Most of the 92 cases of measles confirmed in California are among adults — more than 62 percent. Maybe they or their parents chose not to vaccinate, or maybe those people are allergic to one of the ingredients in the measles vaccine.

But it's also possible that a few of those adults happened to slip through the cracks when the measles vaccine first came to the public.

When doctors began administering the vaccine in 1963, they recommended that only people born after 1957 receive the vaccine. They assumed that people born in 1957 or before had immunity because they'd had the measles. That wasn't a particular stretch, since around then 3 to 4 million people fell ill with the disease each year.

But the ongoing outbreak, which has spread to 14 states, has adults wondering if they really are protected. One of them is Jerry Root, a lifelong resident of Washington, D.C., who says he has no recollection of having had the measles. He called his 90-year-old mother to ask if she could remember whether or not he had received the vaccine. You got all your shots, she said, but couldn't remember any specifics.

"I remember getting shots as a kid all the time," says Root. "I don't remember what they were for." Root worried about measles, especially when he learned that since he was born in 1954, he may not have been required to receive the vaccination to attend school.

"We know that early on there are probably some adults who escaped vaccination and also escaped the disease because of how many people were vaccinated," says Dr. William Schaffner, a professor of preventive medicine at Vanderbilt School of Medicine.

Adults have a few options, according to Schaffner. They can have their blood tested for measles antibodies, which exist in the blood of anyone who has either had measles or received the vaccine. Or they can get the shot; there's no harm in receiving the vaccine an additional time. In fact, most people have had two doses, since that's what's recommended for full coverage.

The measles vaccination was one of the first vaccines required to enroll children in U.S. schools. But when the vaccine was first introduced, it was just 92 percent effective. Clinics were just learning how to administer vaccines and they didn't always store the vaccines properly. "Back in those days you would put the vaccines in the drawers on the doors of refrigerators," says Schaffner.

So officials required that everyone receive a second dose. That has reduced the number of people still lacking immunity to less than 1 percent. Children typically receive the MMR vaccine, which protects against measles, mumps and rubella, at 12 to 15 months, and then a second inoculation at 4 to 6 years.

That's why doctors and the CDC recommend that anyone worried about measles simply receive the vaccine again. That includes college students, people who will be traveling internationally and anyone who doesn't have proof of immunity, like Root.

"If you're really worried and you and your physician are really not sure, then roll up your sleeve," says Schaffner.

Root called his doctor's office and decided it was worth it. "I'm gonna go over to the CVS and get stuck," he says. "I think I can handle it."

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