throughout a population. When fuel abounds, infections can spread quickly. However, as the pool of susceptible people dries up, due to increased vaccination rates or declining population rates, for example, fuel becomes more and more scarce, and the dynamics of the disease change.
Measles is a disease that largely follows this script, the authors said. Countries following the canonical path to elimination start out with a high incidence of the measles, which declines sharply as vaccination programs and changing demographics combine to lower the number of people susceptible to the disease. However, as measles incidence declines, the year-to-year variability in outbreaks becomes higher over subsequent years, before settling into a final elimination stage.
"The canonical path is 'canonical' because it is derived from the laws that govern measles transmission," said Justin Lessler, a Johns Hopkins Bloomberg School of Public Health epidemiologist and co-author of the study.
Measles, caused by the rubeola virus, is one of the most contagious infectious diseases on the planet. According to the Centers for Disease Control and Prevention, a single infected person can spread it to up to 90% of those close by who are not immune. Despite highly effective vaccination efforts over the last half-century, elimination of this dangerous — and potentially fatal disease — in many regions around the globe has proven difficult.
Nevertheless, the World Health Organization (WHO) has established ambitious goals to eliminate measles in all six of its regions by 2020. According to Lessler, measles is a disease that behaves most like theory says a disease should behave, which makes it an ideal candidate for understanding the laws that govern disease transmission and summarizing the complex dynamics into a framework useful for achieving elimination goals.
"We noticed a global reliance on incredibly crude and qualitative frameworks for ranking countries in their progress to elimination that is broadly used to allocate resources and plan vaccination strategies," said co-author Amy Winter, a Johns Hopkins postdoctoral fellow.
The researchers used more than 30 years (1980-2017) of readily available WHO data on measles incidence and vaccination activities in countries spanning the globe to improve these frameworks and identify patterns and similarities in the complex dynamics of measles across countries aspiring to achieve elimination.
Their analysis demonstrated that a country's position on the canonical path could be predicted using demographic data, such as birth and death rates, population size and age distribution, and vaccination coverage.
Countries are rated by each nation's average number of measles cases reported in recent years, said Lessler, and by how much year-to-year variation there is in that number of cases. What emerges is a pattern or path that countries follow over time.
Countries just starting on the canonical path begin in a state of uncontrolled endemic transmission — measles incidence is high and remains high year to year. But as countries begin to implement vaccination strategies and the incidence of measles declines, yearly variability increases, which results in more erratic outbreaks in some years — even if there were none in others — and changes in the age distribution of those most susceptible.
Africa and the Americas illustrate each end of the canonical path. According to the authors, the location of the Americas in 1993 is almost exactly that of Africa in 2008. And while the Americas have — until recently — achieved measles elimination, the majority of Africa's population is just under 50% of their way through the path.
While the method is useful for tracking a country's progress along the path to elimination, it also identifies those that deviate from general expectations. Countries may diverge from this path for both good and bad reasons. Very strong vaccination programs may allow them to move faster toward elimination. However, some have been shown to move backward, as has been the case in some parts of the Americas.
"When we originally looked at the data through 2014, we did not see this backsliding and it looked like many more countries in the Americas had fixed elimination. However, when we added data through 2017, several small to moderate sized epidemics in the region caused the backward progression we see at the end of the time series," said Lessler.
While measles was declared eliminated across the Americas in 2016, recent outbreaks in Venezuela and Brazil have led to the overall loss of the region's WHO elimination status.
"This is likely due to vaccination programs not maintaining the high standards that led to elimination due to social unrest and other factors," said Lessler.
According to the most recent CDC data, a total of 764 cases of measles have been confirmed in 23 U.S. states, which is the greatest number of cases reported since 1994 and since the virus was declared eliminated in the U.S. in 2000.
"Rates of measles in the U.S. are still extremely low, around one in 500,000, even with recent outbreaks, but the worry is that these outbreaks presage more permanent failures in measles control," says Lessler. "And these outbreaks are certainly the results of vaccine refusal."
Vaccine hesitancy can make communities vulnerable to infection — and lead to considerable deviations from the general path — despite being in regions that appear to be well protected if looking at the broader population level.
"Vaccination is the only effective tool we have to control measles, so increasing vaccination rates is the best response," said Lessler. "Understanding who these communities are and why they are not vaccinating is critical."
[Credit for associated image: Paraguay national vaccination campaign in 2014. | Pan American Health Organization]