Posted by: Chris Maloney | October 3, 2014

What Is The Chance Of Developing Measles Encephalitis?

Margaret Harmon wrote a wonderful, terrifying portrayal of her son’s bout of measles encephalitis that was republished in the Kennebec Journal recently. (link here). As a result, I’ve been asked: “can my child, even if they’ve been vaccinated, get this disease?” The short answer is no, very unlikely.

Here’s the longer answer:

While Harmon’s intention is clearly to terrify vaccine resistant parents and to vilify any doctors who might support them, she mentions in passing a simple reality that we cannot avoid. “In epidemics, even vaccinated children can fall ill.” She uses this to point another finger at the dangers of not vaccinating, but it bears repeating. Children, fully vaccinated for the measles vaccine, can still get the measles.

What is the percentage of children, who, having been fully vaccinated, may still get the measles? By our best estimates, measles vaccine is very effective at creating antibodies. But in monitoring outbreaks among a highly vaccinated population, van Boivin, and others found that even fully vaccinated children are susceptible. (paper here).

According to the Cochrane database and based on studies involving over fourteen million children (abstract here) the measles vaccine is 95% effective. If five percent of children grow up, then five percent of the overall population is susceptible to the measles. According to van Boivin, a greater than 95% effective vaccination rate is necessary to eliminate the measles. If we align the two, even when 100% of the population is vaccinated, the measles can still be spread by the 5% who did not respond to full vaccination.

Not to say that we shouldn’t terrify and blame parents who refuse the vaccine to get them to comply, but if Margaret Harmon accomplished her goal of 100% full vaccination she would not accomplish the complete elimination of measles or the risk of measles encephalitis.

So what is the risk of measles encephalitis in the U.S.? According to the CDC: “Analysis of data from an outbreak of measles in the United States during 1989-1991 suggests a rate of 4-11 cases of SSPE (fatal degeneration) per 100,000 cases of measles.”

But above that statistic they have the following: “For every 1,000 children who get measles, 1 or 2 will die from it.” Doing a little math gives two different numbers: one in a thousand vs. the reported 11/100,000 or roughly one in ten thousand.

Let’s look at the more recent reports, not using CDC information from the late 1980s. In looking for recent reports of measles encephalitis, we find this interesting little tidbit: “measles transmission has been eliminated in the United States and the rest of the Americas.” (report 2008-the announcement was first made in 2000) What that means is that there is no transmission inside the U.S. and hasn’t been for years. All measles cases are coming in from outside the U.S. In 2005, there were 66 reported cases in the U.S. In 2014, there have been 592 reported cases, a massive increase over the last fourteen years. (graph here).

There have been no reports of measles encephalitis in the past fourteen years (that I could find). From even the most dramatic statistic, 1 in a 1000, there simply haven’t been enough cases in past years. It is very likely, with the sudden upswing, that we will have a very ill case of measles somewhere in the U.S. this year. I expect it will make the evening news when it does happen.

More locally, what are the risks of getting measles encephalitis in Maine? According to the National Survey (here), Maine’s vaccination rate for MMR is 91% but plus or minus 4.5%. That’s not very reassuring. According to the Maine CDC, there were no cases of measles in Maine since 2004. (here)

So, given the most dramatic number from the U.S. CDC, 1 in 1000, and the most recent data from our state CDC, no cases in ten years, what are the chances your child will develop a horrifying case of measles encephalitis? We can’t ever say zero, particularly with this year’s sudden uptick in imported measles cases. But I would be far more concerned with enterovirus 68 (my blog post on that is here).



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