This year was supposed to mark the point when measles—one of the most infectious diseases on the planet—was finally under control. As a step in its plan to eliminate measles worldwide by 2020, the World Health Organization set a target to reduce the number of cases by 95 percent between 2000 and 2015. The effect has been significant: Measles deaths have fallen from more than 700,000 in 2000 to around 115,000 last year. But for a disease that's easily preventable, 115,000 deaths—the majority of them children under 5 years old—is still too high. And as the WHO reported in November, progress has flat-lined over the past five years, and outbreaks are still common.

What, exactly, went wrong?

At first glance, it seems impossible to pinpoint just one reason for all the various outbreaks around the world. In the Nuba mountains of Sudan, a key factor is the lack of access to humanitarian aid for people trapped in conflict zones; in West Africa, a measles resurgence can be attributed largely to the Ebola epidemic’s crippling effect on local health systems. And in California, the blame rests squarely on the shoulders of anti-vaccine groups for whipping up unfounded fears about the safety of measles-mumps-rubella (MMR) vaccines.

However, all of these seemingly disparate cases—and all other measles outbreaks, for that matter—still have a common underlying cause. Whenever measles strikes, it’s more than just an outbreak of a single disease, or an indication that children aren’t receiving their measles shots; it’s also a warning that immunization coverage in general, for all vaccine-preventable diseases, is lower than it should be. To put it another way: When rates of routine vaccination—children receiving all their shots on schedule, as a preventive measure rather than a reaction to an outbreak—start to fall, the first sign is usually a measles outbreak. In global-health security terms, these outbreaks are the proverbial canaries in the coal mine.

Measles is considered such an integral part of the global-health agenda for precisely that reason: Its contagious nature makes it a good proxy for assessing immunization levels for all vaccine-preventable diseases. But in light of the WHO’s recent report about global progress towards eliminating measles—or, perhaps more accurately, the lack thereof—it’s clear the current approach isn’t working as well as it should be.

Because measles is so infectious compared to other diseases—so contagious, in fact, it’s possible to catch it just by entering a room, hours after an infected person has been there—a much higher level of immunization coverage is required in order to achieve herd immunity. For many common infectious diseases, herd immunity kicks in when 80 to 85 percent of the population has been immunized. But for measles, an outbreak can occur as soon as coverage drops below 90 percent.

Such a high level of contagion means it’s futile to attempt to eliminate the disease from each affected region in isolation. At the beginning of the 21st century, health authorities in the Western Hemisphere came close, reducing the number of cases in the region by 99 percent from where they’d been in 1990—but high levels of immunity weren’t maintained, and the Americas saw new outbreaks a few years later as the virus made its way over from other parts of the world.

In general, though, elimination efforts have generally relied on region-by-region mass measles-vaccination campaigns. These campaigns are often too reactive to be effective over the long term, treating the symptom of a measles outbreak rather than the broader cause: a lack of routine immunizations. While mass vaccination against a single disease is certainly an effective tool in helping health officials respond to outbreaks, it can also draw attention and resources away from more farsighted efforts to increase routine immunization coverage. As a result, it can inadvertently increase the risk of future outbreaks of measles and other infectious diseases.

The elimination of measles, then, may depend on a more sweeping, less targeted strategy. In the remaining five years until 2020, the year that was supposed to mark the total elimination of measles, the WHO and other global-health agencies may be more effective if they focus their resources on routine immunizations that boost population immunity against multiple diseases, rather than playing catch-up with just one. As Tom Frieden, the director of the Centers for Disease Control and Prevention, once noted, “A measles outbreak anywhere is a risk everywhere”—and for more reasons than measles alone.