Many young children traveling abroad aren't receiving the vaccines they need to protect them from measles, a study published Monday in JAMA Pediatrics finds.

Kids are more likely to be exposed to measles when traveling internationally than when they are at home in the United State, said study co-author Dr. Emily Hyle, an assistant professor of medicine at Harvard Medical School. Worldwide, measles cases have risen steadily in recent years.

But children in the U.S. generally aren't slated to be fully vaccinated against the highly contagious virus until at least age 4. That means kids traveling overseas may need the MMR vaccine, which protects against measles, mumps and rubella, before it's usually recommended in the U.S.

Many of those kids aren't getting it.

Hyle and colleagues analyzed data on more than 14,000 kids whose families took them to health clinics specializing in international travel in advance of trips abroad between 2009 and 2018. Some travelers had emigrated from other countries and were visiting family. Others were on mission trips, or simply vacationing.

The study found that the vast majority of babies — 92 percent — qualified for the protective shot. But 44 percent of those infants did not receive it. Neither did nearly two-thirds of the 60 percent of preschool-aged kids who also qualified for the vaccine.

Researchers said it was the doctors themselves who often failed to recognize the children could or should get the MMR vaccine early.

"This underscores the knowledge gaps that exist about MMR vaccination, even among clinicians with expertise in travel medicine," Dr. Regina LaRocque, another study author and an infectious disease investigator at Massachusetts General Hospital, said in a statement.

In the U.S., 1,276 measles cases have been reported this year alone. That's the highest number of cases in this country in nearly three decades, according to the Centers for Disease Control and Prevention.

International travelers are the source for outbreaks in the U.S., Hyle said. "The outbreaks are triggered by either foreign visitors, or returning U.S. travelers who bring the virus back with them."

Measles is one of the most contagious viruses in the world. "Let's say somebody who is ill with measles walks through a room," said Hyle, who is also an infectious disease physician at Massachusetts General Hospital. "For the next two hours, anybody who walks through that same room, who's not vaccinated, has a 90 percent chance of becoming infected."

In general, the CDC recommends two doses of the MMR vaccine, the first after a child's first birthday, and the second between the ages of 4 and 6. But the recommendations change when kids travel out of the U.S., given the increased risk for measles exposure.

In that case, the CDC says any child over 6 months old should get an MMR dose in addition to the two standard recommended doses for extra immunity. And children over a year old should have two doses before traveling, as long as they're separated by about a month.

The new findings underscore the need for additional education among doctors, Hyle told NBC News, particularly when it comes to traveling infants.

Babies are born with a certain level of immunity, thanks to antibodies passed from their mothers. But there's a period of time when older infants may be particularly vulnerable to measles exposure. A study published in November found that newborns' immunity to measles, which is passed on from their mothers, wanes much sooner than previously thought — perhaps within three months.

Cases of measles have been increasing globally in recent years, mostly in areas with low vaccination rates. The World Health Organization estimates more than 140,000 people died from measles last year, worldwide.

Most deaths were among children under age 5. Babies are particularly at risk for complications from the virus, including pneumonia, encephalitis and lasting brain damage.

But the MMR vaccine has been proven to be safe, as well as highly effective. Studies have shown one dose provides 93 percent protection against measles. The second dose boosts immunity to 97 percent.

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