An outbreak of has hit Temple University in Philadelphia with . In response, health authorities are to all students. Most of the students, however, have been previously vaccinated with the MMR (measles, mumps and rubella) vaccine. So why is there an outbreak going on in a highly-vaccinated population?
Before a vaccine was available for mumps, about 186,000 cases of mumps were reported in the United States. Today, the number of reported cases oscillates between 1,000 and 6,000 cases, depending on the number of outbreaks that happen. (Keep in mind that the population in 1967 was about 200 million people and today is it about 320 million.) As a result of this dramatic reduction in mumps, what used to be a fairly common childhood disease is now rare.
We are lucky that it is that rare because mumps doesn’t come without possible severe complications. These can include swelling of the testicles, ovaries, pancreas and brain. In some cases, deafness can result from the infection. Children with mumps are also not able to go to school or daycare, leading to loss of productivity from parents or caregivers who have to stay home with them.
That rarity of cases has led to an interesting phenomenon. Whether you get the disease, , immunity wanes over time if you are not boosted. By boosted, we mean that you are exposed to the virus and your immune system is reminded of what the virus looks like. Your immune system then mounts a vigorous response and enough antibodies and B cells are left for the next time you encounter the virus. Without that boosting, the immune system cells that would create antibodies die off, and your immune system no longer remembers what mumps looks like.
This waning immunity happens over time, and the exact mechanisms of why one person is immune longer than another have not been made fully clear. Furthermore, has shown that the outbreaks happen not because the vaccine is not effective or because the virus has mutated. (It is in terms of mutation.) Because of the lack boosting by natural exposure, it may be necessary to administer a boosting mumps vaccine dose every ten years or so.
It will be interesting to see if vaccination recommendations change as these third doses of the MMR are administered in outbreak situations and it is shown that they stop the outbreak and that those receiving the third dose are protected from future disease. If that is the case, it will be very likely that a recommendation will be made for a third MMR dose in high school or before entering college.