Medical Detectives Explore Mystery of Mumps Outbreaks


The past year was a big year for the mumps virus. With 5,311 cases in 2016, the United States experienced the most mumps infections since 2006. The majority of the outbreaks occurred in the Midwest, however Washington State recorded 486 cases in the past 5 months. Mumps was a very common viral infection prior to the development of a vaccine in 1967, and for over the past 50 years has been well controlled. In attempting to understand the recent increased number of mumps infections, a rational explanation is that the affected individuals had not received all of their routine MMR (Measles, Mumps and Rubella) shots. However, the mumps infections have in fact occurred in populations where individuals have been up to date on their shots, having received the recommended two doses of MMR at around 1 and 4 years of age.  The question remains then, why has the mumps virus made such a commotion this past year?  Has the mumps vaccine become less effective? What does this mean for routine MMR vaccination in the United States? The recent rise in mumps cases has raised intriguing questions such as these for scientists to explore and attempt to answer.


A very refreshing aspect of the scientific process is that when events don’t quite make sense, scientists approach a problem by attempting to observe and note the facts, and then study the possibilities with objective research. With the recent increase in mumps infections, researchers are actively involved in the process of being “medical detectives.” They are looking at the data, evaluating possibilities, and figuring out how to test their ideas. While we do not yet know why the mumps outbreaks have occurred, there are clues that may offer insight into this mumps mystery.


The mumps infections have typically occurred in vaccinated populations, in which people have been living in high density environments, such as college campuses. This close proximity makes transmitting germs easier. Meanwhile, the mumps component of the MMR vaccine is good at preventing mumps infections, but not quite as good as the measles and rubella components. For example, the measles vaccine prevents measles infection with about a 98% success rate. Mumps “vaccine efficacy” is a little lower, at approximately 88%. This slightly lower vaccine efficacy can allow the infection to spread more readily. In addition, after a child receives the second MMR at age 4, the level of mumps protection declines gradually over time, which also increases the possibility of infection. All of these reasons – high density living situations, good but not great vaccine efficacy, and declining immunity over time – may have contributed to the mumps outbreaks.


However, perhaps some the most fascinating reasons why mumps has had a big year may have to do with the “big picture” perspective. Scientists know that some viruses naturally go through cycles over time, where infections increase at somewhat regular intervals. For example, before the advent of the rubella vaccine, rubella epidemics occurred every 7-8 years. It is possible that mumps may have a similar natural history, and perhaps this past year was time for a natural epidemic to occur. If this is the case, the vaccine could have blunted the spike of infections but not prevented it altogether. With this hypothesis in mind, it will be interesting to see if mumps naturally quiets down over the next couple years on its own accord.


In addition, measles and rubella have been completely eradicated from America, such that new cases in The United States are imported from travelers arriving from other countries. Mumps on the other hand has not been eradicated from America, and may be circulating within the country more than previously thought. It is known that approximately one third of mumps infections occur without symptoms, which in turn could enhance spread of the virus without our awareness, or “below the radar.” This phenomenon could have contributed to the increased number of infections. These are some of the thoughts scientists are considering, but they are only ideas at this time. Similarly, it may make sense for a third dose of MMR in the setting of a local outbreak; however, objective data is necessary before making this change.


Lastly, an interesting – and reassuring – observation from the recent mumps outbreaks is that while mumps infection occurred in several thousand people this past year, no complications of the infection have been seen. Before the vaccine was routinely used, mumps infection was associated with a 4% risk of hearing loss, 4% risk of pancreatitis, 30% risk of inflammation of the testes or ovaries, and resulted in approximately 5% of patients becoming hospitalized. This raises the question of whether those recently infected with mumps in fact had enough protection from the vaccine to prevent potentially serious complications. At this time we don’t know, but this is another hypothesis that scientists are exploring.


Medical epidemiologists are doing their fact checking in an attempt to understand the mumps outbreaks better. While science may sometimes be portrayed in the popular media as stodgy or lacking creativity, our current mumps mystery illuminates just how creative science can be with problem solving. Scientists have to “think outside the box” to figure out these problems, both in formulating possible explanations as well as determining how to use the scientific process to test their ideas. In addition, nothing is a given- no matter how good an idea it may seem – until it is supported by the evidence of high quality research. In fact, in science nothing is really ever permanently a “given” as science promotes an ongoing process of evaluating and reevaluating. However, the more research studies that come to the same conclusion, the more likely a concept may be true.


While there have been an increased number of mumps cases this past year throughout the country, and it is not entirely clear why, there are interesting observations – or clues – for researchers to explore. It will be exciting to see what our creative, problem solving medical scientists discover. In the meantime, be sure to encourage MMR childhood vaccination.  A profound consideration is that use of the MMR vaccine has reduced mumps infection by approximately 99% compared to the pre-vaccine era.