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By Julie Steenhuysen

April 28 (Reuters) - Adults in the United States who were vaccinated against measles decades ago may need a new dose depending on when they received the shot and their exposure risk, according to public health experts battling the nation's largest outbreak since the virus was deemed eliminated in 2000.

Up to 10 percent of the 695 confirmed measles cases in the current outbreak occurred in people who received one or two doses of the vaccine, according to the U.S. Centers for Disease Control and Prevention.

The figure illustrates what can happen when a large number of individuals, even those who have been vaccinated, are exposed to the measles. CDC recommends that people who are living in or traveling to outbreak areas should check their vaccination status and consider getting a new dose.

Dr. Allison Bartlett, an infectious disease expert at the University of Chicago Medicine, said the "continued vulnerability to infection" is why high-risk adults such as healthcare workers are routinely advised to get a second dose of the measles vaccine if they have not had one.

But knowing your vaccination status can be tricky, experts said.

"It's complicated and often futile because it's very difficult to resurrect those old records," said Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center.

People vaccinated in the United States since 1989 would most likely have received two doses of the combined measles, mumps and rubella (MMR) shot under federal guidelines, and that is still considered the standard for protection.

Anyone vaccinated between 1963 and 1989 would likely have received only one dose, with many people immunized in the earlier years receiving an inactivated version of the virus. Americans born before 1957 are considered immune as they would have been exposed to the virus directly in an outbreak.

Merck & Co Inc is the sole U.S. provider of the MMR vaccine. The company said in a statement that it has "taken steps to increase U.S. supply" of the vaccine due to the current outbreak.

HIGHLY CONTAGIOUS

The measles virus is highly contagious and can cause blindness, deafness, brain damage or death. It is currently spreading in outbreaks in many parts of the world.

According to the World Health Organization, 95 percent of a population needs to be vaccinated to provide "herd immunity," a form of indirect protection that prevents infection in people too young or sick to be vaccinated. U.S. public health officials have blamed the current outbreak in part on rising rates of vaccine skepticism that have reduced measles immunity in certain communities.

For travelers to outbreak areas abroad, the CDC recommends adults consider getting another dose of MMR unless they have proof of receiving two prior doses, take a blood test showing immunity, or were born before 1957.

In general, the CDC says two doses of the measles vaccine should provide 97 percent protection; one dose should offer 93 percent protection. However, immunity can wane over time.

This has occurred even in adults with two documented doses of the vaccine, said Dr. Michael Phillips, chief epidemiologist at NYU Langone Health, which serves parts of New York City, a hot spot in the U.S. outbreak.

He said in kids, "the vaccine is really effective," but in some adults, memory T-cells, which recognize and attack germs, do not fight the virus as effectively as they once did.

Rapid blood tests are available that can detect whether a person is immune based on the level of measles antibodies, but the tests are not 100 percent reliable.

Adults who have any doubt about their immunity should get another dose, Schaffner said: "It's safe. There's no downside risk. Just roll up your sleeve."

(Reporting by Julie Steenhuysen in Chicago; Additional reporting by Mike Erman and Gabriella Borter in New York; Editing by Lisa Shumaker)

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