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US measles outbreak raises questions about immunity in adults



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

CHICAGO – Adults in the United States who were vaccinated against measles decades ago may need a new dose based on their exposure and their exposure risk, according to the public health battling the nation's largest outbreak since the virus was deemed eliminated in 2000.

Up to 10% of the 695 confirmed measles cases in the current outbreak occurred in people who received one or two doses of vaccine, according to the US 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 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, said the "continued vulnerability to infection" is why high-risk nursing workers are routinely advised 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 are likely to have two doses of 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 have received only one dose, with many people immunized in the earlier years receiving an inactivated version of the virus. The Americans born before 1957 are considered immune as they were exposed to the virus directly in an outbreak.

Merck & Co. Inc. is the sole US provider of the MMR vaccine. The company said in a statement that it has "taken steps to increase US supply" of 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 outbreaks in many parts of the world.

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

For travelers to outbreak areas abroad, the CDC recommends taking another dose of MMR unless they have received two prior doses, a blood test showing immunity, or were born before 1957.

In general, the CDC provides 97% protection for the measles vaccine; one dose should offer 93% protection. However, immunity can wane over time.

This is one of the two documented doses of vaccine, said Dr Michael Phillips, chief epidemiologist at NYU Langone Health, serving New York City, and a hot spot in the US 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 did.

Rapid blood tests are available that can be detected as a person based on the level of measles antibodies, but the tests are not 100% reliable.

Schaffner said: “It’s safe. There's no downside risk. Just roll up your sleeve. ”


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