US measles crisis: Concerns about immunity rise as cases hit 25-year high

Public health experts are now considering a new dose for adults who were vaccinated against measles decades ago, depending on when they received the shot and their exposure risk as the US battles the largest outbreak since the virus was deemed eliminated in 2000.

The United States recorded 78 new measles cases last week for a total of 704 cases so far this year, the greatest number in a quarter century, federal health officials. Some 22 states have recorded cases of the extremely contagious and sometimes deadly disease.

The figures illustrate what can happen when a large number of individuals, even those who have been vaccinated, are exposed to the measles.

HOW CONTAGIOUS IS MEASLES

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 per cent 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 scepticism that have reduced measles immunity in certain communities.

KNOWING YOUR VACCINATION STATUS MAY NOT BE SO SIMPLE

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 are born before 1957 are considered immune as they would have been exposed to the virus directly in an outbreak.

IMMUNITY CAN WANE OVER TIME

For travellers 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 per cent protection; one dose should offer 93 per cent protection. However, immunity can wane over time.

Rapid blood tests are available that can detect whether a person is an 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."