Should you get a measles booster? Here’s what to know.

There have been 1,267 confirmed cases of measles in the United States this year, according to the Centers for Disease Control and Prevention. The agency has also reported three deaths from measles.

As cases continue to emerge, health care experts say the best way to protect against measles is to get vaccinated. The two doses of the measles, mumps and rubella vaccine, or MMR, provide immunity for many years. Most people in the U.S. have gotten these shots as children, but a measles vaccine booster may help some adults.

Here’s what health experts have said about the measles vaccine.

States with reported measles cases

As of Tuesday, July 2, the CDC has said that 37 states have reported measles cases. They are: Alaska, Arkansas, Arizona, California, Colorado, Florida, Georgia, Hawaii, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maryland, Michigan, Minnesota, Missouri, Montana, Nebraska, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Dakota, Tennessee, Texas, Utah, Vermont, Virginia and Washington.

Should I get a booster shot of the measles vaccine?

The answer depends on when you were born, your vaccine records and whether you plan to travel internationally.

People born before 1957 are assumed to have been exposed to measles when they were children and “don’t necessarily have to do anything,” Margot Savoy, a senior vice president at the American Academy of Family Physicians, said last month. Anyone infected with measles, regardless of when they were born, is presumed to have immunity, experts said.

Adults who don’t have presumptive immunity should get at least one dose of the vaccine.

Some adults who received the vaccine between 1963 and 1967 (less than 5%, according to the CDC) may have received a version of the measles vaccine that isn’t as effective, Adam Ratner, director of the division of pediatric infectious diseases at New York University Grossman School of Medicine, said in March. If you fall into that age range and you are not sure what version of the vaccine you received, the CDC recommends another dose.

People born between 1968 and 1989 possibly got one dose of the measles vaccine, instead of the two given to children today. The CDC says one dose is enough for most adults but recommends two doses of the vaccine for anyone planning international travel. And health officials may recommend people get a second dose of the vaccine if there’s a local measles outbreak.

Savoy said she got her second dose of the measles vaccine in high school during an outbreak in the 1990s.

“There are some people who are in their early 50s, late 40s who — if they only ever got one dose of MMR and they never had the measles as an infection — they might benefit from getting a booster dose,” Savoy said.

If you got two doses of the measles vaccine, experts said, a booster isn’t going to provide any additional protection.

A person’s vaccine-induced measles immunity can wane over time, but it’s rare, Ratner said. What can happen is “primary vaccine failure,” when a person doesn’t respond to one dose of the measles vaccine and they’re not protected against the virus, Ratner said. This is why children get a second dose, to ensure they have the antibodies, he said.

There are lab tests that can measure a person’s antibody protection against measles, Savoy said. But the test may not be covered by insurance, and the results are only an approximate measurement of a person’s risk if they are exposed to the virus, she said.

How can an adult find out whether they’re vaccinated?

Most Americans were vaccinated for a number of diseases as children, but experts said it can be impossible for adults to remember the shots they received decades ago, and in many cases, medical records can be hard to find.

“If you’re a 57-year-old adult like me, well, I’m sure that my pediatrician is no longer around,” Daniel Salmon, a professor of international health at the Johns Hopkins Bloomberg School of Public Health, said in March. “For many adults, especially older adults, there may not be a single source for their immunization record.”

Start by asking your health care provider whether they have your vaccination records. You can also contact your state’s health department. States, and some major cities, have vaccine registries.

But there is no national database, and a state’s vaccination records may not be comprehensive for adults, experts said. It can be “very, very difficult” for people to keep track of their vaccinations when they move or travel out of state for health care, Priya Sampathkumar, an infectious-disease specialist at the Mayo Clinic, said last month

“We have a really fragmented health care system,” Salmon said, causing records supporting individual-level vaccines to often be scattered.

Based on a patient’s age and medical history, Sampathkumar said, she can tell someone the vaccines they should have gotten. But then they have to “do some legwork,” she said.

Ask your parents or a caregiver if they have records from when you were a child. Call your past health care providers. Some employers, including the military, require vaccinations and may have records. Schools and universities may, as well.

If your parents didn’t object to vaccines, you can make a “reasonable assumption” that you got the vaccinations required to go to school in the United States, Sampathkumar said.

Sampathkumar said she tells people that when they get a vaccine, they should take note of it in their phone. And if they have access to their records online, through their health care provider, they should download and save the documents.

“I keep a vaccine card,” Emily Smith, an associate professor of global health at George Washington University’s Milken Institute School of Public Health, said in March. “I have my doctor put on it — or the pharmacist put on it — when I get a vaccine on that card.”

If no records can be found, a person can get a blood test to see whether they have antibodies against certain viral infections, said Joseph Teel, a professor of clinical family medicine at the Perelman School of Medicine at the University of Pennsylvania.

And, when in doubt, it doesn’t hurt to just get another dose of a vaccine, Sampathkumar said.