By Tom Ulrich

There’s been a lot of speculation about whether low vaccination rates are feeding the 2015 U.S. measles outbreak, which as I write this stands at 145 cases across seven states. Well, we can stop speculating, because the numbers are in, and measles is taking advantage of pockets of inadequately vaccinated people.

That’s the stark, unequivocal message from a study by epidemiologists at Boston Children’s Hospital, published this week in the journal JAMA Pediatrics.

Measles is highly contagious. In a population with no immunity, one infected person will pass the virus on to between 11 and 18 additional people. Epidemiologists call that rate of spread the virus’s basic reproduction rate, or R0; for perspective, Ebola’s R₀ is 2.

That high level of transmissibility is why so many people in a population – between 96 and 99 percent – have to be vaccinated against measles in order for herd immunity to kick in and keep the virus from spreading.

To calculate the rate of measles vaccination among the clusters of cases that have popped up to date, Maia Majumder, MPH, and John Brownstein, PhD, of Boston Children’s HealthMap team crunched a bunch of numbers:

• the number of cases attributed to the outbreak as of January 28, 2015, as captured through HealthMap and reported by California’s state health department
• measles’ R₀
• the virus’s serial interval (how long it takes for each successive wave of cases to follow the wave before it)
• measles’ effective reproductive rate, or R_E, which describes how readily the virus moves through a population that has at least some vaccine protection

“It’s as though you took everyone exposed to measles in the areas with case clusters, put them in a room and measured the level of vaccine coverage in that aggregate population,” says Majumder, who is an engineering systems PhD student at MIT and a computational epidemiology research fellow at HealthMap.

The coverage rates they calculated – between 50 and 86 percent – fall far below the 96 to 99 percent needed for herd immunity against measles.

The answer when it comes to this outbreak is clear.

“Our data tell us a very straightforward story – that the way to stop this and future measles outbreaks is through vaccination,” says Brownstein, a digital epidemiologist and HealthMap’s co-founder. “The fundamental reason why we’re seeing the number of cases we are is inadequate vaccine coverage among the exposed.

“We hope these data encourage families to ensure they and their loved ones are vaccinated,” he continues, “and help local public health officials in their efforts to control this outbreak.”

See coverage of this study in Time, Forbes, Reuters and NPR Shots.

Curious to see the impact of different vaccination rates on a simulated measles outbreak? Check out HealthMap’s interactive measles vaccination model.