Public health officials say the United States is experiencing a resurgence of measles because some parents are opting out of the recommended vaccines for their kids. The results of a new survey might help explain why that is.

A Pew poll found that 83 percent of adults in the United States view vaccines — such as the measles, mumps and rubella (MMR) vaccine — as safe for healthy children. (Nine percent said they thought such vaccines were unsafe; seven percent said they didn't know.) But those figures change depending on the age of the person you ask.

Ninety percent of adults ages 50 and older are confident in the safety of vaccines, the poll found. That number slides to 81 percent among adults ages 30 to 49, and then down to 77 percent for adults ages 18 to 29, according to the survey, which was released yesterday (Feb. 9). [Measles Outbreak, Measles Vaccine: Top Questions Answered]

Dr. William Schaffner, an infectious disease specialist at Vanderbilt University Medical Center in Nashville, Tennessee, said he wasn't surprised by the poll's results. The older that people are, the more likely they are to take measles seriously and grasp the importance of the measles vaccine, Schaffner said.

"Particularly if you get into the gray-haired group — that group has all seen measles," Schaffner told Live Science. "They know about it and respect it and even, in a sense, fear it.

"If you lived during that era when the vaccine was introduced, you could see measles melt away in front your eyes," Schaffner added. "But today's parents have not seen measles, and they weren't taught about measles in school. You could understand why it is that they're puzzled by it."

Measles is a highly contagious viral respiratory disease. Though its symptoms are somewhat similar to those of a bad cold (with the addition of a characteristic rash), measles can lead to much more serious complications, such as pneumonia and encephalitis (a swelling of the tissues that surround the brain and spinal cord). One or two out of every 1,000 infected children will die, according to the Centers for Disease Control and Prevention (CDC).

Measles was thought to be eradicated in the United States in 2000, but cases have been on the rise. Last year, the nation saw more than 600 measles cases — the most in two decades. In January 2015 alone, there were more than 100 measles cases, most of them linked to an outbreak that began at Disneyland in California in mid-December.

Schaffner said today's younger adults tend to be more skeptical of authority and the idea that they are obliged to do something. Younger people are also more likely to go to the Internet to find information about health — and they might not always land on the most reliable websites, he said.

"If you're wary of authority," Schaffner said, "you won't go to the CDC website or the American Academy of Pediatrics website," which are places with trustworthy information, and where doctors say the MMR vaccine is safe and effective.

The new survey, which was conducted Feb. 5-8 among 1,003 adults, let the respondents who said they thought the MMR vaccine was unsafe to explain why — but no clear theme emerged, the researchers said. Some people said they distrusted pharmaceutical companies. Other people seemed to be suspicious of how vaccines work.

People's levels of education also seemed to be a factor that affected their attitudes about vaccines. Among college graduates, 92 percent of those polled said they view MMR vaccines as safe for healthy children, while 85 percent of those with some college experience said the same, and 77 percent of adults with a high school degree or less agreed.

Schaffner said he believes more work is needed to teach kids about vaccines, especially because most health curriculums for children in middle school and high school no longer include "those old bad diseases that are now gone.

"It's no wonder that if you're not taught about these things in school, when you become a parent you're scratching your head," Schaffner said.

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