Up to 10% of the 695 confirmed measles cases in the US outbreak have occurred in people who received one dose, and even two doses, of the vaccine as children, according to the Centers for Disease Control and Prevention (CDC).

The figures have led public health experts, who are battling the country’s largest measles outbreak since the virus was eliminated in 2000, to advise high-risk adults who were vaccinated decades to get another MMR dose.

Dr Allison Bartlett, an infectious disease expert at the University of Chicago Medicine, says 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.

For people travelling to outbreak areas outside of the US, 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.