Measles Q&A

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A while back, Dr Mary Fernando wrote an article about Measles for the Medical Post, which has since been reproduced at Canada Healthwatch. For the article, she asked me to submit some of the common Measles questions that people send me, which I did, and which she then incorporated into her splendid article.

I thought it would be useful to share with you the complete document that I sent her, just for archival purposes. So here goes: questions about Measles that people send me, and the answers that I give…


Question #1: My kid got 2 MMR vaccines. So why should I care that there are cases now appearing? Don’t the vaccines work?

 One dose of MMR vaccine confers about 93% protection against measles infection. Two doses are supposed to offer about 97% lifetime protection. That’s astoundingly good. But serological studies suggest that immunity can wane at a fairly precipitous rate, perhaps as much as 9.7% per year. This will, of course, vary quite dramatically from person to person.

Let’s back up a moment and consider why vaccination might fail for some people. We tend to think of primary or secondary vaccine failure. Primary failure refers to those unusual instances where a vaccinated person fails to mount an immune response. For MMR, this can be as much as 2-7% of those who received only one dose, and 1% of those who received two doses. Commonly, the estimate is combined to suggest that ~5% of all MMR vaccinated people do not “seroconvert” to produce antibodies against infection.

Secondary vaccine failure refers to instances where antibody levels have fallen over time to the point where “humoral” immunity (the kind that prevents infection, not just serious outcomes) has plummeted significantly. It is estimated that 4-8% of “breakthrough” measles infections among vaccinated people are the result of secondary vaccine failure.

What this means is that even if you are diligently up to date on your MMR shots, there is a small but meaningful chance that you are not actually protected against measles infection. Therefore, the best way to ensure that you or your child do not become infected is to lower the probability that you will be exposed to the virus at all. This means doing everything possible to make sure everyone else in your community has been immunised, either through vaccination or natural recovery.

There are also some people who, for medical reasons, are unable to accept the live MMR injection. Those people most definitely rely upon the rest of us to do our civic duty and to seek immunity. 

For a disease as contagious as measles, and for a vaccine with the efficacy benchmarks of MMR, this means that 95% of people in a given community must be vaccinated or be otherwise immune in order to eliminate the disease from the immediate area. This is the so-called herd immunity threshold.

Like all other population health issues, measles is a community concern, not just an individual concern. But to protect the community –and therefore ourselves– we must take action at the individual level. So it should matter to you that others are not vaccinated, even if you are.


Question #2: I had measles as a kid. So did lots of people I knew. It wasn’t a big deal. Why should we worry?

In recent decades, measles stopped being scary because of two reasons. First, vaccination rates have been so high that cases are rare. Second, health care advancements have made death by measles even rarer.

As vaccination rates drop, everything changes for the worse.

It’s useful to keep a historical perspective. Since mandatory MMR vaccination became the norm in the 1970s, Canada has seen a dozen or so cases per year, perhaps getting as high as a couple of hundred cases in unusual years. Prior to vaccination, though, we saw 50,000 to 100,000 cases per year, at a time when we had a smaller overall population!

The measles death rate fell sharply last century prior to the arrival of vaccination in the 1960s, mostly due to improvements in nutrition, general health, advanced health care, and the availability of antibiotics used for treating the secondary bacterial pneumonia that often results from measles.  

Now, about 0.1% of measles cases will die. In poorer countries, however, that number is as high as 15%, due to a dearth of the aforementioned advantages. And about 20% of unvaccinated measles cases here in wealthy countries end up hospitalised. 

Clearly, if we were to return to pre-vaccination infection rates in Canada, the resulting yearly deaths would dwarf the mortality impacts of other child killers, like leukaemia. More immediately, the hospitalisation increases would be significant, at a time when our healthcare system is severely stressed.

It is unlikely that we would return to the infection levels we saw in the 1950s and before, mostly because the great majority of Canadians are presently well vaccinated. But all it takes is a generation or two of vaccination slippage, and significant pockets of susceptible Canadians will emerge, making the likelihood of regular multiple explosive outbreaks quite high indeed.


But I’m afraid of the MMR vaccine. I hear it causes autism and worse!

No medical intervention is without risk. But the MMR vaccine is one of the most effective and tested paediatric medical tools in global use.

The number one reason given by MMR hesitant parents is their belief that the vaccine will cause autism in their child. This alleged association came into the public consciousness with the 1998 publication of a study by Andrew Wakefield. The paper was subsequently retracted, and has since been deemed fraudulent

Multiple studies have since confirmed that there is no observed causal connection between the MMR vaccine and autism.