Buenaventuramariano via Getty Images A new study shows that more than half of measles cases in recent outbreaks were among people who were unvaccinated.

Measles -- a disease that was completely eliminated in the United States 15 years ago -- has captured national attention recently, thanks to outbreaks among unvaccinated travelers who have contracted the disease abroad. At the same time, anti-vaccination sentiments have found footing among some parents worried over unfounded safety concerns.

Now, a new study shows just how closely the two phenomena are intertwined.

More than half of reported measles cases in recent outbreaks in the U.S. were among unvaccinated individuals -- most of whom were intentionally unvaccinated for philosophical, rather than medical, reasons. In a third of the cases, researchers did not have enough information about the person's vaccination history to tell if they'd been immunized.

A similar pattern held true for individuals who contracted pertussis, or whooping cough, another vaccine-preventable disease that hit its low point in this country in the late 1970s.

While previous studies have examined how anti-vaccination sentiments may contribute to localized outbreaks, Dr. Varun Phadke, a fellow in the division of infectious diseases with the Emory University School of Medicine and an author on the new investigation, told The Huffington Post that this latest investigation is much broader.

"Our study adds to this work by looking at the contribution of vaccine refusal to outbreaks of measles and pertussis in the U.S," he said.

And that contribution, Phadke and his colleagues found, is significant.

In the study, published in JAMA on Tuesday, the researchers analyzed 18 published reports on measles outbreaks in the United States since the country declared it eliminated in 2000. Outbreaks since that time have generally been caused by visitors bringing the disease from overseas, and have grown in recent years, from 63 cases in 2010, to 667 in 2014. In 2015, there were 189 documented cases, many linked to the much-publicized California outbreak that began at a Disney theme park, though those numbers are not yet final.

Overall, the researchers considered more than 1,400 measles cases in their new study, 57 percent of which were in unvaccinated people. (Though measles is generally considered a childhood condition, and many of the cases were young children, there were cases in individuals as old as 84.)

In instances where the researchers had detailed information about why individuals who contracted measles had not been vaccinated, 71 percent cited non-medical reasons, like religious restrictions or philosophical arguments. That, Phadke said, caught his team's attention.

"It is not that surprising that unvaccinated, and therefore non-immune, individuals comprised a substantial proportion of the measles cases in our review," he said. "However, it was notable to us that most of the unvaccinated individuals who developed measles were intentionally unvaccinated."

Fortunately, measles vaccination rates in this country remain relatively high, at about 92 percent of young children. But there is significant geographic variability in vaccination rates, and many states allow for religious and philosophical exemptions.

As for whooping cough, the researchers found that the five largest recent statewide outbreaks included a significant portion of unvaccinated individuals, or people who had not received the full, recommended dose on the recommended timeline -- many for non-medical reasons. But several outbreaks did occur in areas where vaccination rates were high, suggesting that waning immunity may also be at play.

Taken as a whole, the study provides a clear message to public health officials about the importance of promoting vaccination in children, and spreading information about vaccine safety.

For parents, the message is also clear.

"Measles is a highly, highly contagious disease, and there's a tremendous chance of your child being infected if they haven't been protected through immunization," said Dr. Carol J. Baker, executive director of the Center for Vaccine Awareness and Research at Texas Children’s Hospital, who did not work on the new study.

Because it has been eliminated, parents in this country do not remember how bad measles can be, she said, adding that it leads to symptoms such as high fever and red, painful eyes that are extremely sensitive to light. It can also lead to complications, like pneumonia, and even death in extreme cases. Some children cannot get the vaccine, because they have health problems, like cancer, that compromise their immune system, or because they are too young. In that case, Baker said, they depend on others around them to be immunized to ensure their own safety, an idea known as herd immunity. When a certain percent of a population is vaccinated, it helps protect even those who can't be immunized, because it contains the infection.