By Julie Steenhuysen
28. April (Reuters) – Adults in the United States who were vaccinated against measles decades ago may need a new dose depending on when they've been shot and their exposure risk the nation's largest outbreak since the virus hit in 2000 was considered eliminated.
Up to 10 percent of the 695 confirmed measles cases in the current outbreak occurred in people who received one or two doses, according to the US Centers for Disease Control and Prevention.
The figure shows what can happen when a large number of people, including those who have been vaccinated, are exposed to the measles. The CDC recommends that people living in or traveling to areas check their vaccination status and consider a new dose.
Dr. Allison Bartlett, an expert in infectious diseases at the University of Chicago Medicine, said that "persistent susceptibility to infection" is the reason why high-risk adults, such as health professionals, are routinely recommended to receive a second dose of measles vaccine, if they did not have one yet
But knowing the vaccination status can be difficult, experts say.
"It's complicated and often pointless because it's very difficult to revive those old records," Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center.
People who have been vaccinated in the United States since 1
Anyone vaccinations between 1963 and 1989 would probably have received only one dose, and many people who had been immunized in earlier years received an inactivated version of the virus. Americans born before 1957 are considered immune because they would have been directly exposed to the virus in an outbreak.
Merck & Co Inc. is the only US provider of the MMR vaccine. The company stated in a statement that it had "taken measures to increase US supply through the current outbreak".
Measles virus is highly contagious and can lead to blindness, deafness, brain damage or vomiting death. In many parts of the world, it is currently erupting.
According to the World Health Organization (WHO), 95 percent of the population must be vaccinated to provide "herd immunity", an indirect protection that also prevents infections in humans from being vaccinated young or ill. US public health officials have blamed the current outbreak in part for the rising rate of vaccine skepticism that has reduced measles immunity in certain communities.
For travelers traveling to foreign outbreaks, the CDC recommends that adults receive another dose of MMR unless they have evidence. After taking two previous doses, take a blood test that reveals, or has been, immunity born before 1957.
In general, the CDC says that two measles vaccinations should provide 97 percent protection; One dose should provide 93 percent protection. However, immunity may decline over time.
This has occurred even in adults with two documented vaccine doses, Dr. Michael Phillips, chief epidemiologist of Langone Health in New York City, a neighborhood of New York City, a hot spot of the US outbreak.
He said in children, "The vaccine is really effective," but in some adults, memory T-cells that recognize and attack germs do not fight the virus as effectively as they used to.  There are quick blood tests that can tell from the level of measles antibodies that a person is immune. However, the tests are not 100 percent reliable.
Adults who have doubts about their immunity should receive another dose, Schaffner said: "It's safe, there's no downside risk, just roll up your sleeve."
(Report by Julie Steenhuysen in Chicago; Additional coverage by Mike Erman and Gabriella Borter in New York; Editorial by Lisa Shumaker)
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