A new US government study indicates that children and teenagers who have been vaccinated against the mumps virus account for one-third of illnesses in recent years.

The causes are unknown, but experts say that annual childhood immunization is still the best way to prevent mumps, a contagious infection that is usually mild but can cause serious complications.

After the mumps vaccine was introduced in the United States in 1967, cases of infection plummeted by 99% 1967. However, there has been an increase in yearly instances since 2006, particularly among college-aged adults who were vaccinated as children.

That led to speculation that waning immunity was to blame.

According to the latest study, vaccinated children and teenagers have been responsible for a major portion of mumps occurrences in recent years.

The exact number has varied from year to year, but between 2007 and 2019, vaccinated children accounted for over one-third of all mumps cases.

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