The lesson of the 2006 mumps outbreak

If you want to know the single most important class of public health interventions with respect to infectious diseases in the 20th century it wasn't vaccines but provision of clean water and food supplies. But vaccines may be next. With major waterborne diseases like typhoid and cholera under control, the next big category of infectious diseases was the major childhood ailments: measles, German measles (aka rubella), mumps, chickenpox, polio, diphtheria, whooping cough. Some were just memories by the time I came along (diphtheria, pertussis), one was conquered in my younger years, the others were after my time but my kids got vaccinated against them (measles, mumps, rubella). Both missed the chickenpox vaccine (and both got chickenpox "naturally"). For my grandchildren, I am hoping most of those diseases will be historical curiosities. But the largest outbreak of mumps in recent memory -- over 6000 cases in the midwest in 2006 -- shows that some of these diseases still have some fight in them, an effective vaccine notwithstanding.

The first suspicion when an outbreak of mumps across six midwestern states came crashing down on the 18 to 24 year old age group (mostly college students) was that a tendency toward waning vaccination rates was responsible. Mumps is a fairly contagious viral disease and has a high reproductive number (the average number of new cases produced by a single infective case in a completely susceptible population). In order to keep very contagious diseases like mumps from spreading there has to be a fairly high level of population immunity (referred to as "herd immunity"). If the levels drop below a certain proportion of the population outbreaks can occur. Now that CDC and the state health departments that coped with the outbreak have looked at what happened, however, it seems that this was a surprise failure of the vaccination regimen itself, not the population level of vaccination.

Mumps vaccine is usually first given to children 12 to 15 months old combined with measles and rubella vaccines, then given again at ages 4 to 6. It now appears likely the immunity conferred by this two dose schedule is not sufficiently long lasting, since four out of five of the mumps cases had a history of receiving both doses of the mumps vaccine. Still, the high level of population immunity means the virus does not normally circulate in the community, even at low levels, so even with reduced protection from a waning response there would be no outbreak. Outbreaks still depend on the presence of the virus. But many countries, even those with good public health systems like the UK, do not require mumps vaccination for children and the strain involved in the outbreak was likely imported from there by a traveler.

This is an object lesson in the importance of maintaining effective routine public health protections (and in this case points to improving the effectiveness of an existing public health program); and an example of the fragile status of hard won victories in a tightly interconnected world. Even when we believe we have protected our populations, a failure elsewhere on the globe can threaten us.

The mumps virus didn't check passports before deciding to infect someone. The big lesson of the mumps outbreak -- a lesson that shouldn't be missed because of an understandable preoccupation with the implications for the mumps vaccine recommendations -- is that we are all in this together.

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revere: Was there any recommendation made for that age group to re-vaccinate in all other parts of the country? Was there a batch difference between the midwestern states and other parts of the US?

wen: My understanding is that they are reviewing the recommendations to see if this experience requires a third booster (e.g., upon graduating highschool). I don't know of any differences in the vaccine lots themselves and I think this is unlikely. We are just learning more about the natural history of the vaccination.

Sorry you omitted Smallpox from your first paragraph... Perhaps that might cause you to reconsider your overall ranking of the PH impact of "vaccines" (and toxoids, please).

By bingoswami (not verified) on 12 Apr 2008 #permalink

bingoswami: Good point. But the data on the modern rise in world population are pretty clear. The increase is not affected by vaccines and other medical technologies but by sanitation. This in fact makes sense as you are operating on the whole population at once. The demographers have looked at this since McKeown's work four decades ago.

But many countries, even those with good public health systems like the UK, do not require mumps vaccination for children and the strain involved in the outbreak was likely imported from there by a traveler.

I'm confused here. I was under the impression that the MMR is part of the routine immunization schedule in the UK. Are the requirements for immunization prior to school entry more lax in the UK?

Enjoyed the post!

FullFrontal: Thanks. My understanding is that mumps vaccination is not mandatory in the UK. Can any of our UK readers (we have many) confirm this?

That's correct: MMR is part of the standard offered schedule in the UK, but no vaccinations are mandatory for school attendance, or for any other purpose that I know of.

By Mathematician (not verified) on 12 Apr 2008 #permalink

Having worked through the great mumps outbreak of 2006, I'd like to point out a couple of things. 1st, except for virus culturing, lab testing for mumps isn't very good, and culturing, while 100% specific isn't very sensitive. 2nd, many of the diagnoses were made by people (physicians and others) who had never seen a case of mumps. As a result, at the height of the outbreak, we had people with any kind of sore throat being reported as having mumps. My favorite story was about the teen who appeared at the ER with a swollen jaw, was seen by the ER physician and diagnosed with mumps. When he asked about x-rays, the physician asked why and was told that the teen thought his jaw might have been broken when he got hit by the baseball bat.Not saying that there weren't lots of cases of mumps, and that the waning of the vaccine's protection might have contributed. I'm just saying that the 6000 cases may not have all been because of low titers.

MoM: You make an important point. The first step in an outbreak investigation (as I learned it many moons ago) is verify the diagnosis. The NEJM paper used a clinical case definition for mumps:

A case that met the clinical case definition (i.e., an illness with an acute onset of unilateral or bilateral tender, self-limited swelling of the parotid or other salivary gland that lasted at least 2 days, without other apparent cause) and that was not laboratory-confirmed was classified as a probable case. A patient with a confirmed case had a positive laboratory test or met the clinical case definition and was epidemiologically linked to a confirmed or probable case.

Unfortunately lab studies in vaccinated people have limitations in ruling out mumps, so the question of accuracy of the case definition is an open one. Given the relatively high level of 2 dose vaccination coverage, however, and the certainty this was a large outbreak (even if some overdiagnosis occurred) opens the question of the adequacy of the vaccine regimen. We will likely hear more about this. These data should be seen as preliminary.

I quite agree. And I know that in a neighboring state, they had a high percentage of cases that were culture confirmed. Our demographic pattern was somewhat different than those states with the high level of cases in college-age people, and given our lower level of culture positives, it makes me wonder how many of ours were really mumps.If I'm not mistaken, that case definition is right out of the CDC's Case Definitions for Infectious Conditions Under Public Health Surveillance. MMWR 1997:46 (No.RR-10): 39. It is the one we use as well.