Scientists admit the mumps vaccine—one component of the MMR jab—doesn't offer full protection anyway, and its effects wear off in time. This would explain why most of the outbreaks have been reported among teenagers and people in their early 20s, but it doesn't account for the steep rise that's happened in the last few years.

One study of a mumps outbreak among 259 students at the University of Iowa discovered that 98 per cent had received two doses of MMR when they were children.

The researchers from the Centers for Disease Control and Prevention (CDC) concluded that students who had been vaccinated more than 13 years ago were nine times more likely to contract mumps—and the protective effects had diminished to such an extent that they were no better off than those who had never been vaccinated. The risk fell only when the students were given a third dose of MMR, they discovered.

The outbreaks have been happening in close-knit communities, such as in student accommodation and among sports teams. Mumps is a disease of the salivary gland, and is spread through saliva, such as on drinking glasses, cutlery and plates.