Syracuse University mumps outbreak – bad anti-vaccine math

Partially because I’m an alumnus, and partially because I watch new reports about infectious disease outbreaks all over the world, I’ve been following the recent Syracuse University mumps outbreak. As of 13 November 2017, Syracuse University (SU) Health Services has reported 41 confirmed cases and 78 probable cases of the mumps on the SU campus.

One of the age-old tropes of the anti-vaccine statistics world is that kids who have been vaccinated against the mumps (or measles or any disease) are more likely to get mumps (or any disease) than those who are not vaccinated. I squashed this myth before, but you know what happens – the anti-vaccine zombie tropes tend to reappear over and over and over and over again.

Now, the anti-vaccine statistics deniers have jumped into the Syracuse University mumps outbreak with their alternative facts, or should I say alternative math. So, once more unto the breach, dear friends, once more. We will take down this trope.

Remember this about measles

Many people think mumps is a minor disease, something you can overcome with a little bedrest and homeopathic potions. This belief results from a lack of personal history with the disease by parents of today. It has been over 50 years since major mumps epidemics swept through children.

Mumps is caused by the mumps virus. According to the CDC, the initial symptoms of the disease include, fever, muscle pain, headache, and feeling tired. This is similar to many other infectious diseases, including the flu. However, these early symptoms are usually followed by painful swelling of one or both parotid salivary glands.

Mumps symptoms generally occur 16-18 days after initial exposure, and the symptoms resolve after 7-10 days. Symptoms in adults are often more severe than in children (pdf).

The CDC reports that there can be severe complications from the mumps. These include:

  • Meningitis (swelling of the tissue covering the brain and spinal cord), which occurs in about 15% of cases.
  • Pancreatitis, which occurs in about 4% of cases.
  • Deafness (temporary or permanent)
  • Encephalitis (swelling of the brain)
  • Orchitis (swelling of the testicles) in males who have reached puberty, which can lead to permanent infertility
  • Oophoritis (swelling of the ovaries) and/or mastitis (swelling of the breasts) in females who have reached puberty
  • In rare cases, mumps can cause death.

Mumps is highly contagious  spreading quickly in closed quarters. The virus is spread through respiratory droplets in the air.

The Syracuse University mumps outbreak probably spread quickly because of close contact of individuals in dormitories and classrooms.

Anti-vaccine statistics – getting it all wrong

Relying upon a whistleblower lawsuit against Merck’s mumps vaccine, which has been thoroughly reviewed on this blog and, currently, has not been resolved in court, the anti-vaccine group is trying to claim that the Syracuse University mumps outbreak can be completely blamed on the faulty mumps vaccine. And this is where their math completely falls apart.

We will start right at the top. The anti-vaccine radicals try to claim that vaccinated kids are more at risk of mumps than unvaccinated kids. And that the effectiveness of the vaccine is worthless. To this end, they anti-vaccine pseudoscientists use amateurish statistics to make these assertions.

Occasionally, infectious disease outbreaks do show higher incidence in vaccinated children. However, this number ignores the frequency or rate. Those bolded words are completely different, in case you are keeping score.

Incidence represents raw numbers. Maybe out of the 41 confirmed cases of mumps at Syracuse University, 30 were vaccinated and 11 were unvaccinated. You’d look at those numbers, and scream from the rooftops that vaccinated kids were more susceptible to mumps than unvaccinated kids.

Except that’s inaccurate. What matters is frequency, and the vaccinated population is probably 10X larger than the unvaccinated population. So the frequency or rate of mumps infection is substantially higher than in the vaccinated group.

However, in the case of the Syracuse University mumps outbreak, it appears that all 41 confirmed cases were vaccinated, while 0 unvaccinated students caught the disease. Unless you flunked basic statistics (or never took it, as I assume is the case for 99.999% of the anti-vaccine religion adherents), you’d know that it would be a false statement to claim that vaccines are worthless based on these numbers. Far from it.

Syracuse University mumps outbreak statistics

The anti-vaccine crowd uses the raw incidence number to compare vaccinated and unvaccinated populations. However, that is not the appropriate way to examine this outbreak.

