The majority of U.S. travelers who visit a doctor prior to traveling are already immune to measles, primarily from already receiving the appropriate vaccines.

But more than half of the people who are not immune to the potentially-deadly measles germs are not vaccinated during that doctor’s visit, according to a new study in the journal Annals of Internal Medicine.

“It was surprising to see such a high number of missed opportunities for MMR vaccination, even at these specialized pre-travel consultations,” said Emily Hyle, lead author, from the Massachusetts General Hospital Division of Infectious Diseases.

The patient pool from 2009 to 2014 who had visited Global TravEpiNet clinics numbered 40,800. About 85 percent of those patients had immunity to measles.

But 6,612 patients were eligible for the MMR vaccine. Fifty-three percent (3,477) were not vaccinated during that clinic visit. Some 48 percent of those non-vaccinations were due to patient refusal. Of those refusals, 74 percent of patients reported no concern about measles, 20 percent reported vaccine safety concerns, and cost worries accounted for the remaining six percent.

Not all were refusals, however: 28 percent of the patients were not recommended the MMR by the health professionals at the clinic. Another 24 percent of the patients were simply referred to another doctor or location for the shots.

Measles, which was declared eliminated from the U.S. in 2000, still appears domestically through importation where it is still regularly contagious, according to the paper. Sixty-one cases in the U.S. have been reported through April 22 for this year, according to CDC. The peak in recent years was 2014, when 667 cases were counted by the federal agency. That record year was driven by 383 cases reported among the Amish in Ohio, who are predominantly unvaccinated, and also from travelers from the Philippines, which had its own huge outbreak of the disease that year, according to federal data.

“A single case of measles can spark a major outbreak, especially in communities where many people are not immune,” said Reinga LaRocque, the senior author. “Measles is one of the most infectious diseases known – 90 percent of people who are not immune will contract measles from an even-minimal exposure to somone who is infected.”

But there is hope to improve coverage for everyone, Hyle said.

“We can definitely improve how often providers specializing in pre-travel medical advice offer MMR vaccine to eligible travelers and encourage clear discussions with patients about the risks of contracting measles and of spreading the disease after their return to the U.S.,” she said.