Mumps Outbreaks: Why Do we Care and Is the Vaccine Working?

By Jeff Duchin, Health Officer, Public Health – Seattle & King County

Learn about the on-going mumps outbreak in King County.

Why do we care about mumps?

Mumps causes painful swelling of the salivary glands in the cheek and jaw area that usually lasts for 1-2 weeks.

  • Complications of mumps include inflammation of the testes in  boys past puberty and in men, and of the breast tissue and ovaries in females.
  • More serious complications include meningitis (inflammation of the lining of the brain) and encephalitis (brain inflammation).
  • Other complications include hearing loss, and inflammation of the pancreas.

How did the mumps vaccine help?

In the pre-vaccine era, tens of thousands of cases of mumps occurred each month in the U.S., and mumps was the #1 cause of viral encephalitis and a leading cause of viral meningitis. Since the widespread use of the mumps vaccine in the US, there has been a 99% decrease in mumps. However, mumps continues to be very common in other parts of the world where vaccination is not used or vaccination rates are low, and mumps and other vaccine preventable diseases remain a risk for travelers.

Does the Mumps Vaccine Work?

Mumps infection spreads readily in settings where people are in close contact, such as households, schools, and camps. Studies of mumps vaccine effectiveness show an average of 88% protection after two MMR (Measles/Mumps/Rubella) doses.  Yet, in recent years, outbreaks of mumps have been reported in colleges and other settings.  As most young people in the U.S. have received the MMR vaccine, many of the people getting mumps have been vaccinated.

When many people have been vaccinated in a setting where a mumps cases are occurring, most of the vaccinated people are protected, but mumps cases can still occur in a small percentage. This may make it seem like the vaccine isn’t working. However, the occurrence of mumps during outbreaks in highly vaccinated populations does not mean the vaccine isn’t working.

  • Data from many of these outbreaks indicate that mumps vaccination provides significant protection. In several studies, vaccine effectiveness was estimated at approximately 80% and higher, which shows a great benefit to vaccination.
  • In addition, the rates of mumps among vaccinated persons in these outbreaks were low, ranging from 2% to 8%, compared with mumps infection rates of 25% to 49% in unvaccinated persons.
  • In other words, although a minority of vaccinated people may get mumps during an outbreak, the risk is greatly reduced compared to unvaccinated persons. This means many more people would become ill in the absence of vaccination.
  • In addition, fewer and less severe complications are reported in people that have been vaccinated.

Here’s an example from a real outbreak. In a study conducted among children attending a summer camp in New York where a mumps outbreak occurred, 96% of study participants had a history of at least one dose of vaccine, 91% of whom had a two-dose MMR vaccine history. The mumps “attack rates” among two-dose, one-dose, and zero-dose vaccine recipients were 3.6%, 8.7%, and 42.9%, respectively.  In other words, the risk of mumps for fully vaccinated people was less than 10% (one-twelfth, actually) of that for unvaccinated people. If you were vaccinated, you had a 3.6% chance of getting mumps. If you weren’t vaccinated, you had a 42.9% chance of getting mumps!

Mumps vaccine provides meaningful protection, but is not perfect. There are a few possible reasons why some vaccinated people can still get mumps:

  • Primary vaccine failure has been suggested to have a role in mumps outbreaks, but this has not been proven and based on studies, it would be uncommon.
  • Protection may decrease with time since vaccination. This has been suggested in some studies but not others.
  • There may be differences between the vaccine strain and mumps viruses circulating in the community. Some studies suggest this may be playing a role, but no studies have shown this to be true.
  • Failure to store and administer the vaccine correctly (this would be uncommon) could impact the effectiveness of the vaccine.
  • A combination of these factors.
  • Typically, a person is immune from getting mumps again after having had the disease. However, it is important to note that even mumps infection does not provide 100% protection – rarely, mumps can occur even after a natural mumps virus infection.

The bottom line is that mumps vaccination provides meaningful, although not 100% protection from mumps, and greatly reduces the risk for mumps and its complications compared to unvaccinated people. There are on-going investigations as to whether a routine booster-dose of mumps vaccine would be useful as well as the potential role of changes in circulating mumps viruses.