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Mumps vaccine effectiveness — the facts

In two previous articles from the vaccine legal expert, Professor Dorit Rubinstein Reiss, a whistleblower lawsuit against Merck regarding its mumps vaccine was thoroughly discussed. The whistleblowers claimed that Merck provided false information about the effectiveness of the mumps component of the MMR vaccine (which prevents mumps, measles, and rubella infections).

In essence, the whistleblowers claim that Merck, the manufacturer of the MMR vaccine, through either direct falsification or poor study design, may have overstated the effectiveness of the mumps component of the vaccine. Merck has been claiming that the vaccine was approximately 95% effective (meaning at least 95% of children given the vaccine were protected against the disease).

As Professor Reiss wrote, a Federal court dismissed the case, and there was no evidence that Merck provided false information about the effectiveness of the measles component of the MMR vaccine.

So let’s be clear about this so-called whistleblower lawsuit — no evidence has been presented, and that evidence hasn’t been cross-examined. In science, only a cold evaluation of evidence published in respected journals matters, not dismissed lawsuits. Furthermore, the weight of the evidence is both in quality and quantity.

This article will examine the scientific and clinical facts about mumps vaccine effectiveness.

photography of people on grass field
Photo by Anny Patterson on

First, what is mumps?

I still find it incredible that many people believe that mumps is a minor disease. Part of that is because the last major outbreak of mumps amongst children was in 1967 when over 162,000 children were infected, something we don’t see anymore.

Mumps are caused by the mumps virus. The disease itself is known for the swelling of the cheeks and upper chin as a result of the swelling of the salivary glands. Although most individuals who catch the disease recover after 1-2 weeks, the disease can cause some serious complications:

  • The most common complication is inflammation of the testicles (orchitis) in males who have reached puberty; rarely does this lead to fertility problems. However, recent epidemiological research has shown that males who are unvaccinated against mumps have a small but significantly increased risk for infertility.
  • Inflammation of the brain and/or tissue covering the brain and spinal cord (encephalitis/meningitis).
  • Inflammation of the ovaries (oophoritis) and/or breasts (mastitis) in females who have reached puberty.
  • Deafness.

Yes, these complications are rare, generally less than 5%. But if we go back to the time when over 162,000 children were infected by mumps every year (or approximately 270,000 to adjust for today’s USA population), a 5% complication rate would mean around 13,000 children would require additional treatment.

Anti-vaccine activists always seem to dismiss the risks of complications because they live in a world where the mumps vaccine has effectively reduced the incidence of the disease to a few hundred each year.

Examining the effectiveness of the mumps vaccine

Before I start anything, the MMR vaccine (of which, the mumps vaccine is a component) is not linked to autism spectrum disorder, according to over 150 published clinical studies. This is not even a subject of scientific discussion, despite the unfounded and pseudoscientific claims of the anti-vaccine world.

However, this article is about the mumps vaccine effectiveness which was the primary issue pushed by the Merck whistleblowers. Here are some of the unbiased, peer-reviewed, published research on the overall effectiveness of mumps vaccines:

However, there might be a reason why the effectiveness of the mumps component is lower than expected – waning immunity. Although the immune system remembers all antigens theoretically forever, in reality, without constant challenges, immunity could wane over time.

There is strong evidence that the recent increase in mumps outbreaks in the USA may be related to both a lower antibody level over time and incomplete vaccination (meaning getting only 1 of 2 doses). In addition, because of nearly non-existent exposure to the wild virus (thanks to the success of vaccinations), those who only had one dose never get a “natural” booster. They then are at risk of the disease, especially in a closed environment like a college, when someone brings the virus from another country where mumps has larger epidemics or is endemic.

Other evidence indicates that even those individuals with two doses during childhood have waning immunity, and when they become adults, they might become more susceptible to mumps, especially as the time between immunization and exposure to the wild virus increases.

Scientific research has given us a better view of how the immunity against mumps begins to wane over time, irrespective of whatever data comes from the manufacturer of the vaccine, Merck. The research also tells us that even in individuals with waning immunity (the term is not “lost immunity”), the course and severity of the mumps infection is less than in those who were never vaccinated.

Again, let me make this clear. What Merck did or did not do is irrelevant, absolutely unrelated to the safety and effectiveness of the MMR vaccine, specifically for mumps. The scientific evidence clearly shows that effectiveness is extremely high, and Merck’s data for mumps is right on the upper end of the statistical errors for what has been found in independent research. And more than that, waning immunity may be contributing to a slight lower effectiveness, something outside of the research done by Merck.

Nevertheless, because of what we know about waning immunity, it’s possible that the schedule for vaccines may have to be reconfigured. Maybe, like tetanus vaccines, it has to be repeated every 10 years or so, boosting our immunity in a naturally effective manner. Scientific research will give us future choices in either better vaccinations or boosters, so that we keep mumps at bay.

And let’s remember vaccines save lives.


Michael Simpson
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