The United States could be on the verge of larger outbreaks of measles.

One in eight children under the age of 17 is susceptible to the disease and would likely get sick if exposed to the dangerous virus, researchers from Emory University in Atlanta reported Thursday.

The situation is most acute among young children: Nearly a quarter of those age 3 and under are now vulnerable to the highly infectious virus.

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Measles epidemics used to occur regularly in the pre-vaccine era.

Most children survived what was a nasty infection, but some came away with lasting side-effects, such as permanent hearing loss or intellectual disabilities. And for every 1,000 children infected with measles, one or two died.

But the measles vaccine introduced in the 1960s made the virus a rarity. These days, it is news when something like the recent Disneyland outbreak occurs. An exposure event in late December 2014 at the California theme park triggered at least 117 cases in the United States.

Measles cases are now rare because the country has achieved what’s called herd immunity — a phenomenon in which so few people are susceptible that when viruses come here from other parts of the world, they quickly die out.

In order for herd immunity to exist, at least 92 percent of people need to be protected. The Emory team’s research suggests protection rates may dip below that figure. The only age group studied to hit the 92 percent mark was 13- to 17-year-olds.

“Right now we have a little bit of a buffer,” said lead author Robert Bednarczyk in an interview from San Diego, where he presented his study findings at IDWeek, a major infectious diseases conference. “But it’s just that: a little bit of a buffer.”

“If we do start to see immunization levels dip down a little bit and we see those dips sustained, we could start to lose that overall population level immunity,” he continued. “And if measles get introduced, we may start seeing larger and more severe outbreaks.”

To come up with his estimates, Bednarczyk used data from an annual survey conducted by the Centers for Disease Control and Prevention. The National Immunization Survey-Teen asks parents of adolescent children whether and when they immunized their children. The information is checked against medical records.

The CDC recommends that children have one shot from the ages of 12 to 15 months and a booster shot from the ages of 4 to 6 years.

Children over the age of 1 are susceptible to measles if they are not vaccinated at all. And it’s estimated that about 8 percent of those who have had only one dose are still vulnerable to infection. All infants between six months and one year of age are also at risk. (At less than six months, babies have antibodies inherited from their mothers, but those eventually wane.)

Bednarczyk and his colleagues from Emory’s Rollins School of Public Health plugged the survey data into a mathematical model to estimate overall infection risk.

Factoring that about 3 percent of children who are fully vaccinated are nonetheless not protected — a level that still makes the measles shot one of the most effective vaccines — the researchers concluded that about one in eight children age 17 and younger is vulnerable to the virus.

Measles specialist Dr. Neal Halsey, director of the Johns Hopkins Institute for Vaccine Safety in Baltimore, was surprised by that figure. At that level of protection, Halsey said, he would expect more large outbreaks like the one at Disneyland.

That outbreak was blamed on low immunization rates. But it was also compounded by the fact that protection is not evenly spaced.

One of the people infected in California was visiting from Lanaudière, Quebec, and was a member of a religious group that opposes vaccination. The imported case rapidly triggered local transmission. Quebec recorded 159 cases.

The United States eliminated measles in 2000, meaning it stopped the spread of indigenous measles viruses. All cases since then have been triggered by measles viruses imported from other countries.

In recent years, more parents have resisted vaccinating their children against measles, fearing a purported link between the vaccine and autism. That alleged association — initially raised in a since-withdrawn 1998 study in the journal Lancet — has been repudiated.

But the damage to the vaccine’s reputation remains.

This year, health officials reported the first measles death in the United States since 2003, involving a woman in Washington state. And the CDC reported there had been 189 cases this year until Sept. 18 in the country.

That’s better than last year, when the CDC reported 668 cases of measles, the largest tally since elimination in 2000.

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