ATLANTA, Ga. (WPVI) -- With colleges starting again, many parents, and campus health officials, are worried about more mumps outbreaks.
A new study has some clues on why they're happening.
Doctors at the CDC and Emory University found that about 10 per cent of the young adults they tested had no immunity to mumps, even though they were fully vaccinated.
They tested 71 people, aged 18 to 23, in the Atlanta area. All but 2 had received 2 MMR shots, and 80 per cent received their second shot at least 10 years before enrolling in the study.
That was to give researchers a look at what happened to a person's mumps immunity over time.
They were tested for mumps memory B cells, which would normally be capable of producing antiviral antibodies to the mumps virus.
10% of people in the study had no detectable mumps-specific memory B cells.
And the average frequency of the memory B cells in their blood was 5 to 10 times lower for cells making antibodies against mumps, compared with cells making antibodies to measles or rubella.
They also found that the circulating strain of the mumps virus has drifted away from the one in the vaccine.
The strain in the vaccine was originally cultured from the throat of a scientist's daughter in the 1960s.
The study results will be published in the journal PNAS, by the National Academy of Sciences.
Medical experts are split on whether a third dose of MMR vaccine for the general public will be worthwhile.
Past research shows that a third dose boosted antibody response in some with low levels of immunity; however, the antibodies declined by 1% a year, so the effect wasn't long-lasting.
The CDC's immunization practices committee has approved a third dose for those at high risk due to an ongoing mumps outbreak.
New study holds clues on return of mumps outbreaks