U.S. measles outbreak raises questions about immunity in adults

Adults in the United States who were vaccinated against measles decades ago may need a new dose. This, according public health experts battling the nation’s largest outbreak since the virus was deemed eliminated in 2000, would depend on when the adults received the shot and their exposure risk.

Up to 10 per cent 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 US Centres for Disease Control and Prevention (CDC).

The figure illustrates what can happen when a large number of individuals, even those who have been vaccinated, are exposed to measles. CDC recommends that people who are living in or travelling 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 immunised in the earlier years receiving an inactivated version of the virus. Americans born before 1957 are considered immune.