Let’s look at the real numbers. Syracuse University has about 22,000 undergraduate and graduate students (pdf), and about 3200 faculty and staff. That makes around 25,200 individuals in close quarters during this outbreak.

According to the CDC, 91.9% of these individuals probably received the MMR vaccine (against measles, mumps and rubella). So that means, out of these 25,200 individuals, 23,159 were vaccinated, and 2,041 that weren’t vaccinated.

Again, it is claimed that all 41 confirmed cases were vaccinated. Now, if the MMR vaccine were totally worthless, and because mumps is extremely infectious, we’d expect huge numbers in this outbreak. However, we observe a rate of only 0.17% of the predicted vaccinated group who caught mumps.

Moreover, the predicted effectiveness of the mumps vaccine is around 88%, and it’s probably a lot lower as the vaccine has shown waning effectiveness, we would predict that there would actually be 2,779 cases of mumps in the vaccinated population. The observed number of cases, 41, is far below what is predicted.

But you’re still pointing at me and saying, “enough with your wizard math – you’re ignoring the fact that no unvaccinated kids got the mumps.”

That’s true. Unless you actually understood statistics. The problem is that the sample population, 41 infected individuals, is tiny. So there is high variability on our predictions based on real math.

If we take a Bayesian probability, there is a high chance, completely random, that the number of unvaccinated students and staff who contracted mumps could be 0 – on the other hand, it could be 200. That means that this data provides us with no evidence that unvaccinated kids have a superior resistance to the mumps virus. None at all.

Now if we have a sample size of 1000 infected students and staff at the Syracuse University mumps outbreak, and 100% of them were vaccinated, I’d wonder if there’s an issue. I’d be writing about it here.

However, knowing that there are no miracle preventions against mumps in an unvaccinated population, we would predict that 97 unvaccinated students and staff would be contract the disease out of the total of 1000. And yeah, that means we would predict 903 vaccinated students and staff would contract the mumps. And that would not mean that vaccinated individuals are more prone to mumps – those numbers would mean that the incidence of mumps is approximately equal.

But there’s more to this story. The vaccinated group is providing some level of protection, through the herd effect, to both the vaccinated and unvaccinated groups. Remember, there are only 41 infected students, out of a predicted 2,779 cases if everyone susceptible to the mumps (unvaccinated plus those with waning immunity or ineffective vaccine) caught it.

So the unvaccinated group can thank the vaccinated ones. You’re welcome.

Summary of the Syracuse University mumps outbreak

The lack of understanding of small numbers from the anti-vaccine religion would be amusing, if it weren’t for the danger vaccine preventable disease present to the students and staff at SU. Though this outbreak is scary, because the mumps is a dangerous disease, and SU is re-vaccinating all of their students and staff, only 41 cases have been confirmed.

Remember, 41 is a tiny number to sample a population of 25,200 students and staff. Unless you’re into anecdotes, bad statistics, and magical beliefs, you simply cannot make many conclusions (if at all) about vaccinated vs. unvaccinated populations at Syracuse University.

Yes, the mumps vaccine isn’t perfect.  Yes, we have observed waning immunity from the vaccine. But it still protects the vast majority of individuals against a relatively dangerous disease. I just get the feeling that the anti-vaccine religious fundamentalists believe in the Nirvana fallacy– they believe in a false dichotomy that either a vaccine is 100% effective or it’s worthless. Or, a vaccine is 100% safe or it’s completely dangerous.

We have no evidence, based on this Syracuse University mumps outbreak, that the mumps vaccine is worthless. We do have powerful evidence, based on scientific research, that mumps vaccine is very effective.

And let’s remember, mumps can kill. Mumps can harm. And the vaccine saves lives by preventing mumps.

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The Original Skeptical Raptor
Chief Executive Officer at SkepticalRaptor

Lifetime lover of science, especially biomedical research. Spent years in academics, business development, research, and traveling the world shilling for Big Pharma. I love sports, mostly college basketball and football, hockey, and baseball. I enjoy great food and intelligent conversation. And a delicious morning coffee!