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CHICAGO / NEW YORK – In the United States, adults who were vaccinated against measles decades ago may need a new dose depending on when they received the vaccine and their risk of exposure, according to public health experts fighting the biggest epidemic in the country since the elimination of the virus in 2000.
According to the US Centers for Disease Control and Prevention, nearly 10% of the 695 confirmed cases of measles in the current outbreak occurred among people who received one or two doses of the vaccine.
The figure illustrates what can happen when a large number of individuals, even those who have been vaccinated, are exposed to measles. The CDC recommends that people living or traveling in epidemic areas check their immunization status and consider a new dose.
Dr. Allison Bartlett, an infectious disease expert at the University of Chicago Medicine, said that "continued vulnerability to infection" is why high-risk adults, such as health workers, are systematically invited to receive a second dose of measles vaccine if not had one.
But knowing the status of your vaccination can be tricky, experts said.
"It's complicated and often unnecessary, because it's very difficult to resuscitate 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 probably have received two doses of combined measles, mumps and rubella vaccine (MMR vaccine) in accordance with federal guidelines, which is still considered the standard of protection.
Anyone vaccinated between 1963 and 1989 would probably have received only a single dose, with many people having been immunized in the early years who received an inactivated version of the virus. Americans born before 1957 are considered immune because they would have been exposed to the virus directly in the event of an epidemic.
Merck & Co. Inc. is the only US supplier of MMR vaccine. The company said in a statement that it had "taken steps to increase the US supply" of the vaccine due to the current outbreak.
The measles virus is highly contagious and can cause blindness, deafness, brain damage or death. It is currently spreading through hatching in many parts of the world.
According to the World Health Organization, 95% of the population needs to be vaccinated to provide "collective immunity," a form of indirect protection that prevents infection in people too young or sick to be vaccinated. US public health officials have blamed part of the current outbreak for increased vaccine skepticism, which has reduced immunity against measles in some communities.
For travelers traveling to outbreak areas abroad, the CDC recommends that adults consider taking another dose of MMR unless they have proof that they have already received two doses, to do a blood showing immunity or being born before 1957.
In general, the CDC says that two doses of measles vaccine should provide 97% protection. one dose should provide 93% protection. However, immunity can decrease with time.
This has happened even in adults with two documented doses of the vaccine, said Dr. Michael Phillips, chief epidemiologist at NYU Langone Health, which serves part of New York City, a hot spot of the American epidemic.
He told children that "the vaccine is really effective," but in some adults, memory-sensitive T cells, which recognize and attack germs, do not fight the virus as effectively as before.
There are rapid blood tests to detect a person's immunity based on the level of measles antibodies, but these tests are not 100% reliable.
Adults who have doubts about their immunity should receive another dose, said Schaffner: "That's for sure. There is no risk of decline. Just raise your sleeve.
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